Battery labels mix two units. Milliamp-hours (mAh) say how much charge a battery holds; watt-hours (Wh) say how much energy. The two are only comparable at the same voltage, which is why a 10,000 mAh power bank holds far less energy than a 10,000 mAh laptop battery would. This converter switches between them using the pack's nominal voltage.
How to use it
Fill in any two of the three fields — capacity in mAh, nominal voltage, or energy in Wh — and the third is calculated. Use the nominal voltage printed on the pack (for example 11.1 V, 11.4 V or 15.2 V), not the full-charge voltage.
The formula
Wh = mAh × V ÷ 1000, and therefore mAh = Wh × 1000 ÷ V.
Why laptops are rated in Wh
A laptop pack is made of several lithium-ion cells in series. Its nominal voltage is the cell voltage times the number of cells in series, so the same mAh rating means very different energy depending on the configuration. Watt-hours remove that ambiguity, which is why laptop manufacturers and airline rules use them. Pack voltages are worked out by Battery Pack Voltage Calculator (Cells in Series and Parallel).
Worked example: comparing two batteries
A replacement battery is advertised as "4800 mAh" while the original is labelled "56 Wh, 11.4 V". At 11.4 V, 4800 mAh is 4800 × 11.4 ÷ 1000 ≈ 54.7 Wh — slightly less than the original. If the replacement's listing quotes a lower voltage, recalculate with that; a "6000 mAh" listing at 7.6 V is only 45.6 Wh.
Power banks and USB-C
Power banks quote mAh at the internal cell voltage, about 3.6–3.85 V. A 20,000 mAh power bank at 3.7 V holds about 74 Wh. When it charges a laptop at 20 V, conversion losses mean the laptop receives noticeably less than that. Charging power and limits over USB-C are covered by USB-C Power Delivery Calculator.
Where it matters
- Comparing replacement batteries honestly.
- Estimating runtime with Laptop Battery Runtime Calculator.
- Checking travel limits, which are expressed in Wh.
The pack and cell basics are in watt hour and mah.
FAQ
Why does my battery show 11.1 V on one label and 12.6 V on a meter?
11.1 V is the nominal voltage of three cells in series; 12.6 V is their full-charge voltage. Use the nominal voltage for conversions.
Is a higher mAh battery always better?
Only at the same voltage. Compare Wh instead.
How much of the rated Wh can I actually use?
Somewhat less: the pack keeps a reserve, and capacity falls with age. A health percentage in the battery report tells you how much remains.