Battery Size Chart (AA, AAA, C, D, 9V, 18650, 21700)

Understanding the battery size chart — concept and principle

When a remote, a torch or a toy needs new batteries, the label on the old cell may say AA, LR6, HR6, 18650 or CR123A, and it is not always obvious which ones are interchangeable. This chart lists the common household and lithium-ion cylindrical cells with their IEC designation, maximum dimensions, nominal voltage and a typical capacity, so you can match a replacement or compare sizes.

How it is calculated

The chart is a lookup table built from the standards, not a formula. Two naming systems explain it:

• IEC names for primary and NiMH cells: a letter for the chemistry (L = alkaline, H = nickel–metal hydride, C or F = lithium types), R for round, then a size number. AA alkaline is LR6, AA NiMH is HR6.
• Lithium-ion cylinder codes give the size directly: 18650 means 18 mm diameter and 65.0 mm length; 21700 means 21 × 70 mm.
• Dimensions in the chart are diameter × height in mm (for 9 V, height × length × width).

The principle

Voltage comes from chemistry, not from size. A zinc–manganese dioxide (alkaline) cell gives about 1.5 V whether it is AAA or D; NiMH gives about 1.2 V; lithium-ion about 3.6–3.7 V nominal; a 9 V block is six small 1.5 V cells in series. Size mainly sets how much active material fits inside, so a D cell stores several times more charge than an AA of the same chemistry. Capacity in mAh is not a fixed number; it falls as the current drawn rises.

Worked example

Default selection: AA.

1. IEC name: LR6 for alkaline, HR6 for rechargeable NiMH.
2. Size: up to 14.5 mm diameter × 50.5 mm height.
3. Voltage: 1.5 V alkaline, 1.2 V NiMH.
4. Typical capacity: about 2,500 mAh for alkaline at low drain, which is roughly 1.5 V × 2.5 Ah ≈ 3.75 Wh of nominal energy.

How to read the result

Two cells are interchangeable only if size, voltage and terminal shape all match. A NiMH HR6 can usually replace an alkaline LR6, although its lower voltage makes some low-battery indicators trigger early. Compare capacities only within the same chemistry, and compare energy (Wh = V × Ah) across chemistries: an 18650 at 3.6 V and 3,000 mAh holds about 10.8 Wh, roughly three alkaline AAs' worth.

Common mistakes

• Putting a 14500 lithium-ion cell (AA-sized but 3.6 V) into a device designed for 1.5 V AA batteries.
• Mixing old and new cells, or different chemistries, in one device.
• Trusting very high mAh claims on unbranded 18650 cells.

Frequently asked questions

Is an 18650 bigger than an AA?

Yes. It is 18 × 65 mm versus 14.5 × 50.5 mm, and 3.6 V instead of 1.5 V.

Which is bigger, C or D?

D is larger: about 34.2 × 61.5 mm compared with 26.2 × 50.0 mm for C.

Can rechargeable AA batteries replace alkaline ones?

In most devices, yes. They supply 1.2 V instead of 1.5 V, which most electronics tolerate.

Sources: IEC 60086-2, Primary batteries — Physical and electrical specifications · IEC 61960, Secondary lithium cells and batteries for portable applications · IEC 61951-2, Secondary cells and batteries containing alkaline electrolytes — Nickel-metal hydride