mAh to Hours Calculator

What mAh measures

mAh stands for milliamp-hours, and it is a measure of charge, not energy. One mAh means the battery can supply one milliamp of current for one hour. That is why a 5,000 mAh battery is not automatically better than a 3,000 mAh one unless you know how much current each device draws.

The conversion to runtime is a single division: hours = capacity in mAh ÷ current draw in mA. A 5,000 mAh battery running a device that pulls 250 mA gives 5,000 ÷ 250 = 20 hours. Halve the draw to 125 mA and it doubles to 40 hours; double the draw to 500 mA and it falls to 10 hours. Nothing else about the battery changes.

Where people get stuck is comparing batteries of different voltages. A 3.7-volt phone battery and a 12-volt drill battery with the same mAh rating store different amounts of energy. To compare fairly you convert both to watt-hours: Wh = mAh × volts ÷ 1,000. The USB Implementers Forum and the US Department of Energy both use watt-hours as the standard energy unit for exactly this reason.

Worked example: a 5,000 mAh phone battery

Your phone has a 5,000 mAh battery and a typical app draws 250 mA. Runtime is 5,000 ÷ 250 = 20 hours of active use. Switch to something heavy like video recording at 1,000 mA and the same battery lasts just 5 hours.

• Reading (150 mA): 33 hours
• Web browsing (250 mA): 20 hours
• Streaming video (500 mA): 10 hours
• Video recording (1,000 mA): 5 hours

Now check the airline side. Watt-hours = 5,000 × 3.7 ÷ 1,000 = 18.5 Wh. That is well under the 100 Wh carry-on limit, so a spare battery this size can fly in your carry-on bag with no approval needed, though it still cannot go in a checked bag.

Real devices also spend time asleep, drawing only a few milliamps, so a phone that “lasts all day” is usually a blend of high-draw and low-draw hours. The calculator gives you the steady-state figure, which is the honest upper bound for a constant load.

Worked example: a 20,000 mAh power bank

A 20,000 mAh power bank is the common travel size, and it is where the numbers get interesting. Rated capacity is measured at the internal 3.7 volts, but the output your phone sees is 5 volts, and converting up loses energy as heat.

• Rated capacity: 20,000 mAh at 3.7 V
• Watt-hours: 20,000 × 3.7 ÷ 1,000 = 74 Wh
• Delivered at 5 V with 85% efficiency: about 74 × 0.85 ÷ 5 × 1,000 = 12,580 mAh
• Charges for a 4,000 mAh phone: about 3 full charges

That 74 Wh is comfortably under the 100 Wh carry-on limit, so this bank flies without airline approval. Step up to a 30,000 mAh model and you hit 111 Wh, over the limit, which means airline approval is required and some carriers refuse it outright. The FAA's PackSafe guidance spells out the watt-hour thresholds, and they are the number to check before you buy a travel bank.

Battery capacity and runtime at a glance

Runtime for common capacities at different current draws, plus the watt-hours at 3.7 volts:

27,000 mAh is the practical ceiling for a no-approval carry-on battery, since it lands right at 100 Wh. Anything above that needs airline sign-off.

Why real runtime is shorter than the math

The division above gives you a clean theoretical number, but real batteries always fall short. Three things eat the difference. First, conversion loss: a power bank rated at 3.7 volts puts out 5 volts, and stepping up wastes 10 to 20% as heat. Second, cold weather: lithium batteries lose capacity below freezing, often 20 to 30% at 0°C. Third, age: a battery that has been through 500 charge cycles typically holds only 80% of its original capacity.

Screen brightness, radios and background apps also move the current draw constantly, so the “250 mA” you type is an average, not a constant. If you want a realistic estimate, take the calculated runtime and knock off 15% for conversion loss and another 10% if the battery is more than two years old.

Use the result as a planning figure, not a guarantee. If you are sizing a battery for a critical job, a medical device, a camera shoot, a remote sensor, test the real device against the real load rather than trusting any calculator, including this one.

Choosing the right battery for the job

Working backwards from runtime is the fastest way to pick a battery. Decide how many hours you need and how much current the device draws, multiply the two, and add a 30% buffer for losses and aging. A device drawing 300 mA that must run for 12 hours needs 300 × 12 = 3,600 mAh, so buy a 5,000 mAh battery rather than a 3,600 mAh one.

For phones and tablets, capacity is the headline number and watt-hours decide whether it can fly. For tools and appliances, check the voltage as well as the mAh, because a 12-volt 2,000 mAh pack stores far more energy than a 3.7-volt 5,000 mAh one. For anything with a USB-C port, look for a power bank that advertises its watt-hour rating directly, it is the one number that makes comparison honest.

Finally, remember that capacity is a ceiling, not a promise. The number on the label is measured under ideal conditions at room temperature with a steady load, and no real device ever hits that exactly.

One more habit worth building: when a product lists only mAh and not watt-hours, multiply by the voltage yourself before you compare it to anything else. The label rarely prints the voltage, but USB devices are almost always 3.7 volts internally, so that is the number to reach for. Two power banks with the same mAh rating can hold different amounts of energy if one is a 3.7-volt cell and the other is a 12-volt pack, and only the watt-hour figure tells you which one lasts longer.