The 18650 and 21700 are the two most common cylindrical lithium cell formats. Choosing the right format early saves cost and rework at the pack level. Here is how they compare and when to use each.
The formats at a glance
- 18650 — Ø18 × 65 mm. The industry standard: a wide variant range (~2000–4200 mAh in NCM), mature supply and the lowest unit price.
- 21700 — Ø21 × 70 mm. The higher-energy successor: ~3300–5500 mAh across NCM and LMFP.
Energy and capacity
A 21700 holds roughly 30–50% more energy than an 18650. Fewer cells for the same pack energy means fewer welds, simpler BMS wiring and lower assembly cost — often offsetting the higher per-cell price.
Power and internal resistance
Bigger cans have lower internal resistance and shed heat better, so the 21700 generally handles higher continuous current with less temperature rise. For peak-current designs, check the specific model’s discharge rate rather than the format alone — our high-rate (P) and drone (T) 21700 models reach the highest C-rates.
Pack cost and design
- Fewer cells (21700) → lower assembly labor, fewer interconnects, simpler thermal design.
- More mature ecosystem (18650) → widest choice, easiest second-sourcing, lowest cell price.
Which to choose
- Choose 18650 when cost per cell matters most, you need a specific niche variant, or you are matching an existing design — common in power tools and entry-level e-bikes.
- Choose 21700 when you want higher energy density and lower pack cost at scale — e-bikes, premium tools, drones and storage. For storage, pick NCM (E) for maximum energy density or LMFP (MF) for the safest, longest-cycle-life option.
Don’t forget chemistry
Format is only half the decision — you also pick the chemistry. We offer NCM for energy and power across both formats, plus LMFP (manganese-iron-phosphate blended with NCM) in the 21700 for a safer, longer-cycle-life profile that sits between NCM and LFP. See LiFePO4 vs lead-acid for the storage chemistry background.