What this calculator does
Battery capacity gets quoted in several different units depending on the product: phone and torch batteries are usually rated in mAh, larger batteries in Ah, and solar or power-station batteries in Wh. This battery capacity calculator converts between them, which matters because mAh and Ah measure electrical charge while Wh measures energy, and the two are only comparable once you know the battery's voltage.
The relationship is straightforward once voltage is in the picture: energy in watt-hours equals charge in amp-hours multiplied by voltage. Two batteries with the same mAh rating but different voltages store different amounts of energy, which is why comparing capacity by mAh alone can be misleading when the batteries are not the same voltage.
The formula
Choose which unit you are converting from: mAh, Ah or Wh. Enter the capacity value in that unit, along with the battery's nominal voltage, found on the label or datasheet. The calculator converts mAh to Ah by dividing by 1,000, then multiplies amp-hours by voltage to get watt-hours (or runs the conversion in reverse if you start from Wh), giving you the capacity expressed in all three units at once.
| Term | Meaning |
|---|---|
| mAh | Milliamp-hours, a unit of electrical charge equal to one thousandth of an amp-hour. |
| Ah | Amp-hours, a unit of electrical charge: current in amps sustained for a number of hours. |
| Wh | Watt-hours, a unit of energy: amp-hours multiplied by voltage. |
| Nominal voltage | The battery's rated average voltage, used to convert between charge and energy units. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert from | Which unit your known capacity figure is already in. |
| Capacity value | The capacity value itself, in the unit selected above. |
| Nominal voltage (V) | The battery's nominal voltage from its label or datasheet, such as 3.7V for a typical lithium cell or 12V for a lead-acid battery. |
When to use it
Comparing phone and power bank capacity
A phone battery rated at 4,000 mAh and a power bank rated at 20,000 mAh look very different, but if the power bank runs at a lower effective voltage its usable energy in Wh may be less than four times the phone's, so converting both to Wh gives a fairer comparison.
Checking airline battery limits
Airlines often set carry-on battery limits in Wh rather than mAh. Converting a power bank's mAh and voltage rating to Wh shows whether it falls under a typical 100Wh airline threshold before travelling.
Sizing a solar or off-grid battery bank
Off-grid battery banks are often specified in Ah at a fixed system voltage such as 12V or 24V. Converting a Wh energy requirement back to Ah at the system voltage shows what capacity battery bank to buy.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the same mAh rating convert to Wh at different voltages?
Holding the charge rating fixed at 3,000 mAh and varying voltage shows how much the resulting energy in watt-hours changes.
Questions
How do I convert mAh to Wh?
Divide mAh by 1,000 to get Ah, then multiply by the nominal voltage to get Wh. For example 3,000 mAh at 3.7V is 3 Ah × 3.7V = 11.1 Wh.
Why do two batteries with the same mAh store different energy?
Because mAh only measures charge, not energy. Energy also depends on voltage, so a higher-voltage battery stores more energy than a lower-voltage one at the same mAh rating.
Is a higher mAh always a bigger battery?
Only when comparing batteries of the same voltage. Across different voltages, compare Wh instead, since that accounts for both charge and voltage together.
How is this different from a battery runtime calculator?
This calculator only converts between capacity units (mAh, Ah, Wh) at a given voltage. For how long a battery will power a specific load, see the battery runtime calculator, which uses capacity, depth of discharge and load together.
For how long a battery will actually last under load, see the battery runtime calculator.