Battery Life Calculator
Estimate how long a battery lasts from its capacity (mAh or Wh) and the load (mA or W), or the capacity you need for a runtime. Converts mAh and Wh by voltage.
mAh ratings of power banks refer to the internal 3.7 V cells; at 5 V USB output you get roughly 3.7 / 5 × 85–90% of the label figure.
| Usable | Runtime | Hours |
|---|---|---|
| 100% | 10 h 0 min | 10 |
| 90% | 9 h 0 min | 9 |
| 80% | 8 h 0 min | 8 |
| 70% | 7 h 0 min | 7 |
| 50% | 5 h 0 min | 5 |
Battery runtime is stored energy divided by the power a device draws. This calculator works with whatever numbers you have — a capacity in mAh, Ah or Wh and a load in mA, A or W — converts between them using the battery voltage, and allows for the share of capacity you can really use. Switch to "Capacity needed" to size a battery or power bank for a required runtime.
How to use this tool
- Enter the battery capacity and its unit (from the label or spec sheet).
- Pick the nominal voltage; it only matters when mixing mAh and Wh or mA and W.
- Enter the device's average current or power draw.
- Set the usable share: about 85% covers converter losses; use less if you never run the battery flat.
Formula
runtime (h) = capacity (Wh) × usable share ÷ load (W) · Wh = mAh × V ÷ 1000 · W = mA × V ÷ 1000
When both capacity and load are in mAh and mA at the same voltage, the voltage cancels and runtime = mAh ÷ mA. Real batteries deliver less at high currents and low temperatures, and lead-acid batteries lose noticeably more capacity at high loads (Peukert's law).
Example
A 5000 mAh phone battery powering a 500 mA load lasts 5000 ÷ 500 = 10 hours in theory. A 60 Wh laptop battery with an average draw of 15 W and 85% usable runs 60 × 0.85 ÷ 15 = 3.4 hours (3 h 24 min). A 10,000 mAh power bank holds 10 × 3.7 = 37 Wh.
Frequently asked questions
Why does my power bank charge my phone fewer times than expected?
Its mAh rating is at the 3.7 V cell voltage, but USB delivers 5 V or more, and conversion loses 10–20%. A 10,000 mAh bank typically gives 6,000–6,500 mAh to a phone.
What is depth of discharge?
How much of the capacity you use before recharging. Using 80% instead of 100% shortens each run but makes lithium and especially lead-acid batteries last more cycles.
How do I find my device's load?
Check the spec sheet for average (not peak) consumption, or measure it with a USB power meter or a plug-in energy monitor.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/battery-life-calculator/
Cite or embed this tool
kakaoCalc. (2026, November 8). Battery Life Calculator. https://kakao.mobile/tools/battery-life-calculator/
“Battery Life Calculator.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/battery-life-calculator/.
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