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Battery Life Calculator
Use this free battery life calculator to find out how long a battery will run your device. Enter the battery capacity in mAh, Ah or Wh, the current or power your device draws, and an efficiency figure, and it works out the runtime in hours and minutes. It covers phones, flashlights, drones, laptops and LED lights — pick a preset to fill in typical values, then adjust them to match your own battery and device.
Needed to convert between mAh and watts. Phone cell: 3.7 V. Car battery: 12 V.
100% is the ideal figure. Use 80–90% for real-world planning.
How the battery life calculator works
Battery life is a simple division once both sides are in the same units: runtime = capacity ÷ draw. A 3,000 mAh battery feeding a device that draws 500 mA lasts 3,000 ÷ 500 = 6 hours. The complication is that capacity and draw are often given in different units, and the calculator converts them for you using the battery voltage.
There are two kinds of units at play. Milliamp-hours (mAh) and amp-hours (Ah) measure charge — how many milliamps a battery can deliver for one hour. Watt-hours (Wh) measure energy — charge times voltage. The link between them is Wh = mAh × volts ÷ 1,000, so a 3,000 mAh phone battery at 3.7 volts holds 11.1 Wh, while a 3,000 mAh battery at 12 volts holds 36 Wh. The same mAh number can mean very different amounts of energy, which is why watt-hours are the fair way to compare batteries of different voltages.
The same conversion works on the draw side: watts = amps × volts, so a device drawing 500 mA from a 3.7 V battery uses 1.85 W, and one drawing 45 W from a laptop battery uses about 4 amps at 11.1 V. Once both sides are in watt-hours and watts, the division is straightforward: 11.1 Wh ÷ 1.85 W = 6 hours.
Finally, the efficiency setting covers real-world losses. Regulators, converters and wiring waste some energy as heat, so a battery rarely delivers all of its rated capacity to the device. At 100% the calculator gives the ideal figure; at 85% it gives 6 × 0.85 = 5.1 hours for the same phone example. For planning, 80–90% is a realistic setting for most gadgets, and 70–80% for a power bank charging another device through a cable.
Worked example
A phone has a 3,000 mAh battery (3.7 V) and the screen-on draw averages 500 mA. How long will it last?
Step 1: Convert the capacity to energy — 3,000 mAh × 3.7 V ÷ 1,000 = 11.1 Wh.
Step 2: Convert the draw to power — 500 mA is 0.5 A, and 0.5 × 3.7 V = 1.85 W.
Step 3: Divide energy by power — 11.1 ÷ 1.85 = 6.0 hours at 100% efficiency, which is the ideal figure.
Step 4: Apply a realistic efficiency of 85% — 6.0 × 0.85 = 5.1 hours of real use.
Step 5: In minutes, the ideal runtime is 6 × 60 = 360 minutes, and the realistic figure is about 306 minutes.
The same maths explains why a drone with a 5,000 mAh battery only flies for around 35–40 minutes: its motors draw about 8 amps, so 5 ÷ 8 = 0.63 hours, or roughly 37 minutes, before efficiency losses. Doubling the battery would double the flight time, but it would also add weight, which raises the draw — the reason drone makers chase efficiency rather than just bigger batteries.
Tips for longer battery life
The calculator shows the two levers you have: a bigger battery, or a smaller draw. Cutting the draw is usually cheaper and lighter. On a phone, lowering screen brightness, turning off background refresh and using Wi-Fi instead of mobile data can cut the average draw by a third, which adds a third to the runtime. On a flashlight or LED light, a lower brightness mode can stretch the same battery from hours to days.
Battery age and temperature matter too. A lithium battery that has been through hundreds of charge cycles may hold only 80% of its original capacity, and cold weather can temporarily cut usable capacity by a further 20%. If your device dies much sooner than the calculator says, check the battery health in your device settings and test it at room temperature before assuming the calculator is wrong.
FAQ
How do you calculate battery life?
Divide the battery's capacity by the device's current draw, then multiply by the efficiency. A 3,000 mAh battery powering a device that draws 500 mA lasts 3,000 ÷ 500 = 6 hours at 100% efficiency. In the real world, voltage conversion and heat waste some energy, so an 85% efficiency setting gives 6 × 0.85 = 5.1 hours. If the draw is given in watts, first convert the capacity to watt-hours using the battery voltage.
What is the difference between mAh and Wh?
Milliamp-hours (mAh) measure charge, while watt-hours (Wh) measure energy. The two are linked by voltage: Wh = mAh × volts ÷ 1,000. A 3,000 mAh phone battery at 3.7 volts holds 11.1 Wh, while a 3,000 mAh battery at 12 volts holds 36 Wh — the same mAh, but over three times the energy. Watt-hours are the fair way to compare batteries of different voltages.
Why does my battery last less than the calculator says?
Three things shorten real battery life. First, efficiency losses: regulators and converters waste 10 to 20% of the energy as heat. Second, the draw is rarely steady — a phone idling at 100 mA can spike to 1,500 mA when the screen and radio are active. Third, cold weather and old age reduce a battery's usable capacity, sometimes by 20% or more. Use the efficiency setting and a realistic average draw for a planning figure, not a best-case one.
How long will a 10,000 mAh power bank charge my phone?
A 10,000 mAh power bank at 3.7 volts holds about 37 Wh, but boosting that to the 5 volts your phone charges at, plus conversion losses, leaves roughly 30 to 32 Wh of usable energy. A phone with an 11 Wh battery therefore gets about two and a half to three full charges from it. The calculator's efficiency setting covers this: set it to 85% for a typical power bank.
What battery voltage should I enter?
Enter the nominal voltage printed on the battery. Common values are 3.7 volts for a phone or single lithium cell, 7.4 volts for a two-cell pack, 12 volts for a car or lead-acid battery, and 5 volts for USB-powered devices. The voltage only matters when you mix units — capacity in mAh with draw in watts, or capacity in Wh with draw in milliamps — because it converts between charge and energy.