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.
Last reviewed
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
| 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
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.
One is ascending, the other is not
The 21700 is where the industry is investing: electric vehicles, current tool platforms, new torch designs. The 26650 is a legacy size with a shrinking supplier base. If you are choosing equipment rather than replacing a cell, that trajectory matters more than the specifications.
Similar energy, different shape
Their capacities overlap substantially, so this is rarely a performance decision. It is a decision about what your device's tube was machined for, and about which cell you will still be able to buy in five years.
Neither is a household cell
Both are pack cells that happen to be sold loose. They need a lithium-ion charger, they have no protection unless a board was added, and they are not interchangeable with anything in a supermarket. Treat them as components.
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.
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.
21700 vs 26650: common questions
Is the 26650 a direct replacement for the 21700?
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.
What is the difference between the 21700 and the 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. In short: similar capacity, different shape — fit decides it.
Are the 21700 and the 26650 the same size?
No. The 21700 is 21 × 70 mm and the 26650 is 26 × 65 mm.
Which lasts longer, the 21700 or the 26650?
The 26650. It is rated at 4000–5500 mAh against 3000–5000 mAh for the 21700 — roughly 1.2× the capacity, though real runtime depends on how much current your device draws.
Background reading
The wider questions behind these two codes.