Calculate how long a battery will last from its capacity in Ah, mAh or Wh and the load in amps or watts, or find the battery capacity a runtime needs.

From the battery label. Typical values: car battery about 40–70 Ah; phone battery about 3,000–5,000 mAh.

For a parked car’s parasitic draw, enter the measured key-off current in mA; under about 50 mA is usual.

Optional settings
Blank means 100%; reduce to keep a reserve.

Blank means 100%. Avoid double-counting losses in a battery-side current reading.

Battery Drain Time Formula

The basic battery drain time calculation uses battery capacity and current draw. Use capacity in milliamp-hours and current in milliamps to get time in hours.

T = C / I
  • T = battery drain time in hours
  • C = battery capacity in mAh
  • I = average current draw in mA; current is distinct from power in watts

If you need to solve for battery capacity instead of time, use:

C = I × T

If you need to solve for power consumption instead of capacity or time, use:

I = C / T

The calculator supports runtime or required capacity from Ah/mAh with A/mA, Ah/mAh with volts and W/kW, or Wh/kWh with W/kW. Time supports hours, minutes and days. Internal values use Ah, A, Wh, W and hours. With optional usable fraction f and efficiency e, runtime = capacity × f × e ÷ load; required capacity = load × time ÷ (f × e). Blank optional percentages mean 100%.

  • Calculate drain time: enter battery capacity and power consumption.
  • Calculate battery capacity: enter power consumption and desired drain time.
  • Power-based methods: choose Ah and watts with nominal voltage, or Wh and watts. The rearranged current formula above is background; the tool solves runtime or required capacity.

Common Battery Capacity and Current Unit Conversions

Conversion Equivalent Value Used For
1 Ah 1000 mAh Battery capacity
1 mAh 0.001 Ah Battery capacity
1 A 1000 mA Current draw
1 hour 60 minutes Drain time

Typical Current Draw Examples

Device or Load Typical Current Draw Note
Small LED indicator 5 to 20 mA Depends on LED and resistor
Microcontroller board 20 to 100 mA Sleep mode can be much lower
USB-powered small device 100 to 500 mA Varies by device activity
Phone charging load 1 to 2 A Equals 1000 to 2000 mA

Example Calculations

Example 1: Calculate battery drain time

You have a 2000 mAh battery and a device that draws 250 mA.

T = C / I
T = 2000 / 250 = 8

The estimated battery drain time is 8 hours.

Example 2: Calculate required battery capacity

You want a device drawing 0.5 A to run for 3 hours. First convert 0.5 A to 500 mA.

C = I × T
C = 500 × 3 = 1500

The required battery capacity is 1500 mAh, or 1.5 Ah.

FAQ

Why is the real battery life shorter than the calculated drain time?

The result is an ideal estimate. Real battery life can be shorter because of battery age, temperature, voltage cutoffs, discharge rate, converter losses, and the fact that many devices do not draw a perfectly constant current.

Should I use mAh or Ah for battery capacity?

Either unit works if you keep the units consistent. For small batteries, mAh is common. For larger batteries, Ah is common. The conversion is simple: 1 Ah equals 1000 mAh.

Does this calculation work for watts?

Yes. Choose a watts method. Ah capacity requires nominal voltage, using Wh = Ah × V; Wh capacity can be used directly. Match average load power with system efficiency, and avoid double-counting losses if the load is already measured at the battery.