Battery Capacity Calculator

Created by Dominik Czernia, PhD
Last updated: Sep 05, 2022

The battery capacity calculator is an excellent choice if you want to know what battery capacity is or if you need to compute the properties of various batteries and compare them before purchasing a new battery.

We need batteries to power our phones, laptops, and cars, and knowing how to calculate their amp hours is a crucial thing. In the following text, you can read more about this battery charge calculator, learn about the battery capacity formula, and see how you can use this tool as a 12 V battery run time calculator in an example. Stay tuned!

What is battery capacity?

The capacity of a battery is the amount of energy that it can store. A battery's capacity is expressed in amp hours (Ah), which is a measure of electrical current over time. One amp hour equals one amp of current flowing for one hour. The higher the Ah, the longer the battery will last.

We show the battery Ah calculation below, together with a battery amp hours chart at the very end of this article.

We also have an interesting text and calculator about different types of horsepower units, including electrical horsepower. Check the hp to amps calculator to learn more!

How to calculate amp hours

To calculate amp hours, you need to know the voltage of the battery and the amount of energy stored in the battery. Multiply the energy in watt-hours by voltage in volts, and you will obtain amp hours.

Alternatively, if you have the capacity in mAh and you want to make a battery Ah calculation, simply use the equation: Ah = (capacity in mAh)/1000. For example, if a battery has a capacity of 3000 mAh, then its Ah rating would be 3 Ah.

Finally, to calculate the capacity of a battery in amp hours, you can use the current flowing in the battery and the amount of time that the battery can provide power at that current and multiply both values: amp hours = current × time. For example, if you have a 12-volt battery that can provide 1 amp of current for 3 hours, the capacity of the battery is: amp hours = 1 amps × 3 hours = 3 amp hours.

Battery capacity formula

We have already shown various methods explaining how to calculate amp hours (Ah). Let's now see the particular battery capacity formulae:

  1. Ah = watt-hours × voltage
  2. Ah = mAh / 1000
  3. Ah = discharge current × time (hours)

Our battery calculator offers a quick estimation of all the above equations depending on your needs. To use the second equation, enter the battery capacity in Ah, and then switch the units to mAh.

If you already found your answer, you might find our other tools, such as battery cable size calculator or circuit breaker sizing calculator, valuable and useful.

How to use the battery capacity calculator

The tool on this website can work in various ways:

  1. Battery capacity calculator - enter voltage and watt-hours, and you will obtain battery capacity in ampere-hours.
  2. Battery charge calculator (or battery kWh calculator) - enter voltage and ampere-hours to find watt-hours and, thus, the battery charge.
  3. Battery charge time calculator - input C-rate (one C-rate is equal to a battery working for 1 hour with 100 amperes) or battery capacity and discharge current to find how long you need to wait to fully charge or discharge the battery.

Battery amp hours chart

If you prefer a chart over a battery calculator, we have prepared two charts that you can use instead. The first one tells you what capacity your battery has depending on the voltage and watt-hours, while the second one estimates how long your battery will run with a specific capacity and discharge current.



6 V

12 V

24 V

300 Wh

50 Ah

25 Ah

12.5 Ah

600 Wh

100 Ah

50 Ah

25 Ah

1200 Wh

200 Ah

100 Ah

50 Ah

3000 Wh

500 Ah

250 Ah

125 Ah

Discharge current


1 A

3 A

5 A

50 Ah

50 hrs

16 hrs 40 min

10 hrs

100 Ah

100 hrs

33 hrs 20 min

20 hrs

200 Ah

200 hrs

66 hrs 40 min

40 hrs

500 Ah

500 hrs

166 hrs 40 min

100 hrs

Dominik Czernia, PhD
Battery capacity
Battery capacity
Runtime to full capacity
Discharge current
Runtime to full capacity
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