Computers & Laptops

Laptop Charging Time Calculator

How long a laptop battery takes to charge, with the slowdown above 80% accounted for rather than ignored.

9 inputs Free, no sign-up

Your figures

In amp-hours. If your battery is labelled in Wh, divide it by the pack voltage to get Ah - the laptop battery capacity calculator does that conversion.

What the charger actually delivers to the battery. Work it back from the charger's watts and the pack voltage: a 65 W charger into an 11.1 V pack is under 6 A, and the machine itself takes a share of that while you use it.

Stopping at 80% avoids the slow part entirely and is easier on the battery. Many laptops have a setting that does it for you.

Leave blank for this page's 85% assumption. Charging is not lossless - some of the energy becomes heat in the charger, the cable and the battery.

Optional. Add it to see the energy going in and coming off the socket.

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

Capacity divided by charging current is right up to about 80% and optimistic past it. Lithium batteries take a steady current only while there is room; as they fill, the charger holds the voltage and lets the current fall, so the last fifth can take as long as the first four.

This splits the charge at 80% and shows both parts, so a 20–80% charge gets the simple answer — which is correct there — and a full charge gets a slower, honest one.

How to use this calculator

  1. Enter the battery capacity in amp-hours.
  2. Enter the charging current the pack actually receives. It is lower than the charger's rating when you are using the machine at the same time.
  3. Set the starting and target percentages.

The formula

Charge to add = Capacity x (Target - Start) / 100
Below 80%     = that share / (Current x Efficiency)
Above 80%     = that share x 2 / (Current x Efficiency)
Time          = the two added together

The factor of two above 80% is a convention rather than a measurement, and the result says so.

SymbolMeaningUnit
Capacity Battery charge capacity Ah
Current What reaches the battery A

Worked example

A 4.5 Ah pack from 20% to 100% at 3 A, 85% efficient:

20 to 80%  = 2.7 Ah / (3 x 0.85)     = 1.06 h
80 to 100% = 0.9 Ah x 2 / (3 x 0.85)  = 0.71 h
Total                                  = 1.77 h

Stopping at 80% would have taken just over an hour.

Notes and limits

  • Using the laptop while it charges slows the charge down, because the machine takes its power from the same supply.
  • Many laptops limit charging to 80% as a battery-health setting. That is the range where charging is fastest and the cells are least stressed.
  • USB-C Power Delivery negotiates a profile: a 100 W charger and a machine that accepts 45 W charge at 45 W, not 100.
  • A hot battery charges more slowly, and a laptop under heavy load is a hot battery.

What this calculation assumes

  • Charging current is treated as constant below 80%, which heat and a machine in use both break.
  • The taper above 80% is modelled at roughly twice the time per unit of charge. It is an assumption and varies by machine.
  • Charging efficiency defaults to 85% and the result names it.

Frequently asked questions

How long does a laptop take to charge?

It depends on the battery and the charger. A 4.5 Ah pack from 20% to full at 3 A is roughly 1.8 hours once the slowdown above 80% is allowed for. To 80% only, it is about an hour.

Why does the last 20% take so long?

Because the charger stops pushing a constant current and holds a constant voltage instead, letting the current fall away as the cells fill. It protects the battery, and it means the final stretch is far slower per percentage point.

Does a more powerful charger charge faster?

Only up to what the laptop will accept. Beyond that limit the extra capacity does nothing, and the machine negotiates the profile it wants.

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