Cordless tool battery runtime calculator
Convert voltage and amp-hours to watt-hours, and estimate how long a battery will run a tool.
How it works
Energy in watt-hours = nominal voltage × amp-hours. An 18 V 5 Ah pack holds 90 Wh. Runtime ≈ Wh × 0.85 ÷ load watts. The 0.85 covers losses in the pack and electronics. Real tools rarely draw full power all the time, so actual runtime is often longer than the continuous-load figure.
Quick reference
| 12 V 2 Ah | 24 Wh |
| 18/20 V 2 Ah | 36 Wh |
| 18/20 V 5 Ah | 90 Wh |
| 18/20 V 8 Ah | 144 Wh |
| 40 V 4 Ah | 144 Wh |
| 56 V 5 Ah | 280 Wh |
“20 V max” packs have the same 18 V nominal voltage as “18 V” packs.
Questions
Can I fly with tool batteries?
IATA allows lithium-ion batteries up to 100 Wh in carry-on baggage. Batteries of 100–160 Wh need airline approval, and spares must not go in checked baggage.
Are third-party batteries safe?
CPSC has recalled several aftermarket tool batteries for fire and explosion hazards. Check the recall list on this site.
Sources
Injury data for these tools
ER visits in 2025
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