Calculate Amp Hours From Current, Runtime or Watt-Hours

To calculate amp-hours (Ah), multiply current in amps by time in hours:

Amp-hours = amps × hours

If you know watt-hours and voltage instead, use:

Amp-hours = watt-hours ÷ volts

For a cordless-tool battery, use the pack’s nominal voltage, not necessarily the largest voltage printed in the platform name.

Amp-hour formulas

What you know	Formula
Current and time	Ah = A × h
Watt-hours and voltage	Ah = Wh ÷ V
Milliamp-hours	Ah = mAh ÷ 1,000
Amp-hours and average current	Runtime in hours = Ah ÷ A

Convert minutes to hours before calculating:

Hours = minutes ÷ 60

Amp-hours describe electrical charge capacity. They are not the same as amps, which describe current at a particular moment, or watt-hours, which account for both capacity and voltage.

Example: calculate Ah from current and time

Suppose a load draws 2.5 A for 36 minutes.

1. Convert the time: 36 ÷ 60 = 0.6 hours
2. Multiply: 2.5 A × 0.6 h = 1.5 Ah

The load used 1.5 Ah.

When current changes, calculate each interval separately and add the results. A drill drawing 8 A for five minutes and 2 A for ten minutes would use:

• 8 × (5 ÷ 60) = 0.667 Ah
• 2 × (10 ÷ 60) = 0.333 Ah
• Total = 1.0 Ah

This interval method is a simplified version of coulomb counting. Battery gauges can integrate current continuously, while more advanced state-of-charge estimation may also account for battery behavior, temperature and aging (Analog Devices).

Example: calculate Ah from watt-hours

A battery rated at 90 Wh and 18 V nominal has:

90 Wh ÷ 18 V = 5 Ah

This can also be checked in the other direction:

18 V × 5 Ah = 90 Wh

The FAA gives the same watt-hour calculation for power-tool batteries: multiply voltage by amp-hours, after converting mAh to Ah if necessary (FAA PackSafe). Makita likewise lists an 18 V, 5 Ah pack as 90 Wh (Makita).

Use nominal voltage

Do not automatically divide by a platform’s advertised “maximum” voltage. DeWalt, for example, states that its 20V MAX batteries have a 20 V maximum initial voltage measured without a load but an 18 V nominal rating (DeWalt).

For a 90 Wh pack:

• Nominal-voltage calculation: 90 ÷ 18 = 5 Ah
• Using 20 V instead: 90 ÷ 20 = 4.5 Ah

Use the nominal voltage or, better still, the Ah and Wh values printed on the battery label. Do not mix the nominal voltage from one convention with a watt-hour rating based on another.

Estimate drill runtime from Ah

For a steady load, divide capacity by average current:

Runtime = Ah ÷ average amps

A 5 Ah battery supplying an average of 10 A has an idealized runtime of:

5 Ah ÷ 10 A = 0.5 hours = 30 minutes

That means approximately 30 minutes of accumulated operation at that average current—not necessarily 30 minutes from the start to the end of a job. Drilling is intermittent, and current changes with hole diameter, bit condition, material, speed, feed pressure and whether the tool is near a stall.

The arithmetic is therefore a planning estimate, not a guaranteed runtime. Available capacity can also change with discharge rate. This effect is especially significant in lead-acid batteries and generally less pronounced in lithium batteries (Victron Energy). Battery temperature, age, protection cutoffs and conversion losses can further separate calculated runtime from delivered runtime.

Series and parallel battery calculations

For matched batteries or cells in a compatible system designed for the arrangement:

• Series connection: add voltage; Ah remains the same.
• Parallel connection: voltage remains the same; add Ah.

Two 18 V, 5 Ah packs in series form a nominal 36 V, 5 Ah system. Its energy is:

36 V × 5 Ah = 180 Wh

Two matched 18 V, 5 Ah batteries in parallel form an 18 V, 10 Ah system, also totaling 180 Wh. Power-Sonic illustrates the same rule: series raises system voltage without raising Ah, while parallel raises Ah without raising voltage (Power-Sonic).

This is calculation guidance, not a recommendation to modify a tool pack. Cordless-tool batteries require compatible tools, chargers, cell arrangements and battery-management electronics.

Calculated capacity versus measured capacity

The Ah printed on a pack is its rated capacity under specified conditions. To determine delivered capacity, current must be measured throughout a controlled discharge to the correct cutoff voltage, then integrated over time. For a constant-current test, the basic calculation remains amps × hours. Fluke describes controlled discharge testing as a way to determine a battery’s available capacity and compare it with its manufacturer rating (Fluke).

Do not place a multimeter set to current directly across battery terminals; that creates a short circuit. Sealed power-tool packs are better assessed with compatible diagnostic equipment or a controlled load designed for the pack voltage and current.

The shortest reliable method is therefore:

1. Use A × h when current and operating time are known.
2. Use Wh ÷ nominal V when reading a battery label.
3. Use Ah ÷ average A only as an idealized runtime estimate.