Battery life calculator
Enter your battery's Full Charge Capacity and the laptop's average power draw. Don't know the watts? Fill in a line from the Battery usage section of your battery report instead. Everything is calculated in your browser.
Need the capacity numbers first? Open the free battery health checker to read them from your report.
How to use the calculator
- Create a battery report: open Command Prompt and run
powercfg /batteryreport, then open the battery-report.html file it saves. - Copy Full Charge Capacity from the Installed batteries section into the first box. Leave the unit on mWh.
- Enter the laptop's average power draw in watts. If you don't know it, copy one line from Battery usage: the Energy drained value and its Duration. Pick a line where you used the laptop the way you normally do.
- Optionally add Design Capacity to see how long the same use would last on a new battery, or change the start percentage to see how long a partial charge will last.
The battery life formula
A battery's capacity in the Windows report is energy, measured in milliwatt-hours (mWh). Power draw is measured in watts. Dividing one by the other gives time:
- Runtime (hours) = Full Charge Capacity (Wh) ÷ power draw (W)
- Wh = mWh ÷ 1,000, so 27,380 mWh is 27.38 Wh.
- From a partial charge, multiply the capacity by the percentage first: 50% of 27.38 Wh is 13.69 Wh.
- Power draw from the report = energy drained (Wh) ÷ duration (hours). 19,080 mWh in 48 minutes 5 seconds is 19.08 Wh ÷ 0.801 h ≈ 23.8 W.
If your battery label shows milliamp-hours (mAh), you also need its voltage: Wh = mAh × volts ÷ 1,000. Don't divide an mAh figure by watts; the result will be wrong.
Where to find the numbers in your battery report
Full Charge Capacity is in the Installed batteries table near the top. It is what the battery holds today, which is usually less than Design Capacity on a used laptop. The battery health percentage is simply one divided by the other.
The Battery usage section lists each period the laptop ran on battery over the last three days, with its duration and the energy drained. That is real measured use, so it gives a better power figure than a guess. The battery report guide explains every section.
Real example: my office ThinkPad
I checked the calculator against the battery report from the refurbished Lenovo ThinkPad I use at home, generated on 3 November 2026.
| Value from the report | Number |
|---|---|
| Full Charge Capacity | 27,380 mWh (27.38 Wh) |
| Design Capacity | 47,520 mWh (47.52 Wh) |
| Battery usage on 3 November | 19,080 mWh drained in 0:48:05 |
| Power draw worked out from that line | about 23.8 W |
| Calculated runtime from 100% | about 1 h 9 min (about 2 h 0 min on a new battery) |
| Windows' own estimate since OS install | 1:14:05 at full charge, 2:08:35 at design capacity |
The calculator and Windows agree within about five minutes. The small gap is expected: Windows averages every battery session since the operating system was installed, while the calculator uses one session that you choose. If your usual workload is lighter or heavier than that session, pick a different line from Battery usage.
Why real battery life can be shorter or longer
- Your workload changes. Screen brightness, video calls, games, downloads and background updates all change the power draw from minute to minute.
- The laptop rarely runs to exactly 0%. Windows usually warns you and then sleeps or hibernates before the battery is completely empty, so the usable time is a little shorter.
- The capacity figure is an estimate. The battery's own controller estimates Full Charge Capacity, and the number can move between reports, especially on a heavily worn battery.
- Cold and age matter. A worn or cold battery can drop out sooner under a heavy load than its capacity suggests.
If the calculated time looks fine but the laptop shuts down early, see why a laptop battery drains so fast and when to replace a laptop battery.
Calculator or Windows battery life estimates?
The battery report already contains a Battery life estimates table. Use it to see the long-term average. Use this calculator when you want to ask a specific question, such as how long the laptop will last at a workload you measured, from a half-charged battery, or after a battery replacement.
Frequently asked questions
How long will my laptop battery last?
Divide the energy your battery holds in watt-hours by the laptop's average power draw in watts. A battery that holds 27.4 Wh at 23.8 W lasts about 1 hour 9 minutes. Take Full Charge Capacity from your Windows battery report, not the original design figure, because a used battery holds less.
How do I calculate laptop battery life?
Convert Full Charge Capacity from mWh to Wh by dividing by 1,000, then divide by the power draw in watts. The answer is in hours; multiply the decimal part by 60 to get minutes.
How do I find my laptop's power draw in watts?
Open the Battery usage section of your Windows battery report and pick a session where you used the laptop normally. Divide its energy drained in watt-hours by its duration in hours. For example, 19,080 mWh in 48 minutes is about 23.8 W.
Can I use mAh instead of Wh?
Not directly. Milliamp-hours measure charge, not energy, so you also need the battery voltage: Wh = mAh × volts ÷ 1,000. The Windows battery report already shows capacity in mWh, so it is easier to use that.
Does battery health affect battery life?
Yes. Runtime depends on the energy the battery holds today. A battery at 57.6% health holds 57.6% of its original energy, so the same workload runs for about 57.6% as long as it did when the battery was new.
How this tool was verified
The formula is basic energy arithmetic: hours = watt-hours ÷ watts. I checked the calculator against the battery report from my own Lenovo ThinkPad generated on 3 November 2026, where it gave about 1 hour 9 minutes against Windows' estimate of 1 hour 14 minutes. Values are calculated in your browser and are not uploaded or stored.
Senior software engineer in Karachi with more than five years of experience building web applications in ASP.NET Core, C# and JavaScript. He built the free battery health checker and checks each guide against Microsoft and laptop makers’ support documentation.