21700 vs 26650

Both 3.6 V lithium-ion with overlapping capacity. The 26650 is 26 × 65 mm, the 21700 is 21 × 70 mm — different shapes, not different classes.

Quick answer

Both are rechargeable lithium-ion at 3.6 volts and hold broadly similar amounts of energy. The 26650 is wider and shorter; the 21700 is narrower and longer. Neither fits the other's compartment, so there is no real choice to make — buy the one your device is built for. If you are designing around one, the 21700 is the cell the industry is standardising on.

Verdict:: Similar capacity, different shape — fit decides it
Interchangeable:: no

Side-by-side specifications

21700 compared with 26650
Specification	21700	26650
Chemistry	Li-ion	Li-ion
Type	Cylindrical	Cylindrical
Nominal voltage	3.6 V	3.6 V
Diameter	21 mm	26 mm
Height	70 mm	65 mm
Typical capacity	3000–5000 mAh	4000–5500 mAh
Rechargeable	Yes	Yes
Also sold as	INR21700, NCR21700, 2170	INR26650, IMR26650

Size comparison

21700

Ø 21 mm · H 70 mm

26650

Ø 26 mm · H 65 mm

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

Shape, not size class

26 × 65 mm against 21 × 70 mm. The 26650 trades length for width; the volumes are closer than the codes suggest.

Capacity

4000–5500 mAh against 3000–5000 mAh. They overlap, so capacity rarely decides between them — the compartment does.

Where each is going

The 21700 is the current standard for electric vehicles and new tool packs. The 26650 is a narrower market and getting narrower.

Can I use a 26650 instead of a 21700?

Not directly. The 26650 measures 26 × 65 mm against 21 × 70 mm for the 21700, so it will not seat properly in the same compartment.

21700 specs →26650 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.

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