Battery health
Enter both capacities in the same unit (mWh, Wh or mAh - it does not matter which). Scroll down to find these numbers on your machine.
Normal wear. Most people will not notice the difference in runtime.
How the number is worked out
Capacity lost = 100 − Health %
Every lithium-ion battery leaves the factory with a design capacity - the energy it was built to hold. As it ages, the most it can actually hold when full, the full-charge capacity, slowly falls. Health is simply how much of the original capacity is left. That is the same calculation the built-in reports on every operating system use, and the same one desktop battery tools run behind a nicer face.
Because health is a ratio, the units cancel. Windows quotes these figures in mWh, Linux in Wh and macOS in mAh - as long as your two numbers share a unit, the percentage is identical. This tool never sees your machine, so you read the two numbers yourself and type them in.
Finding the two numbers on your laptop
Windows 10 and 11
Open Command Prompt and run:
It saves an HTML file (the path is printed - usually your user folder). Open it and look under Installed batteries for DESIGN CAPACITY and FULL CHARGE CAPACITY, both in mWh. That same report also lists your cycle count lower down.
macOS
For the quick answer, open Settings > Battery > Battery Health and read Maximum Capacity - that is already your health percentage. For the raw numbers, open Terminal and run:
Read DesignCapacity and AppleRawMaxCapacity (both in mAh). On Apple Silicon Macs, use AppleRawMaxCapacity rather than "MaxCapacity", which can be pinned at 100%. Cycle count lives in Settings > General > About > System Report > Power, or via system_profiler SPPowerDataType.
Linux
Most distributions ship upower. Run:
Read energy-full and energy-full-design (in Wh). Helpfully, the capacity line already prints your health percentage directly. No upower? The kernel exposes the same values at /sys/class/power_supply/BAT0/ as charge_full and charge_full_design.
What your health percentage means
There is no single "bad" number, but these bands hold for most laptops:
- 90% and above: Excellent. Typical of a battery in its first year or two.
- 80% to 90%: Healthy. Normal wear - the drop in runtime is rarely noticeable.
- 60% to 80%: Fair. Runtime is meaningfully shorter; fine if you are often near a charger.
- Below 60%: Worn. Runtime is short enough to plan around; a replacement is worth pricing up.
The 80% line matters because many manufacturers define a battery as consumed once it falls below 80% of design capacity within its rated cycle count - that is often the warranty threshold. Falling below 80% after the rated cycles is expected wear, not a fault.
Cycle count, and why it is not the whole story
A charge cycle is one full battery's worth of discharge, however it accumulates - draining from 100% to 50% on two days is one cycle, not two. Laptop batteries are typically rated for 300 to 1000 cycles before reaching roughly 80% health; budget and older machines sit lower, recent MacBooks are rated for 1000. Health percentage is the number that actually reflects the battery in front of you; cycle count is context. A battery can read healthy with a high cycle count, or worn with a low one, depending on heat, charging habits and age.
How this differs from a desktop app
Native tools like the popular Windows battery-health apps read the same design and full-charge figures and show the same percentage - the maths is not the differentiator. What they add is background monitoring, tray alerts and auto-launch, at the cost of an install and, usually, being tied to one operating system. This page trades that convenience for zero install, one link that works on Windows, macOS and Linux, and the fact that your figures never leave the browser. For a quick "how worn is it, should I worry" check, that is usually all you need.
Common questions
How do you calculate laptop battery health?
Divide the current full-charge capacity by the original design capacity, then multiply by 100. Health % = (full-charge capacity / design capacity) x 100. A battery designed for 50000mWh that now tops out at 42000mWh is at 84% health, so it has lost 16% of its original capacity.
What is a good laptop battery health percentage?
Above 90% is excellent and normal for a newer machine. From 80% to 90% is healthy - most people will not notice the difference. 80% is the common service threshold: many manufacturers consider a battery consumed once it drops below 80% of design capacity within its rated cycle count, and warranties are often written around that line. Below about 60% the shorter runtime is usually obvious and a replacement starts to make sense.
How do I find my battery design and full-charge capacity?
On Windows, run "powercfg /batteryreport" in Command Prompt and open the HTML file it creates - it lists DESIGN CAPACITY and FULL CHARGE CAPACITY in mWh. On a Mac, run "ioreg -l -w0 | grep -i capacity" in Terminal and read DesignCapacity and AppleRawMaxCapacity in mAh, or just read Maximum Capacity in Settings > Battery. On Linux, run "upower -i $(upower -e | grep BAT)" and read energy-full and energy-full-design in Wh.
Does it matter whether I enter mWh, Wh or mAh?
No, as long as both numbers use the same unit. Health is a ratio, so the units cancel out. Windows reports mWh, Linux reports Wh and macOS reports mAh - whichever your system uses, put both the design and full-charge figures in that same unit and the percentage comes out the same.
How many charge cycles does a laptop battery last?
Most laptop lithium-ion batteries are rated for somewhere between 300 and 1000 full charge cycles before they fall to around 80% of design capacity. Older and budget machines sit near the low end; recent MacBooks are rated for 1000 cycles. A cycle is one full charge worth of use, so charging from 50% to 100% twice counts as a single cycle, not two.
Should I replace my laptop battery, and is a swollen battery dangerous?
Replace it when the reduced runtime gets in the way of how you use the machine, not at a fixed number - a desk-bound laptop can run at 60% health for years. Stop using the battery immediately if it is swollen, puffy or deforming the case or trackpad: a swelling lithium battery is a safety hazard, should never be punctured or pressed, and needs professional replacement and proper recycling.
Related: convert those capacities with the mAh to Wh calculator, or estimate how long a charge lasts with the runtime calculator.