18650 vs 21700
Both are lithium-ion rechargeable cells with the same nominal voltage (3.6–3.7 V). The 21700 is physically larger and holds significantly more charge per cell. They are not directly interchangeable.
Side-by-side comparison
| Property | 18650 | 21700 |
|---|---|---|
| Diameter | 18.6 mm | 21.0 mm |
| Length | 65.2 mm | 70.0 mm |
| Nominal voltage | 3.6–3.7 V | 3.6–3.7 V |
| Capacity range | 1800–3500 mAh | 3000–5000 mAh |
| Energy per cell | ~6.5–13 Wh | ~11–18.5 Wh |
| Typical runtime advantage | — | +15–50% over comparable 18650 |
| Device support | Very wide — vast majority of Li-ion devices | Newer high-performance devices |
| Availability | Excellent — widely stocked | Good — growing rapidly |
Which should you choose?
Your device dictates the format — you cannot substitute one for the other without an adapter and only in compatible devices. If your flashlight or device accepts both formats, choose 21700 for longer runtime per charge. Choose 18650 if you need wide compatibility or already have a charger and spare cells.
Adapter: 21700 device → use 18650
Some 21700 devices (particularly flashlights from Olight, Nitecore) ship with a plastic sleeve adapter that allows an 18650 cell to fit in the larger 21700 tube. This works electrically, but reduces runtime compared to a same-capacity 21700 cell.