Guides
18650 vs 21700
The number is just the dimensions. What it really tells you is how long your light runs and how easy it will be to keep alive in ten years.
By Scooter M. · Published September 2, 2026
Short answer
Sofirn SC31 Pro
Two thousand published lumens out of a standard 18650 you can buy anywhere for a few dollars, in a light that costs less than most brands charge for a keychain. The cell is the whole argument: when it dies in three years you replace a battery, not a flashlight.
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Disclosure — we earn a commission on qualifying purchases made through the Amazon links on this page, at no extra cost to you. It does not change which lights we pick or what we say about them. The full version.
How we rank. Published ANSI FL1 output divided by the live retail price, with sealed-battery lights marked down because a light you cannot re-cell is a light with an expiry date. Units we claim to have tested: 0 — we compile manufacturers' published figures and show the arithmetic instead. How we work this out.
What the numbers mean
The four-digit names are dimensions in millimeters. An 18650 is 18mm across and 65mm long; a 21700 is 21mm across and 70mm long. The larger cell holds more energy, so it runs longer at the same output, at the cost of a thicker and heavier light. Both are standard formats you can replace for a few dollars.
| Cell | Diameter | Length | Roughly the size of | Typical use |
|---|---|---|---|---|
| 16340 | 16 mm | 34 mm | A CR123A | Keychain and mini lights |
| 14500 | 14 mm | 50 mm | An AA battery | Compact everyday-carry lights |
| 18650 | 18 mm | 65 mm | A slightly fat AA | The category standard for full-size lights |
| 21700 | 21 mm | 70 mm | A little larger again | High-output lights and work headlamps |
Why the format matters more than the lumen figure
A light built around a standard cell can be kept alive indefinitely. When the battery tires in three or five years you buy a new one for a few dollars and the light is as good as it was. A light with a sealed cell cannot be revived by an ordinary owner, so it has a service life rather than a lifespan.
That distinction changes the arithmetic on every value calculation on this site, and it is why battery architecture is weighted so heavily in our rankings. Two lights with identical lumens-per-dollar figures on the day you buy them can differ by a factor of three in what they cost over a decade.
Which to choose
Choose 18650 if
- You want the most widely available cell in the world — it has been the standard in laptops and tools for two decades.
- You want a light that is comfortable in a pocket. Sofirn publishes 59 grams without the cell for the SC31 Pro; a 21700 light is noticeably chunkier.
- You want the cheapest replacements and the widest choice of chargers.
Choose 21700 if
- You need sustained high output, where the extra capacity translates directly into runtime — Fenix publishes a 3,400 mAh 21700 in the HM65R V2.0 headlamp.
- The light lives on a belt or in a bag rather than a pocket, so the extra bulk costs you nothing.
- You want to carry one spare that lasts a whole shift rather than two smaller ones.
Choose 14500 or 16340 if
Size is the constraint. Wurkkos publishes 32 grams for the TS10 V2 on a 14500 and one hour fourteen minutes at 1,400 lumens — impressive for the format and honestly short. Sofirn publishes 1,100 lumens from a 16340 in the SC21 Pro. Both are the right answer when the alternative is not carrying a light at all.
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1. Best lumens per dollar overall
Sofirn SC31 Pro
Lumens per dollar
Computed the moment we have a live price for it
Two thousand published lumens out of a standard 18650 you can buy anywhere for a few dollars, in a light that costs less than most brands charge for a keychain. The cell is the whole argument: when it dies in three years you replace a battery, not a flashlight.
- Max output
- 2,000 lm
- Runtime
- Not published per mode
- Battery
- Replaceable cell
- Water rating
- IPX8
What it gets right
- Standard 18650 cell — replacements cost a few dollars and are sold everywhere
- IPX8, the strongest water rating in this price class
- USB-C charging built into the light, so no separate charger to lose
- Anduril firmware gives real ramping control rather than four fixed steps
What it gets wrong
- Sofirn publishes no per-mode runtime figures on the product page
- Anduril is genuinely confusing for the first week
- Tint varies between batches, which is the price of this price
Published specification
- Maximum output: 2,000 lumens — Sofirn published specification
- Beam distance: 226 m — Sofirn published specification
- Battery: 1 x 18650 Li-ion, user replaceable — Sofirn published specification
- Charging: USB-C, 2.5 hours with a 2A adapter — Sofirn published specification
- Water rating: IPX8 — Sofirn published specification
- Weight: 59 g without battery — Sofirn published specification
- Length: 117.5 mm — Sofirn published specification
- Emitter: SST40, 5000K or 6500K — Sofirn published specification
- Per-mode runtime: Not published on the product page
Skip this one if
you want to press one button and get one brightness. Anduril has a shortcut for everything and a menu for everything else, and if that sounds tiring you will be happier with a three-mode light.
Buy it if: anyone who intends to keep the light for longer than the battery lasts, and does not mind reading a manual once.
- Output 9.0
- Battery 9.5
- Build 8.0
- Usability 7.0
- Overall 8.6
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2. Best color quality per dollar
Wurkkos FC11
Lumens per dollar
Computed the moment we have a live price for it
A 90-CRI emitter at a price where nobody else bothers, which means colors look like colors rather than like a parking lot. Same replaceable 18650 story as the Sofirn, plus a magnetic tail that turns it into a work light.
- Max output
- 1,300 lm
- Runtime
- Not published per mode
- Battery
- Replaceable cell
- Water rating
- IPX7
What it gets right
- 90 CRI at a price where 70 CRI is normal — skin, wiring and paint read correctly
- Magnetic tailcap sticks it to anything ferrous and frees both hands
- Standard 18650, USB-C charging in the light
- Two mode groups, so you can pick a simple ramp or a stepped set
What it gets wrong
- IPX7 rather than IPX8 — fine for rain, not for a river
- High CRI costs raw lumens; the Sofirn is visibly brighter on turbo
- No per-mode runtime published on the product page
Published specification
- Maximum output: 1,300 lumens — Wurkkos published specification
- Beam distance: 144 m (157 yd) — Wurkkos published specification
- Battery: 1 x 18650 Li-ion, user replaceable — Wurkkos published specification
- Water rating: IPX7 — Wurkkos published specification
- Weight: 65 g +/- 1 g without battery — Wurkkos published specification
- Length: 116.23 mm — Wurkkos published specification
- Emitter: Nichia 519A / LH351D, 90 CRI — Wurkkos published specification
- Body: 6061 aluminum, type III hard anodized — Wurkkos published specification
Skip this one if
the only number you care about is peak output. You are paying part of this light's budget for color accuracy, and if you will never notice that, the money is better spent elsewhere.
Buy it if: anyone who uses a flashlight to look at things closely — wiring, paint, engine bays, a face — rather than to light up a field.
- Output 7.5
- Battery 9.5
- Build 8.0
- Usability 8.0
- Overall 8.4
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3. Most output with a removable 21700
Fenix HM65R V2.0
Lumens per dollar
Computed the moment we have a live price for it
Twenty-two hundred lumens, 226 meters, IP68, a 2-meter impact rating and a removable 3,400 mAh 21700. It costs several times what a budget headlamp costs and it is the only one here you can carry a spare cell for.
- Max output
- 2,200 lm
- Runtime
- Not published per mode on the retail page
- Battery
- Replaceable cell
- Water rating
- IP68
What it gets right
- Removable 21700, so a spare cell turns a four-hour night into an eight-hour one
- IP68 — the strongest water rating in this roundup
- Independent spot and flood control, which is what work actually needs
What it gets wrong
- Its lumens-per-dollar figure will be poor against any budget lamp
- Heavy on the head compared with the ultralight class
- No per-mode runtime on the page we read
Published specification
- Maximum output: 2,200 lumens, spot and flood combined — Fenix published specification
- Beam distance: 741 ft (226 m) — Fenix published specification
- Battery: 3,400 mAh 21700, removable, USB-C — Fenix published specification
- Water rating: IP68 — Fenix published specification
- Impact: 2 m — Fenix published specification
- Body: Magnesium alloy — Fenix published specification
- Per-mode runtime: Not published on the retail page we read
Skip this one if
you are buying on value. This is a work tool priced as one.
Buy it if: night shifts, cave work and anyone who has run a headlamp flat halfway through a job.
- Output 9.5
- Battery 9.0
- Weight 5.5
- Usability 8.5
- Overall 8.4
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4. Best pocket light under two ounces
Wurkkos TS10 V2
Lumens per dollar
Computed the moment we have a live price for it
Thirty-two grams, three 90-CRI emitters, and one of the very few budget lights whose maker publishes the runtime at maximum output: one hour fourteen at 1,400 lumens. Publishing that number at all puts Wurkkos ahead of several brands ten times its size.
- Max output
- 1,400 lm
- Runtime
- 1 h 14 min
- Battery
- Replaceable cell
- Water rating
- IPX8
What it gets right
- Wurkkos publishes runtime at maximum output, which most brands quietly do not
- Thirty-two grams — light enough that you stop noticing it in a pocket
- Standard 14500 cell, replaceable
- Documented automatic step-down instead of an undocumented one
What it gets wrong
- A 14500 is a small cell; sustained output is not this light's job
- RGB auxiliary LEDs are a toy, and they draw from the same cell
- Anduril again, with the same learning curve
Published specification
- Maximum output: 1,400 lumens — Wurkkos published specification
- Runtime at maximum: 1 h 14 min — Wurkkos published specification
- Runtime at 1 lumen: 6 h 43 min — Wurkkos published specification
- Battery: 1 x 14500 Li-ion, user replaceable — Wurkkos published specification
- Water rating: IPX8, 2 m — Wurkkos published specification
- Weight: 32 g +/- 1 g without battery — Wurkkos published specification
- Length: 71.5 mm — Wurkkos published specification
- Thermal behavior: Automatic step-down from turbo to high — Wurkkos published specification
Skip this one if
you need something to run for an hour on high without getting hot. A light this small has nowhere to put the heat, and the step-down is not optional.
Buy it if: the person who wants a real light on them at all times and will not carry anything heavier than a pen.
- Output 8.0
- Battery 8.0
- Build 7.5
- Usability 7.5
- Overall 8.2
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5. Best keychain light with a replaceable cell
Sofirn SC21 Pro
Lumens per dollar
Computed the moment we have a live price for it
Eleven hundred lumens from a keyring-sized body with a 16340 you can actually swap. Almost every rival at this size seals the battery in, which turns the light into a two-year consumable.
- Max output
- 1,100 lm
- Runtime
- Not published per mode
- Battery
- Replaceable cell
- Water rating
- None published
What it gets right
- A replaceable cell in a keychain-sized light, which is genuinely rare
- Magnetic tail, so it doubles as a hands-free task light
- USB-C in the light rather than a proprietary dock
What it gets wrong
- Sofirn publishes neither runtime nor a water rating for this model
- 1,100 lumens out of a 16340 is a very short-lived 1,100 lumens
- Anduril on a light this small means a lot of clicking
Published specification
- Maximum output: 1,100 lumens — Sofirn published specification
- Beam distance: 131 m — Sofirn published specification
- Battery: 1 x 16340 Li-ion, included and replaceable — Sofirn published specification
- Charging: USB-C into the light — Sofirn published specification
- Fittings: Magnetic tailcap and pocket clip — Sofirn published specification
- Water rating: Not published on the product page
- Per-mode runtime: Not published on the product page
Skip this one if
you want a published IP rating. Sofirn does not state one for this model, and we are not going to guess on your behalf.
Buy it if: someone who wants keychain size without accepting a disposable battery.
- Output 7.5
- Battery 8.5
- Build 7.0
- Usability 7.0
- Overall 7.8
We show a price only when we have pulled a live one. #ad How we earn
Buying cells without being sold a lie
- 1.Ignore any cell claiming a capacity far above the format's realistic maximum. A genuine 18650 in the 3,000 to 3,500 mAh range is normal; five-figure claims are fiction.
- 2.Buy from a seller you would buy anything else electrical from. Counterfeit cells are common and an overstated capacity means the protection circuit is being asked to do a job it was not designed for.
- 3.Decide whether you need a protected cell. Some lights include low-voltage protection in the driver, some do not; when in doubt, protected is the safer choice and slightly longer.
- 4.Check whether your light needs a button-top or flat-top cell. This catches people out and is usually stated in the manual.
- 5.Buy two. A spare turns a flat light into a ten-second inconvenience.
Units we claim to have tested: 0
Every publisher in this category says they tested twenty flashlights. We have not tested any, and we say so. What we do instead: compile the manufacturer's own published ANSI FL1 figures, pull the price the light is selling for right now, and divide one by the other. Every number on this page is one you can check yourself in about two minutes.
Common questions
Is a 21700 better than an 18650?
It holds more energy, so it runs longer at the same output — but it makes the light thicker and heavier. For a pocket light the 18650 is usually the better trade; for a work headlamp or a high-output handheld where runtime is the constraint, the 21700 is. Both are standard replaceable formats, which matters far more than the choice between them.
Can I use a 21700 in an 18650 flashlight?
No. A 21700 is 3mm wider and 5mm longer, so it will not physically fit a body machined for an 18650. Some lights ship with an adapter allowing a smaller cell in a larger tube, but never the reverse. Use the format the manufacturer specifies.
What about 18650 vs 21700 vs 26650?
A 26650 is larger again — 26mm across and 65mm long — and appears mainly in big high-output lights where bulk is acceptable. It is far less common than the other two, which means fewer cells and fewer chargers to choose from. For most buyers the real choice is between 18650 and 21700.
Can you replace the battery in a rechargeable flashlight?
It depends entirely on the model, and it is the single most consequential thing to check before buying. Lights built around a standard cell — an 18650, a 21700, a 14500, a 16340 — take a screw-off tailcap and a five-dollar replacement. Lights with a built-in cell, which manufacturers describe as 'integrated' or 'non-removable', cannot be re-celled by an ordinary owner: when the battery reaches the end of its cycle life the whole light is refuse. Energizer, for example, states plainly that the Vision HD Rechargeable uses non-removable lithium-ion batteries.
Sources
- Sofirn published product specifications — read 2026-09-01
- Wurkkos published product specifications and user manuals — read 2026-09-01
- Fenix published product specifications — read 2026-09-01
- Lumen & Ledger method — how every figure on this page is derived
Who writes this
Scooter M. — I am an enthusiast who is genuinely into this. I read the manuals, compile the published specs, and do the math. No lab coat, no test bench, no drawer of free samples. What I can tell you is exactly what a light claims on paper, exactly what it costs right now, and what those two numbers mean when you divide one by the other.
Read next
How long do rechargeable flashlights last?
Cycle counts, calendar aging, and the question that decides it: can you re-cell it?
Best rechargeable flashlights under $50
Where the value curve is steepest, and the three things $50 should buy you.
The best rechargeable flashlights for the money
The flagship ranking: every pick scored on output over price, with the arithmetic shown.
How long to charge a rechargeable flashlight
The arithmetic, the variables, and why your charger is probably the slow part.
Sofirn vs Wurkkos
The two brands that own the value tier, compared on the specs each one publishes.
The Ledger
Every light we cover in one ranked table, sorted by lumens per dollar.