18650 vs 21700
Both 3.6 V lithium-ion. The 21700 is 21 × 70 mm against 18 × 65 mm, holding roughly 50% more energy.
Last reviewed
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
| 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
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.
Where you meet each
The 18650 is inside older laptop packs, most enthusiast torches, e-bike batteries and countless power banks. The 21700 is in newer electric vehicles, recent power tool packs and the current generation of high-output torches. Both are pack cells first and loose purchases second.
Buying loose cells
The market for both is full of wildly overstated capacity claims — anything advertising 9900 mAh in an 18650 is fiction, since the physical ceiling is around 3500. Buy known cell makers from sellers who name the actual cell model, and treat an unbranded wrapper as unknown.
Protected cells are longer
A protection board adds a couple of millimetres, so a protected 18650 can measure 69 mm rather than 65 and will not fit a tube machined for the bare cell. That trips people up more often than the difference between 18650 and 21700 does.
Can I use a 21700 instead of an 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.
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.
18650 vs 21700: common questions
Is the 21700 a direct replacement for the 18650?
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.
What is the difference between the 18650 and the 21700?
Both 3.6 V lithium-ion. The 21700 is 21 × 70 mm against 18 × 65 mm, holding roughly 50% more energy. In short: 21700 for capacity, 18650 where the tube is already cut for it.
Are the 18650 and the 21700 the same size?
No. The 18650 is 18 × 65 mm and the 21700 is 21 × 70 mm.
Which lasts longer, the 18650 or the 21700?
The 21700. It is rated at 3000–5000 mAh against 2000–3500 mAh for the 18650 — roughly 1.5× the capacity, though real runtime depends on how much current your device draws.
Background reading
The wider questions behind these two codes.