Sofirn SK40

1lumen selects and reviews products personally. We may earn affiliate commissions through our links, which help support our testing.

Sofirn SK40 Specifications & measurements

Brand & Model	Sofirn SK40
Flashlight category	Duty/General Purpose
Battery config.	1*21700
Switch type	Rear, Side
Dimensions:
Length	146 mm
Head diameter	40 mmn
Weight with battery	205 g
LED & Beam
LED type	Luminus SFT-70
High CRI	No
Focusable beam	No
Features
Onboard charging	Yes
Power bank	No
Specifications and measurements
Specified output	3,200 lm
Measured output (30sec)	2,520 lm
Max. measured output	2,664 lm
Specified beam distance	580 m
Measured beam distance (30sec)	604 m
Specified beam intensity	84,583 cd
Measured beam intensity (30sec)	91,225 cd
Overall rating	5 Stars
Review publication date	July 2025

If you’re a flashlight hobbyist, or just someone who likes bright flashlights, but doesn’t have the funds to plop down on a higher-output Acebeam, Olight, or Fenix light, you’ve probably familiarized yourself with Sofirn. Alongside Convoy, Wurkkos, and Astrolux, they have made a name for themselves and amassed some impressive street cred in the low-cost, high value game.

Their lineup is also pretty impressive, and they’ve dabbled in the duty/tactical-role lights a bit, but never really produced one that could ‘keep up’ with the mainstream leader lights, until now. Sofirn released the SK40, a single 21700 high output tube light with a dual-switch UI aimed at less-casual users and more towards professional users.

That’s a super-tall order indeed, but Sofirn put their best foot forward with this one, and at least on paper, it really has some pro-level hardware and specs. The specs are what matter here, and we all know Sofirn, but hey, maybe I’ll be surprised here.

What's in the package

Sofirn lights have come in really nice, presentable and up-level packaging of late. The SK40 is no exception, following the SR15’s lead with a white, rigid box with a flip-open magnet closure lid and an Olight-like presentation. The light sits in dense microcell foam with the bits on the side. Here’s what’s inside:

• Sofirn SK40
• Sofirn branded 5000 mAh 21700 (loaded in the light)
• O-rings
• USB C to C cable
• Lanyard
• Manual

I’m always glad to see a full load-out, ready-to-go kit included with a budget light at a time when higher-end (and cheap) brands are cheaping out and leaving out USB cables and O-rings. The SK40 includes a nice C to C cable for charging, and the cell was sitting at 3.8 volts, which isn’t great for storage, but the cell was isolated with an insulator, and that higher voltage gets you impressive out-of-the-box output.

Let’s address the elephant in the room: Sofirn is marketing this as a tactical/duty light. Having reviewed lots of those from the major brands, I’ve found it’s a tall order for any flashlight, let alone a budget one, to get right, but here we are. The SK40 is a single 21700 tube light with dual switches for tactical and duty use (on paper).

The design is reminiscent of the SR15 and Speras EST Plus, with a wider head and longer tailcap to accommodate the rear mechanical switch. There’s some shallow heat sinking on the head, and circular, pointed grooves milled in the tube for grip, similar to the Olight Warrior X series. Otherwise, the design is pretty utilitarian and unremarkable.

It’s too big for EDC, but you can clip or tote the SK40 easily. The handling manners are sound. It’s easy to grip and the controls are easy to access in all grip positions. The switchgear consists of a large forward clicky out back and a head-mounted side e-switch for mode changes. The forward switch has a somewhat smooth, but grippy rubber boot and nice positive clicks and feedback.

The button is flanked by two ‘ears’ with good-sized lanyard holes, and it helps orient the thumb over the switch. You get a friction-fit, removable one-way (bezel-down only) pocket clip for retention. It’s a semi-deep carry clip and seems durable, but not the most sturdy I’ve seen.

The side switch has a firm rubber cover with a central LED indicator for on-state and battery condition. It feels good: A low action, but with tactile feedback and muted clicks. More importantly though, it’s easy to find by feel. Opposite the switch is the USB C charge port. It’s sealed with a rectangular, deep rubber cover that’s been plucked from the Sofirn/Wurkkos parts bin. That’s a good thing because it makes a nice seal and is easy to open, but not so easy as it would open unintentionally. 

For build quality, Sofirn is a budget brand, and doesn’t gloss over that fact, but their lights have become better and better over the years, adding more uplevel features found on higher priced lights. The SK40 is no exception, and the build quality, fit, and finish are on-point with no anomalies or defects.

Functionality is perfect as well and all this suggests a higher price point than this comes in at. Pricing for the SK40 depends on the vendor, but direct from Sofirn it’s around $45 US with a battery. That’s a steal considering the competitors run $100-$140.

The 60601-T6 aluminum host is nicely machined with no defects, gaps, or machine marks anywhere, and the finish is equally nice. It’s advertised as black type III HA, and semi-matte. Sofirn finishes are pretty durable, but not like something from Fenix or Olight.

The light breaks down easily: Tube, tailcap, and the tailswitch PCB is retained with a cool machined aluminum retainer. There’s two springs at work: One for the driver and tail like there should be in a light meant for more than searching behind the couch for a cat toy. The tail spring is also nice and stout, with an inner and outer spring for a low-resistance current path and durability. 

The rear threads are fully anodized, rectangular cut units that are really smooth and nicely lubed. There’s o-rings sealing the ends of the tube, and with a tightly sealed charge port, the IP rating (on paper) is IP68, so pretty good for a budget light. However, I doubt it’s been tested, so how far that IP68 goes is up to you.

For the warranty,it’s decent and depends on where you buy your SK40. However, here’s what Sofirn has to say (from Sofirn): AT SOFIRN, WE BELIEVE IN OUR PRODUCTS.

If your Sofirn product has any defect as the result of the materials or workmanship we want to make it right!

Within 30 days of purchase: Contact the original seller for repair or replacement.

Flashlights: Within 1 year of purchase: Contact Sofirn for repair or replacement via

Email: [email protected]

Facebook group: sofirn deals group

Facebook page: www.facebook.com/sofirnlight

Facebook Messenger: Sofirn Flashlight USA

Batteries: Within half a year of purchase: Contact Sofirn for repair or replacement via

Email: [email protected]

Facebook group: sofirn deals group

Facebook page: www.facebook.com/sofirnlight

Facebook Messenger: Sofirn Flashlight USA

In the first instance we attempt to resolve the issue. In the unlikely event of a faulty product we may advise customers to return the product to Sofirn for repair or replacement.

Please Note:

This warranty does not cover normal wear and tear, modifications, misuse, negligence, accidents, improper maintenance, or repair by anyone other than an Authorized retailer or Sofirn itself.

Tactical and duty-ready lights need high output and solid midrange levels of throw, so Sofirn grabbed what I feel is the right LED for the job, one that’s garnered some solid street cred (Fenix and Olight use it), the Luminus SFT-70. I don’t know if this is the gen 1 or gen 2, but all SFT-70s are 5050 size, domeless quad die emitters.

They’re lateral design LEDs with bonding wires for the dies, but they’re potted in the substrate for added durability and current handling with an efficiency boost at higher outputs. It’s similar to the XHP50.3 HI, but the LES is smaller, so you get a bit higher intensity and lumen density for more cd/mm². 

It runs on 6 or 12 volts, and comes in CW 6500K. For the bezel, it’s a bead blasted stainless steel unit that seems a bit small for serious smashing duty, with crenulations that extend a ways over the hardened AR coated mineral glass lens so it’s protected well.

The reflector is a deep, smooth unit for a nice blend of side illumination and lots of reach. The beam has a nice even spill and plenty of side illumination with a brighter corona and a large bright center spot. You get a lot of throw with this configuration, and it makes for a versatile beam.

More details: 

• LED Type: Luminus SFT-70
• High CRI: No
• Focusable beam: No

Spectral measurements: 

I used an Opple Lightmaster Pro to measure the flashlight at 2 meters distance. 

Mode:	CCT:	CRI Ra:	duv
Turbo	6691K	68.8	0.0043
Low	5585K	70.3	0.0076

Dimensions: 

Sofirn SK40	Millimeters	Inches
Length	146 mm	5.5 in
Head diameter	40 mm	1.6 in
Body diameter	25 mm	1 in

Dimensions are rounded to the nearest millimeter and the nearest tenth of an Inch.

Weight: 

Sofirn SK40	Weight in grams	Weight in oz
Without battery:	136 g	4.8 oz
With included 21700 battery	205 g	7.2 oz

Weight is rounded to the nearest gram and tenth of an Oz.

Flashlight size comparison with its competition:

Group 1: Fenix TK17, Streamlight Sting 2020, Sofirn SK40, Acebeam P17 Defender, Olight Warrior X4

Group 2: Klarus XT11GT Pro V2, Sofirn SR15, Sofirn SK40, Speras EST Plus, Convoy M21B

Group 3: Acebeam P17 Defender, Sofirn SK40, Olight Warrior X4

The driver is going to be a constant current boost unit since the single 21700 can’t provide the 6 volts the SFT-70 needs. For years, all you could get from Sofirn was a linear/FET driver, but no more, and that’s great because it’s a premium feature usually found on more expensive lights.

In addition, it’s pretty much mandatory for anything other than chasing off the raccoons gorging themselves on your cat’s food at night. A light meant for working requires a fully regulated driver for uninterrupted brightness regardless of battery state. Also, in my opinion, there’s no reason why a mainstream light shouldn’t have a constant current driver onboard.

For the UI, Sofirn went with a fairly generic dual-switch affair with four modes, Turbo, and three Strobes: Strobe, S.O.S, and Beacon. These are broken down into two lighting modes: Tactical and Duty. Tactical accesses two modes, with instant Turbo, and Duty mode for more everyday tasks with access to all lighting modes plus instant Turbo and blinkies. To switch between the two modes, from ON, press and hold the side switch for about four seconds. The main light will blink twice to acknowledge the switch.

Available modes: 

• Eco, Low, Med, High, Turbo 

Available blinky modes:

• Strobe, S.O.S, Beacon flasher. 

From OFF Tactical Mode:

• Fully click tailswitch: Constant on in Turbo
• Half press tailswitch: Momentary on in Turbo
• Double tap rear switch: Momentary Strobe
• Hold side switch and click rear switch: Eco

From ON Tactical Mode:

• Fully click tailswitch: Turns off
• Click side switch: Activates Turbo, clicking again activates Medium. Turbo is the first mode by default
• Double click side switch: Activates Strobe. Clicking the side switch again advances to S.O.S, and double clicking again switches to Beacon (repeat to cycle back to Strobe)

From OFF Duty Mode:

• Fully click rear switch: Constant on in last used mode
• Half press rear switch: Momentary on in last used mode
• Half press rear switch and click side switch: Turbo
• Hold side switch and click rear switch: Eco

From ON Duty Mode:

• Fully click rear switch: Turns off
• Half press rear switch: Momentary on in last used mode
• Single click side switch: Cycles through the lighting modes Low-Med-Turbo-Low (ascending order)
• Press and hold side switch: Long-pressing for more than one second activates Strobe. Long press the side switch again to exist Strobe to the last used mode

Mode memory:

• Yes in Duty mode only

Shortcuts:

• To Strobe: from OFF in Tactical mode, double tap the rear switch. From ON in Duty mode, triple click side switch.
• To Eco: From OFF in Tactical and Duty modes, press and hold rear switch and click the side switch
• To Turbo: From OFF in Tactical mode, click the rear switch. From ON in Duty mode, double click the side switch or from OFF, double tap the rear switch.

Strobe/blinkies

•  Strobe, S.O.S, Beacon flasher. To access blinky modes, from OFF in Tactical mode, half press the rear switch and click the side switch. From ON in Tactical mode, double-clicking the side switch, and in Duty mode, triple click the side switch. Strobe activates by default, and double clicking the side switch again advances through S.O.S and Beacon flasher. Click and hold the side switch to exit to the last used (memorized) mode. 

Lock-out mode: 

• None, but the light can be mechanically locked by unscrewing the tailcap ⅛ turn.

PWM

• None visible by the naked eye

Additional/summary info on the UI: 

• Overall, I like this UI. It reminds me of the Wurkkos FC12C UI. For a budget light claiming to be duty and tactical-ready, it’s actually not far off from being a great dual purpose UI. However, it’s still not the same level as a leader brand because it’s just too busy with shortcuts and hidden modes. However, for everyday users, it’s totally serviceable. It’s easy to use as well, and doesn’t require a ton of button clicking for the shortcuts. 
• I like the Turbo-Medium output rotation in Tactical mode, and the instant-on Turbo and somewhat easily accessible (but also nicely obscure) Strobe are also nice. Half pressing the rear switch in any mode gives momentary operation in the last used mode.
• There’s also obligatory LVP and thermal regulation on board as well. Sofirn specifies the ceiling at 55 C.

The SK40 is a 21700 size light and Sofirn includes their generic black wrapped 5000 mAh 21700 flat top cell with the light. It’s a good cell, probably a BAK or Lishen under the wrapper and good for the 10-11 amps the boost driver pulls on Turbo. Thanks to the dual springs, I was able to fit long protected button and flat tops in the tube. The longest 21700s from Acebeam with built-in USB C were a tight fit and I’d probably pass on those anyway. 

The SK40 also has onboard USB C charging. Sofirn sets it at 5 volts 2 amps, so pretty standard and nowadays pretty slow for a modern flashlight. I’m wondering why they didn’t use the 3 amp charging on the TS25 and Wurkkos TS26S here since a 5 Ah battery could benefit from that. The side switch LED shows both charging state and battery condition for five seconds at turn-on.

On a 45 watt USB C to C charging adapter, I saw around 7.5 watts max input, or 5 volts and around 1.6 amps. That’s not great, and the charging (like on the Wurkkos FC12C) isn’t great either. The switch LED turned green to indicate the charging was finished, but the light was still taking about 350 mA, so I let it sit and boil a bit longer to see if it would terminate to 0 volts, 0 mA, but it didn’t.

Charging sort of tapers off and drops to about 75 mA and trickle charges the battery until you disconnect the charger. I let it run for over 5 hours before unplugging it, and the battery had added 4921 mAh.

Battery level indicator:

• Solid green light: 70% to 100%
• Solid red light: 30% to 70%
• Flashing red light: 0% to 30%

Batteries	Details
Compatible batteries	1*21700
Incompatible batteries	None I had
Low voltage protection/warning	Blink
Onboard Charging:
Onboard charging	Yes
Power bank functionality	No
USB-C to USB-C charging	Yes
Max power of USB charger (port)	45 watts
Max power measured	7.5 watts / 5 V 1.5 A
Final battery voltage (charged)	4.17 V
Avg. onboard charge time	5h 39 min

Lumen measurements

How Lumens are Measured: Understanding ANSI FL1 Standards

About Lumen measurements: The ANSI FL1 standards specify that output in lumens should be measured 30 seconds after turning on, as this is the standardized time for measuring brightness according to the industry standard. This is why we focus on this part in our measurements. The ANSI FL1 standards require an ambient temperature of 22 ± 3°C. We record the ambient the ambient temperature to identify potential reasons for any observed discrepancies.

Lumens are measured in my 50 cm integrating sphere with a Digi-Sense 20250-00 data logging luxmeter. The sphere has been calibrated with a Convoy S2+ measured to 260 Lumens and the figures are within 10% of actual. Current is measured with my Thinside B18B+ multimeter with 14 gauge wires in the meter and my Fy19 clamp meter with a loop of 12 gauge wire for over 500 mA. Outputs measured using the included fully charged 5000 mAh 21700.

Mode	Amps at start	Specified output	Turn on	30 sec.	10 min.
Eco	16.6 mA	10 lm	6 lm	6 lm	-
Low	254 mA	150 lm	161 lm	161 lm	-
Medium	900 mA	500 lm	497 lm	497 lm	497 lm
High	2.9 A	1,500 lm	1,253 lm	1,246 lm	994 lm
Turbo	11.5 A	3,200 lm	2,664 lm	2,520 lm	936 lm

*Turn-on and 30 second measurements are done manually, while the 10-minutes readings comes from the runtime graph. This is our standard approach, though some reviewers may choose a different approach.

Ambient temperature during testing:

• 21 °C 

Parasitic drain:

N/A couldn’t measure (plus it’s a clicky switch)

The outputs look great too! They’re down from spec (except Medium), but it’s still competitive with the higher-end brands.

Battery Life: Runtime graphs

How Runtimes are Measured: Understanding ANSI FL1 Standards

About ANSI FL1 runtime standards: The runtime is measured until the light drops to 10% of its initial output (30 seconds after turning on). This does not mean that the flashlight is not usable anymore. The last column shows how long the light actually works till it shuts off. If there is a + symbol, it means that the test was stopped at that particular point, but the light was actually still running. This happens on certain occasions, with certain drivers, firmware, or batteries.

Lumens are measured in my 50 cm integrating sphere with a Digi-Sense 20250-00 data logging luxmeter. The sphere has been calibrated with a Convoy S2+ measured to 260 Lumens and the figures are within 10% of actual. I use a Digi-Sense 20250-92 data logging thermocouple for the temperature measurements. The probe is affixed to the head using kapton tape and uses the same 5 second sampling rate for logging. Runtimes were conducted using the included fully charged 5000 mAh 21700

Mode	Specified runtime	Runtime (ANSI FL1)	Time till shut off*
Medium	4h 50min	5h 12min	5h 12min
High	2h 15min	2h 3min	2h 10min
Turbo	1h 51min	1h 48min	2h 10min

*The last column reflects the total runtime until the light turns off. A "+" symbol indicates that the test was stopped at that point, but the light continued to run. This typically occurs with certain drivers, firmware, or battery types.

The runtimes look really good for a sub $50 flashlight and honestly it did better than I expected. Yes, it’s under spec for brightness, but the runtimes are bang-on, and the outputs between them are nicely regulated. Turbo has a roughly 30 second step down, and a big step at 80 seconds down to around 1,000 Lumens where it sits for around 90 minutes, which is pretty good.

You get more than 2,000 Lumens for 70 seconds, and Turbo can be reactivated. High mode maintains over 1,000 Lumens for almost 8 minutes, and you get 2 hours of uninterrupted near-1,000 Lumen output. Medium ran for over 5 hours at nearly 500 Lumens. All modes run until the output drops low, then shuts off. The light was still usable after each test, but only at Low to Medium output. 

Temps on Turbo were kept in check and never went over 60 C, dropping to around 52 C, and the tube was hand-friendly during the entirety of the Turbo run.

Sofirn Flashlight Runtime Comparison

For the comparison, I rounded up some other 21700 size tactical/duty lights. In sustained output, it’s up there with the Acebeam, Olight Warrior X4 (also a SFT-70 light, along with the Fenix TK20R UE and V2). In fact, it matches the Olight, which costs almost $100.00 more. The Streamlight is the leader here, but it also costs $160. Well done, Sofirn!

Peak beam intensity and beam distance measurements

About Peak beam intensity: Understanding ANSI FL1 Standard

About peak beam intensity: The calculated value of distance in meters at which the flashlight produces a light intensity of 0.25 lux. (0.25 lux is about the brightness of a full moon shining on an object). This means that the intensity has decreased so much, it becomes difficult to see darker objects, or objects that don't reflect light. The columns ‘Meters’ and ‘Yards’ use rounded numbers.

Beam distances are measured using a Uni-T UT383S luxmeter measured indoors at 5 meters.. Measurements taken at 30 seconds using the included fully charged 5000 mAh 21700. The light was allowed to cool and recharge between the High and Turbo tests.

Mode	Specified intensity	Specified beam distance	Candela measured	Meters	Yards
Eco	244 cd	31 m	250 cd	32 m	35 yd
Low	5058 cd	142 m	4,650 cd	136 m	149 yd
Medium	16,950 cd	260 m	17,550 cd	265 m	290 yd
High	40,421 cd	410 m	43,175 cd	415 m	454 yd
Turbo	84,583 cd	580 m	91,225 cd	604 m	660 yd

Ambient temperature during testing:

• 21°C 

The beam distance testing shows pretty fantastic performance from the SK40. At turn on, I got 97,400 cd, which is really good for a 40 mm head. This is more than enough reach for just about every situation outside search and rescue stuff. I’m exceeding the Turbo, High, and Medium specs, but a good amount. These figures more or less match the Olight Warrior X4, and beat the Fenix TK21R V2, Acebeam P17, and P18. Pretty impressive for a $45 flashlight.

Camera settings and distance: Photos taken with a Canon EOS R100 with Canon RF-S 18-45 mm STM lens. The fence is 95 meters and 45 meters distant. The camera is set to 0.3s, F5 ISO1600 and 5000K WB. For the 45 meter shots, the camera is set to 0.3s, F4.5 ISO 1600 and 5000K WB. The flashlights are set to their High modes for the 45 meter shots.

Beamshots of the following flashlights compared:

• Sofirn SK40
• Fenix TK20R V2
• Fenix PD35R ACE
• Olight Warrior Ultra
• Klarus XT11GT V2 Pro
• Fenix TK17

Please note that the following beamshots are mainly intended to showcase the beam pattern and beam quality, rather than overall performance. These images are typically taken directly after activation, and in different seasons or weather conditions, and therefore do not fully represent its overall performance. In few cases the camera settings are even different and therefore not meant for performance comparisons. For accurate performance metrics, such as output, beam distance, and runtimes, you need to look at the performance section of this review.

Disclaimer: This flashlight was sent to us for review at no cost by Sofirn. We have not been paid to review, nor have we been holding back on problems or defect

Get 10% off at Sofirnlight.com with our unique coupon code: 1lumen10

1lumen selects and reviews products personally. We may earn affiliate commissions through our links, which help support our testing.