Enter the battery capacity and your device’s current draw to get the estimated runtime — with a real-world derating factor.
What Is a Battery Life Calculator?
A battery life calculator divides capacity by load: 2,000 mAh ÷ 100 mA × 0.7 = 14.00 hours. The 0.7 derating is the honest part — real batteries never deliver their label capacity (temperature, age, discharge rate and cut-off voltage all eat into it). Ideal maths says 20 hours; reality says ~14.
How to Use
- Enter the battery capacity in mAh.
- Enter the load current in mA.
- Pick the derating factor.
- Click “Calculate Runtime”.
Formula
Hours = capacity ÷ load × derating.
Worked Example
2,000 mAh, 100 mA, 0.7 derating: 2,000 ÷ 100 × 0.7 = 14.00 hours.
How It Works, In Plain Words
The maths is one division and one honest adjustment. Capacity in milliamp-hours is a budget of charge: how much current the battery can supply, and for how long, added together. Load current in milliamps is the rate your device spends that budget. Dividing capacity by load gives the ideal runtime — the worked example’s 2,000 mAh ÷ 100 mA = 20 hours if the battery delivered every drop of its label rating. Real batteries do not, which is what the derating factor is for: it multiplies the ideal answer by the fraction of the label capacity you can actually use. At 0.7, realistic for an ordinary load, the example gives 14.00 hours. The 0.85 option suits a light, gentle load, and 1.0 shows the theoretical ceiling for comparison.
Two practical points hide inside those three inputs. First, the load should be the device’s average draw, including its busy moments, not just its idle sip — a gadget that sleeps most of the time and wakes to transmit spends its budget at the average of the two. Second, when the runtime reaches a day or more, the tool also shows it in days, because long, slow drains are easier to plan in days than in hundreds of hours.
Real-World Uses
A hobbyist building a sensor node checks whether the battery will carry it between visits: enter the cell’s mAh and the circuit’s average draw, keep the realistic 0.7 derating, and the answer decides whether the design is viable or the battery needs to grow. An RC or portable-project builder does the same before a session, learning in advance whether the pack comfortably covers the planned run or will fade part-way through. And anyone eyeing a power bank’s big label number uses the derating honestly: type in the capacity, a typical device’s draw and 0.7, and the runtime becomes the believable figure — closer to the worked example’s 14 hours than the headline 20 — which is the number worth planning a trip around.
Benefits
- Realistic derating built in (0.7 default)
- Days shown for long runtimes
- mAh/mA unit consistency
- Useful for IoT, RC and portable projects
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes
- Using the label capacity with no derating — reality is 70–85%
- Mixing mAh with Ah (1,000× off)
- Forgetting self-discharge on long standby (months)
Common Mistakes to Avoid
The derating exists because the commonest mistake is trusting the label: as the mistakes list puts it, reality is 70–85% of it, so running the numbers at 1.0 plans for a battery that does not exist. Unit slips come next — a capacity quoted in Ah entered as mAh, or the other way round, throws the answer out by a factor of 1,000, which is why this tool keeps everything in mAh and mA. For standby devices, remember self-discharge: over months, a battery loses charge sitting idle, so a runtime counted in months is optimistic even before the load draws anything. Finally, enter the average load, peaks included; a device measured only at idle will always outlast its estimate on paper and disappoint in the hand.
FAQ
How long will a 2,000 mAh battery last at 100 mA?
About 14.00 hours realistically (20 ideal).
What is the derating factor?
The fraction of label capacity actually usable — 0.7 is realistic, 1.0 is ideal.
Why not the full 20 hours?
Temperature, age, discharge rate and cut-off voltage all reduce usable capacity.
mAh vs Wh?
Wh = mAh × V ÷ 1,000 — watt-hours compare batteries of different voltages.
Does cold weather matter?
Yes — cold can cut usable capacity significantly; keep batteries warm.
Can I use this for a battery pack?
Yes — work out the pack’s total capacity in mAh first, then use that figure with your load current. If the pack’s voltage differs from the one the mAh rating assumes, compare energy in watt-hours instead: Wh = mAh × V ÷ 1,000, since mAh alone only compares batteries fairly at the same voltage.
Why does the tool show days as well as hours?
For long, slow drains the hours become hard to picture, so once the runtime reaches a day the tool adds the days figure alongside the hours. It is the same estimate in two units — handy for standby devices expected to run for months, where self-discharge starts to matter as well.