Battery Capacity Calculator

Created by Bogna Szyk
Reviewed by Małgorzata Koperska, MD and Steven Wooding
Last updated: Mar 04, 2023

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) – see the hours calculator.

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.


What is the capacity of a battery?

The capacity of the battery tells us what the total amount of electrical energy generated by electrochemical reactions in the battery is. We usually express it in watt-hours or amp-hours. For example, a 50Ah battery can deliver a current of 1 amp for 50 hours or 5 amps for 10 hours.

How long does it take to fully charge a 200Ah battery?

5 hours, assuming that you have a 12 V 200 Ah car battery and a charging rate is 0.2C. To find it:

  1. Calculate the capacity of your battery: Q = 12 V × 200 Ah = 2,400 Wh.
  2. Calculate the runtime to full capacity: t = 1/0.2 = 5 hours or 300 minutes.

What factors affect battery capacity?

Factors that affect battery capacity are the discharging current, internal resistance, state of charge, and temperature. The higher the discharge current and temperature during charging and operation, the shorter the battery life.

How can I measure battery capacity?

To measure a battery's capacity, use the following methods:

  1. Connect the battery to a constant current load I.
  2. Measure the time T it takes to discharge the battery to a certain voltage.
  3. Calculate the capacity in amp-hours: Q = I×T.


  1. Do the same, but use a constant power load P.
  2. Calculate the capacity in watt-hours: Q = P×T.

What is the C rating of a battery?

The C rating determines the rate at which the battery discharges. The higher the discharge rate (i.e., higher C ratings), the lower the total capacity of the battery. For example, if you have a 60Ah battery rated at 1C, this means that it is capable of delivering 60 A of current continuously in 1 hour.

Bogna Szyk
Battery capacity
Watt-hours or joules
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