Battery Life Calculator
Runtime from capacity and current draw.
Enter your values
Enter the battery capacity in mAh.
Enter the average draw in mA.
Enter the voltage in V.
Enter the usable capacity in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Runtime
- 13.3 hours
- Runtime in days
- 0.55
- Energy
- 18.5 Wh
- Power draw
- 1.18 W
Quick answer
Battery runtime is capacity divided by average current, reduced by efficiency losses.
Hours = capacity (mAh) ÷ draw (mA) × efficiency
At a glance
| What it does | Runtime from capacity and current draw. |
|---|---|
| Category | Technology |
| Inputs needed | Battery capacity, Average draw, Voltage, Usable capacity |
| Main output | Runtime |
| Formula | Hours = capacity (mAh) ÷ draw (mA) × efficiency |
| Cost | Free — no sign-up, no download |
How to use the Battery Life Calculator
- 1Enter battery capacity in mAh.
- 2Enter average current draw in mA.
- 3Adjust usable capacity for real-world losses.
Inputs explained
- Battery capacity(mAh)
- Enter the battery capacity in mAh.
- Average draw(mA)
- Enter the average draw in mA.
- Voltage(V)
- Enter the voltage in V.
- Usable capacity(%)
- Enter the usable capacity in %.
Worked example
Using the values the calculator loads with:
Inputs
- Battery capacity5000 mAh
- Average draw320 mA
- Voltage3.7 V
- Usable capacity85 %
Results
- Runtime13.3 hours
- Runtime in days0.55
- Energy18.5 Wh
- Power draw1.18 W
Frequently asked questions
Why is real runtime lower?
Cold, ageing cells and peak loads all cut usable capacity.
How do I convert to Wh?
Multiply mAh by voltage and divide by 1000.
What do I need to enter into the Battery Life Calculator?
Just 4 values: battery capacity, average draw, voltage, usable capacity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Battery Life Calculator work out the answer?
Battery runtime is capacity divided by average current, reduced by efficiency losses. It applies the formula Hours = capacity (mAh) ÷ draw (mA) × efficiency and shows the working so you can check each step by hand.
Is the Battery Life Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.