Convert milliamp-hours to watt-hours with Wh = mAh × V ÷ 1,000. For example, 10,000mAh at 3.7V equals 37Wh. Use the battery’s nominal voltage, not the 5V USB output label. The calculator below also converts Wh back to mAh.
Enter the nominal battery voltage from the label. Accepted range: 0–1,000,000mAh, 0–10,000Wh and 0.01–1,000V.
Result:
Result:
Formula and Units
The calculator rejects negative capacity, zero voltage and values outside its stated ranges; this prevents an empty field or impossible voltage from being presented as a valid conversion.
Wh = (mAh ÷ 1,000) × V. Reverse it with mAh = Wh × 1,000 ÷ V. Watt-hours measure stored energy; watts measure the rate of power delivery. A 65W output rating and a 65Wh battery describe different things.
Which Voltage Should You Use?
Use the nominal battery or pack voltage printed on the label or datasheet. A USB power bank may output 5V, 9V or more after conversion while its internal cells are rated near 3.6–3.7V. Multiplying the advertised mAh by USB output voltage overstates stored energy.
Common Conversions
| Capacity | Nominal voltage | Energy |
|---|---|---|
| 5,000mAh | 3.7V | 18.5Wh |
| 10,000mAh | 3.7V | 37Wh |
| 20,000mAh | 3.7V | 74Wh |
| 10,000mAh | 3.85V | 38.5Wh |
| 10,000mAh | 7.4V | 74Wh |
Use the result to estimate charge counts in the 10,000mAh guide or 20,000mAh guide.
Three Common Conversion Mistakes
- Using USB output voltage: the internal cell rating and output profile describe different stages.
- Comparing mAh across voltages: 10,000mAh at 7.4V stores twice the energy of 10,000mAh at 3.7V.
- Confusing W with Wh: watts describe power rate; watt-hours describe stored energy.
Estimate Device Runtime from Wh
A planning formula is hours ≈ usable Wh ÷ device watts. The example deliberately chooses 80% as a sensitivity assumption, not a measured product efficiency: 37Wh × 0.8 ÷ a steady 5W load = about 5.9 hours. Replace 80% with measured delivered energy for a specific battery, and treat the result as a budget because real loads change over time.
What the Number Means for Flights
FAA guidance uses Wh for lithium-ion limits and says power banks belong in carry-on baggage. Batteries up to 100Wh are the normal limit; larger 101–160Wh batteries involve airline approval and additional restrictions. Check the airline and destination rules rather than relying only on the calculator.
Battery-Label Audit Worksheet
Use the conversion as a reconciliation check across the RFQ, sample, artwork and shipment file. A correct calculation does not prove cell quality or finished-product performance, but a mismatch is a clear stop signal for document review.
| Record | Required value | Audit action |
|---|---|---|
| Capacity basis | mAh/Ah and the stated nominal voltage | Reject an unexplained voltage assumption |
| Calculated energy | Wh with the formula and rounding rule | Compare with the finished label |
| Product identity | Exact model/SKU and revision | Prevent evidence reuse across models |
| Rated output | Separate declared usable/output capacity if provided | Do not treat cell Wh as delivered Wh |
| Change log | Cells, pack configuration, BMS and artwork | Recalculate after a critical change |
For passenger-air planning, use the current carrier and authority rules cited below. For a finished-product RFQ, carry this worksheet into the relevant power-bank specification.