Battery Life Calculator
Estimate how long a battery will last: enter its capacity in mAh, the current your device draws in mA, and a real-world efficiency factor to see the runtime in hours, minutes and days.
Estimated battery life (hours)
12.75
- Ideal runtime, no losses (hours)
- 15
- Estimated runtime (minutes)
- 765
- Estimated runtime (days)
- 0.53
- Usable capacity after derating (mAh)
- 2,550
That's about 12 h 45 min. Battery life = capacity ÷ load current: milliamp-hours divided by milliamps gives hours directly. The efficiency factor derates the ideal figure to allow for real-world losses (high-drain Peukert effect, cut-off voltage, temperature, converter losses).
How to use this calculator
Enter the battery’s capacity in milliamp-hours (mAh) — it’s printed on the cell, the label, or the spec sheet (a phone battery is roughly 3000–5000 mAh, an AA cell around 2000–2800 mAh). Enter the current your device draws in milliamps (mA) while it runs; if you only know the power in watts, divide by the battery voltage to get amps, then multiply by 1000 for mA. Finally set an efficiency (derating) factor: leave it at 100% for the theoretical maximum, or use 70–90% for a realistic estimate that accounts for the losses every real battery has. The calculator returns the estimated runtime in hours, minutes and days, alongside the ideal (loss-free) figure and the usable capacity.
How the calculation works
A battery’s capacity in milliamp-hours tells you how much charge it holds: 1 mAh is one milliamp flowing for one hour. Because charge equals current times time, dividing the capacity (mAh) by the current the device draws (mA) gives the runtime directly in hours — the "milli-" units cancel. That is the ideal figure. In practice a cell never delivers its full rated capacity: at higher currents the usable capacity shrinks (the Peukert effect), the device stops at a cut-off voltage before the battery is truly flat, and temperature, self-discharge and any voltage-converter losses all take a share. The efficiency factor scales the ideal runtime down to a realistic estimate. The same maths works with energy units too — watt-hours divided by watts also gives hours.
Worked example
A 2000 mAh battery powering a device that draws 200 mA: 2000 ÷ 200 = 10 hours in the ideal case. Apply a 70% derating factor for real-world losses and the estimate becomes 10 × 0.70 = 7 hours. Halve the current draw to 100 mA and the ideal runtime doubles to 20 hours (14 hours after derating); double the capacity to 4000 mAh and it doubles again. Runtime scales directly with capacity and inversely with current draw.
Frequently asked questions
How do I calculate battery life from mAh?
Divide the battery’s capacity in milliamp-hours (mAh) by the current your device draws in milliamps (mA), and the answer is the runtime in hours. For example, a 3000 mAh battery driving a 250 mA load lasts 3000 ÷ 250 = 12 hours in the ideal case. Because real batteries deliver less than their rated capacity, multiply that figure by an efficiency factor (typically 0.7–0.9) for a realistic estimate.
Why is my real battery life shorter than mAh ÷ mA?
The simple division gives the theoretical maximum, which real cells rarely reach. At higher discharge currents the usable capacity falls — this is the Peukert effect. Devices also shut off at a cut-off voltage while the battery still holds some charge, and cold temperatures, self-discharge and the inefficiency of any voltage regulator or DC-DC converter all reduce the delivered energy. The efficiency (derating) factor in this calculator accounts for those losses; 70–90% is a common range.
What efficiency or derating factor should I use?
It depends on the load. For a gentle, steady drain well within the battery’s rating, 85–90% is reasonable. For a high-current load relative to the battery’s size, the Peukert effect bites harder and 60–75% may be closer. If you have no better information, 80% is a sensible middle estimate. Set it to 100% only when you want the pure theoretical figure — the answer you’d get if the battery delivered every last bit of its rated capacity, which it won’t.
How do I use this with a battery rated in watt-hours (Wh)?
The same principle applies with energy units: runtime in hours equals the battery’s energy capacity in watt-hours (Wh) divided by the device’s power draw in watts (W). If your figures are in mAh and volts, convert to watt-hours with Wh = (mAh ÷ 1000) × volts. To use this calculator’s mAh/mA fields instead, make sure both the capacity and the load are referenced to the same voltage — the ratio is what matters.
Does temperature affect battery life?
Yes. Most batteries lose usable capacity in the cold — a lithium-ion cell can deliver noticeably less below freezing — and very high temperatures accelerate self-discharge and long-term degradation. Manufacturers rate capacity at around 20–25°C, so if your device runs much colder or hotter than that, expect the real runtime to fall short and choose a lower efficiency factor to reflect it.
Is this the same as charging time?
No. This estimates how long a charged battery lasts under a load. Charging time is a different calculation: roughly the battery capacity divided by the charger’s current (also adjusted for efficiency, since charging is not 100% efficient either). A 3000 mAh battery charged at 1500 mA takes a bit over two hours, for instance — but the shape of the charge curve means the last portion is slower.