Battery Capacity Calculator (mAh to Wh)
Convert battery capacity (mAh), voltage (V) and energy (Wh), and estimate a theoretical run time from the power consumption.
Formulas
- Energy (Wh) = capacity (mAh) × voltage (V) ÷ 1,000. In reverse, capacity (mAh) = Wh × 1,000 ÷ V.
- Usable energy = energy × efficiency (%) ÷ 100
- Capacity at the output voltage (mAh) = usable energy × 1,000 ÷ output voltage (V)
- Run time (hours) = usable energy ÷ power consumption (W)
The calculation uses exact fractions, and rounding happens once, when a value is displayed.
Nominal voltage, output voltage and efficiency
- Nominal voltage (the battery's voltage): a typical voltage of the battery (for example, about 3.6 to 3.7 V for a lithium-ion cell). It differs by battery type and product, so this calculator has no default value: enter the value from the specifications. The same mAh gives a different Wh depending on the voltage.
- Output voltage: the voltage a power bank or similar device outputs after stepping the battery voltage up or down (5 V, 9 V and so on). Do not read the battery's mAh rating as the capacity at the output. At a different voltage, the same energy (Wh) is a different number of mAh.
- Conversion efficiency: some energy is lost in stepping the voltage up or down and in charging and discharging. The value differs by product, so you enter it. The default of 100% is the theoretical case with no losses (no typical value is suggested).
Examples
- 10,000 mAh at 3.7 V → 37 Wh
- 50 Wh at 3.7 V → about 13,513.51 mAh
- 37 Wh, 5 W load, 100% efficiency → 7 h 24 min
- 37 Wh, 5 W load, 85% efficiency → 6 h 17 min 24 s (37 × 0.85 ÷ 5 = 6.29 hours)
- A 10,000 mAh, 3.7 V battery used at an output of 5 V, 100% efficiency → 7,400 mAh at the output (6,290 mAh at 85% efficiency)
Edge cases
- A capacity of 0 gives 0 Wh and a run time of 0 seconds (not an error).
- A voltage of 0 is an error (Wh and mAh cannot be converted). Negative values are errors too.
- With a power consumption of 0 W, the run time is "Not available" (you cannot divide by 0; it is not an infinite time).
- The efficiency must be greater than 0 and at most 100.
- Numbers can have up to 9 digits before the decimal point and 6 decimal places. Use a period as the decimal separator; commas are accepted only as thousands separators (1,234.5).
Notes
- The run time is a theoretical estimate. It does not guarantee how long a device will really run.
- This calculator does not cover airline carry-on rules for batteries.
- For the relation between voltage, current and power, use the Ohm's Law Calculator.
Frequently asked questions
What is the difference between mAh and Wh?
mAh measures electric charge (capacity), and Wh measures energy. Multiplying mAh by the voltage and dividing by 1,000 gives Wh. Wh makes it easier to compare batteries with different voltages.
Can a 10,000 mAh power bank deliver 10,000 mAh at 5 V?
No. The 10,000 mAh is at the battery's own voltage (such as 3.7 V). At 5 V the capacity is smaller, by the ratio of the voltages and by the conversion losses (see the examples above).
Why is the real run time shorter than the calculation?
Aging, temperature, discharge conditions, a device's standby power and conversion losses all shorten it. The result here is a theoretical value.