Battery Life Calculator
Runtime with a derating factor, because the ideal figure never happens.
Work out Battery Life. Runtime with a derating factor, because the ideal figure never happens. Gravity is an input, not an assumption.
0.7 to 0.85 is typical for consumer cells.
Realistic life (hours)
16
0.67 days · ideal figure 20 h
Capacity divided by draw gives 20 hours, and that figure is always optimistic. Real cells lose usable capacity to self-discharge, temperature, internal resistance and the voltage cutoff of whatever they are powering — so the derating factor is applied openly rather than left implicit in a number that will not be met.
How the Battery Life Calculator works
Capacity divided by draw gives the ideal runtime, and that figure is always optimistic. Real cells lose usable capacity to self-discharge, temperature, internal resistance and the voltage cutoff of whatever they power — so a derating factor is applied openly rather than hidden.
Also known as: how long will my battery last · mah to hours calculator · battery runtime calculator · battery capacity calculator
Frequently asked questions
How do I calculate battery life?
Divide capacity in mAh by draw in mA. A 2,000 mAh battery at 100 mA lasts 20 hours in theory, and around 16 in practice.
What derating factor should I use?
0.7 to 0.85 for most consumer cells. Lower for cold conditions, high discharge rates, or devices that cut off well above the cell's minimum voltage.
Why doesn't my battery last as long as the maths says?
Capacity ratings are measured under gentle, warm, laboratory discharge. Real use is none of those, and the gap is routinely 15–30%.
Does a higher current draw reduce total capacity?
Yes — this is Peukert's effect. Drawing harder gets less total energy out of the same cell, which is one of the things the derating factor stands in for.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/battery-life-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Battery Life Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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