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Battery Life Calculator (mAh or Wh to Hours)
Estimate how long a battery will run a device: enter the capacity in mAh or Wh and the device's current or power, set the usable share for real-world losses, and get the runtime in hours and minutes, plus the energy in Wh.
Battery Life Calculator (mAh or Wh to Hours): Runtime is the capacity divided by the load, times the usable share: 3,000 mAh at 200 mA is 15 hours in theory, and 12 hours 45 minutes at 85%. Capacities at different voltages are compared in watt-hours, mAh × V ÷ 1,000, so a 5,000 mAh cell at 3.7 V holds 18.5 Wh. Runs 100% locally in your browser with zero server file uploads.
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- Home & family tools
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Runtime is capacity divided by the load, times the share of the capacity you can really use: a 3,000 mAh battery powering a 200 mA device lasts 15 hours in theory and about 12¾ at 85%. Losses come from voltage converters, cold, age, and the cut-off that protects lithium cells. Compare batteries of different voltages in Wh, mAh × V ÷ 1,000; a power bank's mAh is usually quoted at the 3.7 V cell, not the 5 V output.
Finding the current
Device specifications give a power in watts or a current in milliamps; a USB power meter shows the real draw. Average the current over the way you use the device, since screens, radios, and motors vary.
Cars and electric vehicles
For an electric car battery, charging time and cost are worked out with the EV charging calculator.
How to use it
- Choose mAh and mA, or Wh and W.
- Enter the capacity and the device's draw, and the usable share.
- Read the runtime.
Privacy & limitations
Everything is calculated in your browser.
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Frequently asked questions
Why is the real runtime shorter?
Voltage converters waste some energy, cold and age reduce capacity, and devices stop before the battery is truly empty to protect it; 80 to 90% is a fair usable share.
How many phone charges is my power bank?
Fewer than mAh alone suggests: a 10,000 mAh bank at 3.7 V holds 37 Wh, and after conversion losses it delivers roughly 25 Wh, about two charges of a 12 Wh phone battery.
Can I take it on a plane?
Airlines usually allow power banks under 100 Wh in cabin baggage; check your airline's rules.
Free tool · runs in your browser · no account required