18650 vs 21700 Cells Explained

Open almost any cordless tool pack and you'll find round lithium-ion cells. Most are one of two sizes: 18650 or 21700. Here's what the numbers mean and why it matters.

1. What the numbers mean

18650: about 18mm across and 65mm long.
21700: about 21mm across and 70mm long.

The size is all the number tells you. Capacity, current rating and quality depend on the cell itself.

2. How they compare

Capacity: 18650 cells are typically about 2,000 to 3,500mAh. 21700 cells are typically about 3,000 to 5,000mAh.
Power: the larger 21700 can usually deliver more current and runs a little cooler doing it, which suits hungry tools.
Size and weight: 21700s are bigger and heavier, so packs are bulkier for the same cell count.

3. How they show up in tool packs

An 18V or 20V Max pack has 5 cells in series (5S). Capacity comes from the cells and how many sets are in parallel:

5 cells, 1 row (5S1P) of 18650s: a slim pack, typically 1.5 to 3Ah.
10 cells, 2 rows (5S2P) of 18650s: a bigger pack, typically 4 to 6Ah.
5 cells, 1 row (5S1P) of 21700s: a compact pack, typically 4 to 5Ah, popular for newer high-output packs.

Work out any pack's layout with the Cell Layout calculator.

4. If you're rebuilding a pack

Use the same size cell the pack was designed for. A 21700 won't fit an 18650 pack, and an 18650 is loose in a 21700 pack.
Use high-drain cells with a current rating that suits the tool. Power tools need high-drain cells.
Use matched new cells: same make, model and batch. Don't mix old and new.
Buy from a reputable supplier. Fake and re-wrapped cells with inflated ratings are common.

See Is a Battery Pack Rebuild Worth It? before you start.