About the mAh to Wh Calculator
mAh to Wh Calculator converts battery capacity between milliamp-hours (mAh), watt-hours (Wh), kilowatt-hours and amp-hours. Because a mAh figure only makes sense together with a voltage, you enter both: the tool multiplies capacity by voltage to get energy, or divides energy by voltage to get capacity. Buttons give the common battery voltages, such as 3.7 V for lithium-ion cells and 12 V for lead-acid. A battery pack mode adds cells in series (which adds voltage) and in parallel (which adds capacity) and shows the total. You can also enter a load in watts, with the efficiency of any converter and the part of the capacity you can use, to estimate how long the battery will run. The results are estimates: real run time depends on temperature, age and how the load varies. Everything is worked out in your browser.
Updated September 2026
How to use the mAh to Wh Calculator
- 1Choose mAh to Wh, Wh to mAh or Battery pack.
- 2Enter the capacity and pick the battery voltage (3.7 V for most phone and power bank cells).
- 3For a pack, enter the number of cells in series and in parallel.
- 4Optionally enter the load in watts, and the efficiency and usable share, to see the run time.
- 5Read the energy or capacity in the result box.
Why mAh is not enough
Battery makers love mAh because the numbers look large, but capacity in mAh is only comparable between batteries of the same voltage. A 3.7 V, 10,000 mAh cell and a 12 V, 10,000 mAh battery do not hold the same energy: the second holds over three times as much. Watt-hours, which include the voltage, describe how much work a battery can do.
That is why airline rules, laptop batteries and electric bike batteries are given in Wh. Use this calculator to put different batteries on the same scale before comparing them.
Series and parallel packs
- Series: cells connected end to end add their voltages, and the capacity in mAh stays the same.
- Parallel: cells connected side by side add their capacities, and the voltage stays the same.
- A pack named 3S2P has 3 cells in series and 2 in parallel, or 6 cells in total.
- Four 3.7 V, 3,000 mAh cells as 2S2P give 7.4 V and 6,000 mAh, or 44.4 Wh.
- Always use a proper battery management system with lithium cells.
Estimating run time
Enter the power of your device in watts. Then set the efficiency to allow for the loss when the battery voltage is converted, and the usable share when the battery should not be emptied fully, such as 80 percent for a lead-acid battery that is best kept above a fifth charge. A camping fridge drawing 45 W from a 100 Ah, 12 V battery holds 1,200 Wh, and with 50 percent usable it runs for about 13 hours.
Cold weather, old cells and heavy loads all shorten the real run time, so leave a margin.
Related tools
To find the current or power of a load, Watts, Amps and Volts Calculator converts them, Ohm's Law Calculator relates voltage, current and resistance, and Energy Converter switches between joules, kilowatt-hours and other units. Electricity Cost Calculator shows what charging costs, and Power Converter changes watts to horsepower.
Be careful with batteries: never short them, never charge a lithium battery with the wrong charger, and follow the maker's limits on charge and discharge current.
Features
- mAh, Ah, Wh and kWh in one place
- Voltage buttons for common batteries
- Battery pack mode for series and parallel cells
- Run time from a load in watts
- Efficiency and usable capacity settings
- Readable time such as 2 h 30 min
- Runs in your browser
Frequently asked questions
How do I convert mAh to Wh?
Multiply the milliamp-hours by the voltage and divide by 1,000: Wh = mAh × V ÷ 1000. A 5,000 mAh phone battery at 3.85 volts holds 19.25 Wh. You need the voltage, because mAh alone does not tell you how much energy a battery has.
How do I convert Wh to mAh?
Multiply the watt-hours by 1,000 and divide by the voltage: mAh = Wh × 1000 ÷ V. A 60 Wh laptop battery at 11.1 volts is about 5,405 mAh. If you change the voltage, the mAh changes too, but the energy stays the same.
Why does my 20,000 mAh power bank not charge a 4,000 mAh phone five times?
Power banks quote the capacity of their cells at 3.7 volts, but they deliver power at 5 volts or more, and the conversion loses energy. With an efficiency of about 70 to 85 percent, a 20,000 mAh bank gives closer to three to three-and-a-half charges of a 4,000 mAh phone. Set the efficiency in the tool to see the effect.
What voltage should I use?
Use the nominal voltage printed on the battery. A single lithium-ion cell is 3.6 to 3.7 V, a nickel-metal-hydride AA is 1.2 V, a car battery is 12 V, and packs are a multiple of the cell voltage, such as 7.4 V for two cells in series or 11.1 V for three.
How do I work out how long a battery will last?
Divide the energy in watt-hours by the power of the load in watts, then allow for losses. A 50 Wh battery running a 10 W device lasts about 5 hours in the ideal case, and around 4 hours with 80 percent efficiency. The run time box in the tool does this.
What can I take on a plane?
Airlines usually limit spare lithium batteries by watt-hours: often up to 100 Wh without approval and up to 160 Wh with it. Convert your battery here to see where it falls, and check with your airline, since rules vary and change.