Mobile & Electronics

Charging Time Calculator

How long a battery takes to charge, with the slowdown above 80% accounted for instead of ignored.

9 inputs Free, no sign-up

Your figures

The figure printed on the battery or in its specifications.

What the charger actually delivers to the battery. A 5 V 3 A charger can deliver up to 3 A, though a phone often accepts less.

Stopping at 80% avoids the slow part entirely, and is easier on the battery.

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

Add it to see the energy going in and coming out of 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

Dividing capacity by charging current gives an answer that is roughly right up to 80% and badly optimistic past it. Lithium batteries charge at a steady current only while there is room; as they fill, the charger holds the voltage and lets the current fall away, so the last fifth can take as long as the first four.

This splits the charge at 80% and shows both parts. Charge from 20% to 80% and you get the simple calculation, because that is the range where it is actually correct.

How to use this calculator

  1. Enter the battery capacity from the phone's specifications.
  2. Enter the charging current. The charger's rating is the ceiling; what the phone accepts is often less, and its own charging screen may tell you.
  3. Set the starting and target percentages. 20–80% is the fast range.
  4. Read the estimate, and the split between the fast part and the tapering part.

The formula

Charge to add    = Capacity x (Target - Start) / 100
Up to 80%        = that portion / (Current x Efficiency)
Above 80%        = that portion x taper factor / (Current x Efficiency)
Estimated time   = the two added

The taper factor here is 2, meaning the region above 80% is treated as taking about twice as long as the same capacity would at full current. That is an assumption, not a measurement, and it varies by device — but assuming no taper at all is worse, because it is wrong in a predictable direction on every full charge.

SymbolMeaningUnit
Capacity Battery capacity mAh
Current Charging current delivered A
Efficiency Share reaching the battery %

Worked example

A 5,000 mAh battery at 3 A, from 20% to 100%, 85% efficient:

Charge to add = 4,000 mAh
20% to 80%    = 3,000 mAh at 3 A x 0.85  = 1 h 11 min
80% to 100%   = 1,000 mAh, tapering       =    47 min
                                  Total   = 1 h 58 min

The same charge stopping at 80% takes 1 h 11 min — and the battery lives longer for it.

Notes and limits

  • An estimate, not a guaranteed time. It assumes the charger delivers your current for the whole charge, which depends on the cable, the adapter and the phone all agreeing.
  • Charging to 80% rather than 100% is faster per percent and easier on the battery. Many phones now offer it as a setting for exactly this reason.
  • Heat slows charging. A phone in the sun, or in use while charging, will take longer than this.
  • A phone rarely accepts the charger's full rating for long. The high figures in advertising apply to a brief window early in the charge.
  • For what a charge costs, see the charging cost calculator.

What this calculation assumes

  • The charger delivers the current entered for the whole of the constant-current phase.
  • Above 80% the current tapers; that region is allowed for at roughly twice the time, which is an assumption.
  • Heat, a phone in use, and cable limits all lengthen a real charge beyond this estimate.

Frequently asked questions

How long does it take to charge a 5,000 mAh battery?

At 3 A from 20% to 100%, roughly two hours once the slowdown above 80% is counted. Stopping at 80% takes about 1 hour 11 minutes. Dividing capacity by current alone would suggest well under an hour and a half, which is why that calculation misleads.

Why does the last 20% take so long?

Lithium batteries charge at constant current until they approach full, then the charger holds the voltage steady and the current falls away. That protects the battery, and it means the final stretch takes disproportionately long.

Should I charge to 100%?

Not routinely, if you can avoid it. Keeping a lithium battery between roughly 20% and 80% causes less wear than repeatedly filling it, and the range is also where charging is fastest. Charge fully when you need the endurance.

Why is my phone charging slower than the charger's rating?

The rating is a ceiling. A cable not rated for the current, the two ends failing to agree a fast-charge mode, a hot battery, using the phone while charging, or being above 80% all reduce it.

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