# Battery Capacity Calculator

URL: https://speedycalculator.com/calculators/other/battery-capacity/
Last updated: 2026-10-30

> The battery capacity calculator converts between amp-hours, watt-hours and voltage – enter any two to get the third – and uses the C-rate or current to find the charge or discharge current, power and the time to fully charge or empty the battery.

## Inputs

- Calculate
- Battery voltage (V, mV)
- Stored energy (Wh, kWh, mWh, kJ, J)
- Battery capacity (Ah, mAh)
- Charge or discharge rate
- C-rate (C) — 1C empties the battery in 1 hour, 0.5C in 2 hours, 2C in 30 minutes.
- Charge or discharge current (A, mA, µA)

## Results

- Battery capacity
- Stored energy
- Voltage
- Electric charge
- C-rate
- Charge or discharge current
- Power at that current
- Time to fully charge or discharge
- Time in minutes

## Formula

```
E (Wh) = V × Q (Ah) · Q = E ÷ V · I = C-rate × Q · t (h) = 1 ÷ C-rate · charge (coulombs) = Q × 3600
```

## Explanation

Battery capacity is quoted in two ways. Amp-hours (Ah or mAh) tell you how much charge the battery holds – how many amps it can supply for how many hours. Watt-hours (Wh) tell you how much energy it holds, which also depends on voltage. A 12 V, 100 Ah lead-acid battery and a 3.7 V, 324 Ah bank of phone cells both store 1,200 Wh, even though their amp-hour figures look very different.

Watt-hours are the fair way to compare batteries of different voltages, and airlines use them: power banks above 100 Wh need approval to fly. Amp-hours are handy when you know a device's current draw. The conversion is simply energy = voltage × capacity.

The C-rate describes how hard a battery is charged or discharged relative to its capacity. At 1C a 2.2 Ah battery delivers 2.2 A and is empty in an hour; at 0.5C it delivers 1.1 A for two hours; at 2C it delivers 4.4 A for 30 minutes. Datasheets limit the C-rate, and real batteries deliver somewhat less at high rates.

### How to use the Battery Capacity Calculator

- **Calculate**
- **Battery voltage**
- **Stored energy**
- **Battery capacity**
- **Charge or discharge rate**
- **C-rate** — 1C empties the battery in 1 hour, 0.5C in 2 hours, 2C in 30 minutes.
- **Charge or discharge current**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose what to calculate and enter the other two quantities. Then give the C-rate or the current. The calculator multiplies or divides by voltage to convert between Ah and Wh, multiplies capacity by C-rate for the current and takes 1 ÷ C-rate for the run time.

### Worked example

A 12 V battery stores 26.4 Wh. Its capacity is 26.4 ÷ 12 = **2.2 Ah** (7,920 coulombs). Discharged at 1C it supplies 2.2 A, or 26.4 W, and is empty after 60 minutes; at 0.5C it supplies 1.1 A for 2 hours.

### Typical battery capacities

BatteryVoltageCapacityEnergyAA alkaline1.5 V2.5 Ah3.75 Wh18650 Li-ion cell3.6 V3 Ah10.8 WhSmartphone3.85 V5 Ah19.25 WhLaptop11.4 V5 Ah57 WhCar starter battery12 V60 Ah720 Wh

### Inputs explained

FieldWhat to enterCalculateChoose one: Capacity (Ah) from energy and voltage, Energy (Wh) from capacity and voltage, Voltage from energy and capacityBattery voltageUnits: V, mVStored energyUnits: Wh, kWh, mWh, kJ, JBattery capacityUnits: Ah, mAhCharge or discharge rateChoose one: I know the C-rate, I know the currentC-rateIn C. 1C empties the battery in 1 hour, 0.5C in 2 hours, 2C in 30 minutes.Charge or discharge currentUnits: A, mA, µA

### What each result means

- **Electric charge** = [Battery capacity]×3600
- **Charge or discharge current** = [C-rate]×[Battery capacity]
- **Power at that current** = [C-rate]×[Battery capacity]×[Voltage]
- **Time to fully charge or discharge** = 1÷[C-rate]
- **Time in minutes** = 60÷[C-rate]

## Worked example

Inputs — Calculate: Capacity (Ah) from energy and voltage; Battery voltage: 12 V; Stored energy: 26.4 Wh; Charge or discharge rate: I know the C-rate; C-rate: 1 C.

Results — **Battery capacity = 2.2 Ah**; Stored energy = 26.4 Wh; Voltage = 12 V; Electric charge = 7,920 coulombs; C-rate = 1 C; Charge or discharge current = 2.2 A; Power at that current = 26.4 W; Time to fully charge or discharge = 1 h 0 min; Time in minutes = 60 min.

## Key terms

- **Amp-hour (Ah)**: Charge delivered by 1 ampere flowing for 1 hour (3,600 coulombs).
- **Watt-hour (Wh)**: Energy delivered by 1 watt for 1 hour (3,600 joules).
- **C-rate**: Charge or discharge current divided by the rated capacity.
- **Nominal voltage**: The average voltage of a cell during discharge, e.g. 3.6–3.7 V for lithium-ion.

## FAQ

**How do I convert mAh to Wh?**
Wh = mAh × V ÷ 1000. A 5,000 mAh phone battery at 3.85 V holds 19.25 Wh.

**How do I convert Wh to Ah?**
Ah = Wh ÷ V. A 1,200 Wh battery at 12 V is 100 Ah.

**What does C-rate mean?**
Current relative to capacity: 1C on a 2 Ah battery is 2 A, which empties it in about 1 hour.

**How long does a 200 Ah battery take to charge?**
At 20 A (0.1C) about 10 hours plus 10–20% for charging losses and the slower final stage.

**Why do I get less capacity than rated?**
High discharge rates, cold temperatures, ageing and the cut-off voltage of your device all reduce usable capacity.

**Can I take a power bank on a plane?**
Most airlines allow power banks up to 100 Wh in hand luggage; 100–160 Wh needs approval.


## Sources

- [Battery University – BU-402: What is C-rate?](https://batteryuniversity.com/article/bu-402-what-is-c-rate)
- [Ampere hour – Wikipedia](https://en.wikipedia.org/wiki/Ampere_hour)