Free battery life calculator. Enter capacity (mAh) and average current draw (mA) to estimate runtime in hours, minutes and days of real use.
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Battery runtime is the capacity divided by the average current draw: a 3000 mAh battery feeding a 500 mA load runs about 6 hours. Dividing runtime hours by the hours you use the device each day gives the days of real-world use; multiplying by 60 gives minutes. This is an ideal estimate - cold weather, battery age, high discharge rates and voltage-converter losses shorten actual runtime.
The whole calculation comes down to one division. A battery's capacity in mAh tells you how much charge it stores, and your device's current draw in mA tells you how fast that charge leaves. Divide the first by the second and you get runtime in hours:
Runtime (hours) = battery capacity (mAh) / average current draw (mA)
Why does this work? One milliamp-hour is exactly the charge delivered by one milliamp flowing for one hour, so the units cancel cleanly into hours. A 3000 mAh battery feeding a 500 mA load runs 3000 / 500 = 6 hours. Double the capacity or halve the current, and the runtime doubles.
Worked example - Arduino project: a 2000 mAh pack powering a board that draws 150 mA gives 2000 / 150 = 13.3 hours of continuous run time. Worked example - LED flashlight: a 2500 mAh lithium cell driving a 1000 mA LED gives 2500 / 1000 = 2.5 hours, or 150 minutes. This is the ideal figure; real-world losses (below) shorten it.
Current draw - measured in milliamps (mA) - is the single most important input, because runtime is inversely proportional to it. Double the current and the battery life halves. But be careful to use the average current, not the peak printed on the label.
Many devices draw current in bursts: a Wi-Fi module may pull 200 mA while transmitting and 0.01 mA while asleep. What matters is the time-weighted average over a full cycle, which you can work out as (active current × active time + sleep current × sleep time) / total time. A sensor that draws 80 mA for 5 seconds and 0.2 mA for 55 seconds averages only (80 × 5 + 0.2 × 55) / 60 = 6.85 mA.
How to find your device's current draw: check the datasheet under supply current or power consumption; measure it with a USB power meter or bench supply; or derive it from wattage - current (mA) = watts × 1000 / volts. Typical ranges: fitness trackers 5-30 mA, smartwatches 15-80 mA, smartphones 150-400 mA with the screen on, tablets 100-500 mA.
Hours of runtime is only half the story - what you really want to know is how many days a charge will last you. That depends on your usage pattern: enter how many hours per day the device actually runs, and the calculator converts hours into days of real use.
Days of use = runtime hours / hours used per day
Example: a 3000 mAh battery with a 500 mA draw gives 6 runtime hours. Used 2 hours a day, that is 6 / 2 = 3 days per charge. Used 8 hours a day, it is only 0.75 days - you would need a bigger battery or a recharge.
If you leave daily usage at zero, the calculator assumes the device runs continuously and reports calendar days (runtime hours / 24). Minutes are always hours × 60, handy for short-burst devices like camera flashes or RC toys.
The ideal formula assumes a perfect battery in perfect conditions. Several real factors shorten battery life, and knowing them keeps your expectations honest - and explains why a battery can drain so fast in the real world.
Temperature: cold slows the chemical reactions inside cells. A lithium cell that delivers full capacity at room temperature may deliver only 70% at 0 C, and cold weather can cut runtime by 30-50% in extreme cases.
Discharge rate: very high current draws reduce the effective capacity of the cell, especially in lead-acid batteries. A battery rated at a slow discharge will deliver less at a 2C draw.
Battery age: capacity fades with use. Lithium cells typically lose roughly 20% of their rated capacity after about 500 full charge cycles.
Usable capacity: device cut-off voltages and voltage-converter losses (boost/buck regulators, USB power banks) mean you rarely get 100% of the nameplate rating. Engineers often plan on 70-90% usable capacity; this calculator shows the ideal figure so you can apply your own margin.
Battery chemistry: lithium-ion, NiMH, alkaline and lead-acid all have different discharge curves and self-discharge rates.
Three worked examples show the formula in action across typical devices.
Smartphone: 4000 mAh battery, 800 mA average draw = 4000 / 800 = 5 hours of continuous active use. At 4 hours of screen time per day, that is about 1.25 days per charge.
Power bank check: a 20,000 mAh power bank at 3.7 V stores 20,000 / 1000 × 3.7 = 74 Wh of energy. Charging a 10 W device gives 74 / 10 = 7.4 hours of charge time. Remember to compare power banks in watt-hours, not mAh, when their voltages differ.
IoT sensor: a 2400 mAh cell feeding a sensor that averages 6.85 mA (bursty draw averaged over a minute) gives 2400 / 6.85 = 350.4 hours - about 14.6 days. Tiny sleep currents are what make coin-cell sensors run for months.
Every example above is the ideal figure; apply a 0.8 multiplier for a conservative real-world estimate.
Using the battery life calculator takes seconds. Enter the battery capacity in milliamp-hours (mAh) - the number printed on the battery or its datasheet, from 1 up to 100,000. Enter the average current draw in mA (0.1 to 50,000); for pulsing devices, average it over one full cycle as explained above. Finally enter how many hours per day the device runs, up to 24.
You get three results: runtime in hours, runtime in minutes, and runtime in days of your actual usage pattern. A quick sanity check before you trust the answer: a 2000 mAh battery at 100 mA should give about 20 hours. If your result is far from that ratio, double-check that you entered milliamps and not amps - entering 0.25 when you mean 250 mA is the most common input mistake.
Common input mistakes to avoid: using peak current instead of average current, confusing mAh with Wh (watt-hours include voltage and are not directly comparable), and assuming 100% of the rated capacity is usable.
A few practical tips help you stretch every charge. Reduce the current draw: dim screens, disable unused radios like Wi-Fi and Bluetooth, and let microcontrollers sleep between readings - sleep modes are the biggest lever for IoT devices. Use efficient voltage converters: cheap boost/buck modules waste energy as heat.
Choose the right battery size for the job instead of oversizing; a right-sized cell plus a 20-30% design margin beats a heavy, expensive pack. Keep batteries at moderate temperatures - cold weather shortens runtime and heat accelerates aging - and store spares at partial charge in a cool place.
Finally, when comparing batteries, compare energy in watt-hours (mAh × volts / 1000) rather than mAh alone: a 5000 mAh 12 V battery holds far more energy than a 5000 mAh 3.7 V phone cell.
Runtime in hours = battery capacity (mAh) / average current draw (mA). A 3000 mAh battery feeding a 500 mA load gives 3000 / 500 = 6 hours. Divide the hours by the hours you use the device per day to get days of use. This calculator does all three steps for you.
mAh stands for milliamp-hours. It measures how much charge a battery stores: a 1000 mAh battery can supply 1000 mA for one hour, 500 mA for two hours, or 100 mA for ten hours. Dividing the mAh rating by your device's current draw gives the runtime in hours.
Check the device or module datasheet under supply current or power consumption. You can also measure directly with a USB power meter or a bench power supply with a current readout. For a device rated in watts, convert with: current (mA) = watts × 1000 / volts.
Real-world losses shorten the ideal figure: cold weather slows the cell chemistry, old batteries hold less charge, high current draws reduce effective capacity, and voltage converters waste energy as heat. Engineers typically plan on 70-90% of the nameplate capacity being usable.
Yes. Cold weather slows the chemical reactions inside the battery, so a lithium cell can deliver only 70% of its rated capacity near freezing. Heat, in turn, speeds up self-discharge and long-term degradation.
Reduce the average current draw - dim the screen, disable unused radios, and use sleep modes on electronics. Choose efficient voltage converters, keep the battery at moderate temperature, and add a 20-30% capacity margin instead of sizing exactly to the ideal runtime.
mAh (milliamp-hours) measures charge; Wh (watt-hours) measures energy and includes voltage: Wh = mAh × volts / 1000. Two batteries with the same mAh but different voltages store different amounts of energy, so always compare batteries in watt-hours when voltages differ.
Yes. Most devices draw current in bursts - for example, a Wi-Fi module pulling 200 mA while transmitting and almost nothing while asleep. Runtime depends on the time-weighted average current over a full cycle, not the peak value.