How do I convert mAh to hours of runtime?

Divide capacity by average current: 3000 mAh at 200 mA is 15 hours on paper. Apply a usable-capacity factor of 80 to 90% for a realistic figure — about 12.75 hours in that example.

What is the difference between mAh and Wh?

mAh measures charge and only makes sense alongside a voltage; Wh measures energy and can be compared across different voltages. Multiply amp-hours by volts to get watt-hours — 3 Ah at 3.7 V is 11.1 Wh.

What does the C rate mean?

It is the discharge current as a multiple of capacity. A 3000 mAh cell drawn at 3000 mA is running at 1C; most capacity ratings are measured at 0.2C to 1C, and going much above that reduces the energy you actually get out.

Why does duty cycle matter so much for battery life?

A sensor that wakes for one second a minute averages almost entirely its sleep current. Cutting the sleep draw from 1 mA to 10 µA can take a device from weeks to years, even though the active current never changes.

How many charges will a power bank give my phone?

Multiply the bank's capacity by about 65% to account for conversion and charging losses, then divide by the phone's battery capacity. A 10,000 mAh bank refills a 4,000 mAh phone roughly 1.6 times.

Battery Life Calculator

Turn capacity, voltage and current draw into a runtime you can trust, with derating and duty cycle taken into account.

The Battery Life Calculator runs entirely in your browser. Your figures are never uploaded.

Open the Ohm's Law Calculator

About Battery Life Calculator

Runtime is capacity divided by draw, but the naive version of that sum is always optimistic. No battery delivers its rated capacity into a real circuit — regulator losses, self-discharge and the cut-off voltage take a share, which is why a usable-capacity figure of 80 to 90% belongs in the calculation. The second correction is the duty cycle: most devices spend nearly all their life asleep, so what matters is the weighted average of the active and sleep currents rather than the peak. This calculator applies both, converts a power figure in watts into current at your pack voltage, reports the discharge C rate, and tells you how many times a power bank of that size will actually refill a phone.

Features

Runtime from capacity, voltage and load, in hours, days or years
Usable-capacity derating with presets per battery chemistry
Duty cycle averaging of active and sleep current
Draw entered as milliamps or watts, converted at the pack voltage
Stored energy in watt-hours and the discharge C rate
Warning when the C rate is beyond what a cell handles well
Capacity needed to hit a target runtime, worked backwards
Power bank charge count with realistic conversion losses
Everything computes in your browser

How to use the Battery Life Calculator

Enter the battery capacity in mAh and its nominal voltage
Pick a chemistry preset to set a realistic usable-capacity figure
Enter the active draw, the sleep draw and the share of time awake
Read the runtime and the figures behind it

Example

Input

3000 mAh at 3.7 V · 200 mA active · 85% usable

Output

11.1 Wh · 2550 mAh usable · 12 h 45 min at 0.067C

The derating is why 3000 mAh at 200 mA gives 12.75 hours, not 15.

Common errors & troubleshooting

The device dies well before the calculated runtime. — Peak draw is usually higher than the average you entered, and cold weather cuts usable capacity sharply. Measure the current under real load, and drop the usable-capacity figure to 70% in winter conditions.
A 10,000 mAh power bank does not charge a 4,000 mAh phone twice. — The bank's cells are 3.7 V and USB output is 5 V, so the conversion alone loses about 20%, and the phone's own charging circuit takes more. Two to two and a half charges is the realistic answer, not 2.5 on paper.
The C rate warning appeared on a small cell. — Drawing more than about 1C means you are pulling harder than the rating was measured at, so the delivered capacity falls below the label. Use a larger pack or cells rated for high discharge.

Frequently asked questions

How do I convert mAh to hours of runtime?: Divide capacity by average current: 3000 mAh at 200 mA is 15 hours on paper. Apply a usable-capacity factor of 80 to 90% for a realistic figure — about 12.75 hours in that example.
What is the difference between mAh and Wh?: mAh measures charge and only makes sense alongside a voltage; Wh measures energy and can be compared across different voltages. Multiply amp-hours by volts to get watt-hours — 3 Ah at 3.7 V is 11.1 Wh.
What does the C rate mean?: It is the discharge current as a multiple of capacity. A 3000 mAh cell drawn at 3000 mA is running at 1C; most capacity ratings are measured at 0.2C to 1C, and going much above that reduces the energy you actually get out.
Why does duty cycle matter so much for battery life?: A sensor that wakes for one second a minute averages almost entirely its sleep current. Cutting the sleep draw from 1 mA to 10 µA can take a device from weeks to years, even though the active current never changes.
How many charges will a power bank give my phone?: Multiply the bank's capacity by about 65% to account for conversion and charging losses, then divide by the phone's battery capacity. A 10,000 mAh bank refills a 4,000 mAh phone roughly 1.6 times.

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