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mAh to Wh Calculator
Capacity in milliamp-hours only becomes comparable once it is turned into watt-hours — and that requires the battery's nominal voltage. This converter goes both ways and prints the formula, so the number can be checked.
Convert capacity
Choose a direction, then enter the capacity and the cell nominal voltage.
RequiredCapacity in mAh and the cell voltageOptionalNothing else — it is one multiplication
Result
Checks
What to do next
The voltage is the part people get wrong. 10,000 mAh at 3.7 V is 37 Wh. The same 10,000 mAh at 5 V would be 50 Wh — a figure that describes a different battery, not yours.
The formula
Wh = mAh × V ÷ 1000
mAh = Wh × 1000 ÷ V
Dividing by 1000 simply converts milliamp-hours to amp-hours. The voltage multiplies the current capacity into an energy figure, which is what watt-hours measure. Because energy is the product of the two, mAh on its own cannot be compared between batteries of different voltages.
Worked example
10,000 mAh × 3.7 V ÷ 1000 = 37 Wh. Going back: 37 Wh × 1000 ÷ 3.7 V = 10,000 mAh. The calculator performs that reverse step on your own input and shows the result, which catches a mistyped voltage immediately.
Which voltage belongs in the formula
| Value seen on a label | What it describes | Use in this formula? |
|---|---|---|
| 3.7 V / 3.85 V / 3.6 V | Nominal voltage of one lithium-ion cell, or of the pack built from them | Yes — this is the one |
| 5 V / 9 V / 12 V / 15 V / 20 V | USB Power Delivery output profiles the port offers | No — an output rail is not a battery voltage |
| 100–240 V | AC input range of a wall charger | No — that is mains input, not stored energy |
| 12 V / 24 V | Lead-acid pack or a car socket | Only for that specific pack, not for USB gear |
If a pack is built from several cells in series, the printed nominal voltage already reflects the whole pack. Use the pack figure; do not multiply by the cell count yourself.
Why watt-hours matter more than mAh
- Air travel limits are written in Wh. Many airlines and aviation authorities limit spare lithium batteries by watt-hours, which is why the figure is printed on the case.
- Charge counts need energy, not current. The number of full charges a pack delivers is an energy comparison, so both sides of the division must be in Wh.
- Marketing uses whatever number is bigger. A pack advertised as 20,000 mAh holds about 74 Wh at 3.7 V. Neither figure tells you the usable output, which is always lower.
Rated capacity is not the same as headline capacity
Many packs print two numbers: the headline cell capacity (for example 10,000 mAh) and a rated capacity (for example 6,000 mAh at 5 V) that describes what comes out of the USB port after conversion. They are not interchangeable. The headline figure describes stored energy; the rated figure describes delivered output at a particular voltage. Converting the rated figure with 5 V is arithmetically valid but answers a different question — this site keeps the two uses separate and does not mix them in one calculation.
Round-tripping out loud
Every conversion here is checked by converting the result back with the same voltage. If the input and the reverse-check differ by more than rounding, the voltage was almost certainly mistyped. Displayed values are rounded; the stored value is not truncated before the check.
Related tools
- Power bank calculator — turn the Wh figure into an estimated number of charges.
- 65W vs 100W charger — for output wattage rather than stored energy.
- How we calculate — method, rounding and limits.
Formula last reviewed 6 November 2026. No product recommendations on this page.