Battery Capacity Calculator

Created by Bogna Szyk
Reviewed by Małgorzata Koperska, MD and Steven Wooding
Last updated: Sep 12, 2022

If you want to convert between amp-hours and watt-hours or find the C-rate of a battery, give this battery capacity calculator a try. It is a handy tool that helps you understand how much energy is stored in the battery that your smartphone or a drone runs on.

Additionally, it provides you with step-by-step instructions on how to calculate amp-hours and watt-hours, so you will be able to perform all of these calculations by yourself, too!

Wondering how long your electrical device will run on this battery? Check out the battery life calculator or battery size calculator!

What are battery amp hours?

The primary function of a battery is to store energy. We usually measure this energy in watt-hours, which correspond to one watt of power sustained for one hour.

If we want to calculate how much energy – in other words, how many watt-hours – is stored in a battery, we need information about the electric charge in the battery. This value is commonly expressed in amp-hours – amps (units of electric current) multiplied by hours (units of time).

Battery capacity formula

As you might remember from our article on Ohm's law, the power P of an electrical device is equal to voltage V multiplied by current I:

P = V × I

As energy E is power P multiplied by time T, all we have to do to find the energy stored in a battery is to multiply both sides of the equation by time:

E = V × I × T

Hopefully, you remember that amp hours are a measure of electric charge Q (the battery capacity). Hence, the final version of the battery capacity formula looks like this:

E = V × Q,


  • E – Energy stored in a battery, expressed in watt-hours;
  • V – Voltage of the battery; and
  • Q – Battery capacity, measured in amp-hours.

How to calculate amp hours?

Let's assume you want to find out the capacity of your battery, knowing its voltage and the energy stored in it.

  1. Note down the voltage. In this example, we will take a standard 12 V battery.

  2. Choose the amount of energy stored in the battery. Let's say it's 26.4 Wh.

  3. Input these numbers into their respective fields of the battery amp hour calculator. It uses the formula mentioned above:

    E = V × Q

    Q = E / V = 26.4 / 12 = 2.2 Ah

  4. The battery capacity is equal to 2.2 Ah.

Battery capacity calculator: advanced mode

If you open the advanced mode of this battery capacity calculator, you can compute three other parameters of a battery.

  1. C-rate of the battery. C-rate is used to describe how fast a battery charges and discharges. For example, a 1C battery needs one hour at 100 A to load 100 Ah. A 2C battery would need just half an hour to load 100 Ah, while a 0.5C battery requires two hours.

  2. Discharge current. This is the current I used for either charging or discharging your battery. It is linked to the C-rate with the following equation:

    I = C-rate × Q

  3. Runtime to full capacity. It is simply the time t needed to fully charge or discharge the battery when using the discharge current, measured in minutes. You can calculate it as t = 1/C.

Bogna Szyk
Battery capacity
Watt-hours or joules
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