Battery Life Calculator
Estimate battery runtime in hours from capacity (mAh or Wh) and device current draw. Compare battery options for laptops
Battery Specifications
Device Power Consumption
Environmental Factors
Battery Life
Battery Life
10.1 hours total
Power Consumption
Average draw
Daily Energy
Energy per day
Usable Capacity
68.0% of rated
Efficiency
Temperature effect
Capacity Analysis
Power Saving Tips
How it works
Battery life estimates how long a device runs on a charge by dividing the battery's capacity by the current the device draws. Capacity is in milliamp-hours (mAh) and draw in milliamps (mA), so the result is in hours — discounted a bit for real-world inefficiency.
Runtime
Hours ≈ (capacity mAh ÷ draw mA) × efficiency
- capacity
- battery rating in mAh
- draw
- device current draw in mA
- efficiency
- ~0.7–0.85 for real losses
Worked example
- Battery = 3,000 mAh
- Device draws 250 mA, efficiency 0.8
- 3,000 ÷ 250 = 12 hours ideal
- × 0.8
≈ 9.6 hours of real-world runtime.
Good to know
- Apply an efficiency factor (often 0.7–0.85) — heat, voltage conversion, and aging all cut into the ideal figure.
- Higher current draw (brighter screen, more processing) shortens runtime proportionally.
- Batteries lose capacity as they age, so an old pack won't hit its rated mAh.
Can't find the exact calculator you need?
Describe it in one sentence — get a working, math-verified calculator in seconds. Free, no signup.
Build your own calculatorRelated Calculators
Frequently Asked Questions
How do I calculate battery life?
Divide the battery capacity by the device's current draw: a 3,000 mAh battery powering a 250 mA load lasts 3,000 ÷ 250 = 12 hours in theory. Multiply by an efficiency factor of about 0.7–0.85 to account for real-world losses.
What is the difference between mAh and Wh?
Milliamp-hours (mAh) measure charge and only compare batteries at the same voltage; watt-hours (Wh) measure energy and are comparable across any voltage. Convert with Wh = mAh × V ÷ 1000 — a 3.7 V, 3,000 mAh phone battery holds about 11.1 Wh.
Why is real battery life shorter than the calculated value?
Voltage conversion losses, heat, screen brightness peaks, radio activity, and battery aging all eat into the rated capacity. That is why an efficiency factor of 70–85% gives much more realistic estimates than the raw division.
Does battery capacity degrade over time?
Yes. Lithium-ion batteries typically retain around 80% of their original capacity after 300–500 full charge cycles, and degradation accelerates with heat and routinely charging to 100%. An older battery will not deliver its label rating.