18650 vs 21700

Both 3.6 V lithium-ion. The 21700 is 21 × 70 mm against 18 × 65 mm, holding roughly 50% more energy.

Quick answer

Both are rechargeable lithium-ion cells at 3.6 volts. The 21700 is 3 mm wider and 5 mm longer, and holds roughly half again as much energy. They are not interchangeable — a 21700 will not go into an 18650 tube, and an 18650 rattles loose in a 21700 one. New designs increasingly use the 21700; the 18650 remains far easier to buy.

Verdict:: 21700 for capacity, 18650 where the tube is already cut for it
Interchangeable:: no

Side-by-side specifications

18650 compared with 21700
Specification	18650	21700
Chemistry	Li-ion	Li-ion
Type	Cylindrical	Cylindrical
Nominal voltage	3.6 V	3.6 V
Diameter	18 mm	21 mm
Height	65 mm	70 mm
Typical capacity	2000–3500 mAh	3000–5000 mAh
Rechargeable	Yes	Yes
Also sold as	INR18650, ICR18650, NCR18650	INR21700, NCR21700, 2170

Size comparison

18650

Ø 18 mm · H 65 mm

21700

Ø 21 mm · H 70 mm

These cells do not share a footprint, so one will not sit correctly in the other's compartment.

The code is the size

18650 is 18 mm across and 65 mm long; 21700 is 21 × 70 mm. Three millimetres of diameter is decisive in a machined tube — no adapter makes the larger cell fit.

Capacity

3000–5000 mAh against 2000–3500 mAh. The extra volume is why electric vehicles and newer power tools moved to the 21700.

Availability

The 18650 has decades of production behind it and is cheaper and easier to source. The 21700 is where new designs are heading.

Can I use a 21700 instead of a 18650?

Not directly. The 21700 measures 21 × 70 mm against 18 × 65 mm for the 18650, so it will not seat properly in the same compartment.

18650 specs →21700 specs →

Check before you swap. Batteries with similar dimensions are not always interchangeable. Always verify voltage, chemistry and the requirements printed in your device manual before fitting a replacement.

Related comparisons

LR44 vs SR44SR44 for anything precise, LR44 for anything cheap CR2032 vs CR2025CR2032 lasts longer; CR2025 fits tighter holders AA vs AAASame voltage, entirely different size CR2032 vs CR2016CR2032 unless the holder is genuinely too shallow CR2025 vs CR2016CR2025 where it fits, CR2016 only where it must CR2430 vs CR2450CR2450 for runtime, CR2430 where the holder is shallow CR2032 vs CR2450Not interchangeable — the CR2450 is a size class up CR2032 vs CR2430No — near-identical thickness, wrong diameter CR2025 vs CR2450Not interchangeable — different size class entirely LR44 vs 357357 for watches and meters, LR44 for everything cheap SR44 vs 357The same cell — SR44 is the standard code, 357 the retail number LR44 vs AG13The same cell — AG13 is the generic code for an LR44 LR44 vs A76The same cell — A76 is Energizer's code for the LR44 357 vs 303Same size and chemistry — originally high-drain against low-drain AA vs CSame voltage, about three times the runtime AA vs DSame 1.5 V, roughly six times the runtime C vs DD for double the runtime, C where a D will not fit AA vs AAAANot interchangeable — the AAAA is a quarter of the cell AAA vs AAAAOne letter apart in the name, about half the cell 14500 vs AASame shape, more than double the voltage — never swap them 10440 vs AAAAAA-shaped, 3.6 V — not a substitute 16340 vs CR123ASame slot, different voltage — check the device is rated for both 18650 vs 2665026650 for capacity, but far fewer devices take one 21700 vs 26650Similar capacity, different shape — fit decides it 18650 vs 14500Same chemistry, entirely different size class 18650 vs AADifferent size, different voltage, different job CR123A vs 18650Many torches take either — two CR123A stack to an 18650's length CR123 vs CR123AThe same cell — CR123A is the current designation CR2 vs CR123ABoth 3 V lithium, but the CR2 is much the smaller cell A23 vs A27Same 12 V and nearly the same length — the A23 is the fatter, longer-lasting one