Computers & Laptops

Laptop Battery Capacity Calculator

Watt-hours, amp-hours and milliamp-hours for a laptop battery, from its voltage and capacity.

5 inputs Free, no sign-up

Your figures

The mAh or Ah figure on the battery label. If yours is marked only in Wh, the laptop battery runtime calculator takes that directly.

From the label. Laptop packs are commonly 11.1 V or 11.55 V for three cells in series and 14.4 V or 15.4 V for four - but read yours rather than assuming, because the arithmetic depends on it entirely.

Leave blank for this page's 90% assumption, which the result names.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

Milliamp-hours measure charge, not energy. A 5,000 mAh pack at 3.7 V and a 5,000 mAh pack at 11.1 V hold three times different amounts of energy, which is why laptop batteries are labelled in watt-hours and phone batteries are not.

Give the capacity and the voltage and this gives all three figures.

How to use this calculator

  1. Read the capacity from the battery label, in mAh or Ah.
  2. Read the voltage from the same label. This is the figure that turns charge into energy and it is not assumed for you.
  3. Leave the efficiency blank unless you have a better figure.

The formula

Wh  = Ah x Volts
Ah  = mAh / 1000
Usable Wh = Wh x Efficiency

The watt-hour figure is the one that compares between batteries. Two packs with the same mAh and different voltages are not the same battery.

SymbolMeaningUnit
Capacity Charge the battery holds mAh or Ah
Voltage Nominal pack voltage V

Worked example

A 4,500 mAh pack at 11.1 V:

Ah = 4500 / 1000  = 4.5
Wh = 4.5 x 11.1    = 49.95

Which is why that battery is sold as "50 Wh".

Notes and limits

  • The voltage on a label is the nominal figure. A lithium cell is nearer 4.2 V full and 3.0 V empty, and the nominal value is the average the capacity was measured at.
  • Aviation rules are written in watt-hours for exactly this reason, so a spare battery's Wh figure is the one that matters at an airport.
  • A battery's real capacity falls with age. Your operating system's battery report shows the current full-charge capacity against the design figure.
  • The efficiency here covers losses in the machine's own regulation, not charging losses - those are on the charging cost page.

What this calculation assumes

  • The nominal voltage is used throughout; a real cell's voltage changes as it discharges.
  • The capacity is the rated figure, which an aged battery no longer holds.

Frequently asked questions

How do I convert mAh to Wh?

Divide the mAh by 1,000 to get amp-hours, then multiply by the battery's voltage. 4,500 mAh at 11.1 V is 49.95 Wh.

Why are laptop batteries rated in Wh and phone batteries in mAh?

Because laptop packs come in several voltages, so mAh alone would not compare between them. Phone batteries are almost all single-cell at about 3.7 V, which makes mAh a usable shorthand there and a misleading one elsewhere.

What voltage should I use?

The one printed on your battery. Guessing changes the answer proportionally, which is why this page requires it rather than filling one in.

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