Battery Life Calculator

TL;DR: To calculate battery life, divide the battery capacity in milliampere-hours (mAh) by the device’s average current consumption in milliamperes (mA); the result is the duration in hours, using the formula Battery Life (hours) = Capacity (mAh) ÷ Consumption (mA).

What Is the Battery Life Calculator?

The Battery Life Calculator is a free online tool that estimates how long a battery will last under a specific load. You provide two essential numbers: the total energy stored in the battery (measured in milliampere-hours, or mAh) and the average current your device draws (measured in milliamperes, or mA). The calculator instantly returns the estimated runtime in hours.

This tool is indispensable for hobbyists building custom electronics, engineers validating power budgets, drone pilots estimating flight endurance, and everyday users wondering why their phone or wireless headphones drain so quickly. Real-world context matters: a 3,000 mAh smartphone battery will not last the same amount of time if you are streaming video (drawing ~500 mA) versus leaving it idle in airplane mode (drawing ~50 mA). The calculator removes guesswork by applying a basic physical law.

The principle is straightforward: every electronic device consumes a certain amount of electrical current per hour. The battery supplies that current until its stored charge is depleted. By dividing the total charge (capacity) by the rate of consumption (load), you find the total time. This calculator is designed for DC-powered devices, batteries, and power banks, and it works for any size—from tiny coin cells to large lithium-ion packs.

How to Use the Calculator

Using this tool takes less than five seconds. Follow these numbered steps:

1. Enter the battery capacity: Locate the input field labeled “Battery Capacity (mAh).” Type the total capacity of your battery. This number is usually printed on the battery itself or in the device’s spec sheet. For example, a typical smartphone battery is 4,000 mAh; a AA alkaline battery is about 2,000 mAh.
2. Enter the current consumption: Find the input field labeled “Current Consumption (mA).” Enter the average current your device draws while operating. If you do not know this number, check the power adapter (if it outputs 5V and 2A, that is roughly 2,000 mA) or use a USB power meter to measure actual draw.
3. Press “Calculate”: Click the calculate button or press Enter. The result will appear instantly in the output field labeled “Estimated Battery Life (Hours).”
4. Interpret the result: The number you see is the maximum theoretical runtime in hours under constant, steady load. For example, if the calculator returns “8.0,” your battery will last roughly 8 hours at that exact consumption level.

The calculator is restricted to these two inputs. It does not require voltage, temperature, or discharge curve data, keeping the interface simple and fast. However, for deeper analysis, you will need to manually apply correction factors, which are covered in the Tips section below.

Formula and Calculation Method

The core logic behind this tool is a direct, linear division. The formula is written as:

Battery Life (hours) = Battery Capacity (mAh) ÷ Current Consumption (mA)

In plain language: you are calculating how many hours the available charge can sustain the required current draw. Since capacity is measured in milliampere-hours (a unit of charge) and consumption is measured in milliamperes (a unit of current per second), dividing one by the other cancels the time dimension in the denominator, leaving only hours.

Let us walk through the concrete example shown on the calculator’s description: a battery with 4,000 mAh capacity and a device that consumes 500 mA.

Step 1: Write down the capacity: 4,000 mAh.
Step 2: Write down the consumption: 500 mA.
Step 3: Divide: 4,000 ÷ 500 = 8.
Result: The estimated battery life is 8 hours.

This means that if you connect a 4,000 mAh battery to a load drawing 500 mA continuously, the battery will deliver power for exactly 8 hours before being fully discharged. The logic is identical to a car’s fuel gauge: if a tank holds 10 gallons and the engine burns 2 gallons per hour, you can drive for 5 hours.

The mathematical relationship holds true regardless of battery chemistry—whether it is lithium-ion, nickel-metal hydride, or alkaline. The formula assumes ideal conditions: constant current draw, no internal resistance losses, and no voltage cutoff before the last milliampere-hour is extracted. In reality, most devices shut off before 100% discharge, but the formula provides a solid ceiling estimate for planning purposes.

Practical Examples

The utility of this calculator becomes clear when applied to real-world scenarios. The table below shows three realistic situations, the resulting calculation, and what the number means in practice.

Notice how the third row differs: the sensor node does not draw 40 mA continuously—it spikes to 200 mA for a second, then sleeps at 5 mA. The average is 40 mA, which the calculator handles correctly. This is the correct way to use the tool: input the average, not the peak.

Another useful scenario involves a power bank. If a 10,000 mAh power bank charges a phone with a 3,000 mAh battery, the calculator (using 10,000 mAh capacity and 1,500 mA phone charging draw) outputs 6.6 hours. In practice, you get about 3 full phone charges because charging efficiency is ~85%, not 100%. The calculator gives you the raw number; your job is to apply the efficiency multiplier.

Tips for Accurate Results

To get the most reliable estimate from this calculator, you must understand its limitations and avoid common mistakes. The two biggest errors are ignoring variable consumption and ignoring battery degradation.

• Use average, not peak consumption: Most electronics do not draw constant current. A Wi-Fi module bursts at 300 mA but idles at 20 mA. To use the calculator correctly, you must determine the average over a full usage cycle. Multiply the current by the duty cycle. For example, if a device draws 200 mA for 25% of the time and 50 mA for 75% of the time, the true average is (200 × 0.25) + (50 × 0.75) = 87.5 mA. Use 87.5 mA in the calculator, not 200 mA.
• Account for battery degradation: A battery’s rated mAh is valid only when it is brand new. After 300–500 full charge cycles, a lithium-ion battery typically retains only 80% of its original capacity. If your phone battery was 4,000 mAh new but is two years old, use 3,200 mAh as the input for a more honest estimate.
• Never mix up mAh and Ah: The calculator expects milliampere-hours and milliamperes. If your datasheet says “4 Ah,” you must enter 4,000 mAh. If your device draws “0.5 A,” enter 500 mA. Forgetting to convert by a factor of 1,000 will give you a result that is off by a thousandfold.
• Understand the unit gotcha with voltage: The mAh unit does not include voltage. Two batteries can both be 2,000 mAh, but one is 1.5V (AA) and the other is 3.7V (li-ion). The watt-hour (Wh) energy is different, but the calculator ignores this. As long as you are comparing the same nominal voltage (e.g., two phones with 3.7V batteries), the mAh division is valid.
• Subtract the cutoff voltage: Most devices will stop working when the battery reaches a cutoff voltage, typically 3.0V for li-ion, even though there is still 5–10% charge remaining. The calculator does not model this. Reduce the capacity input by 10% for a more conservative, real-world estimate.
• Temperature matters: Cold temperatures increase internal resistance and reduce usable capacity. At 0°C, a li-ion battery may only deliver 70% of its rated mAh. If you plan to use the device outdoors in winter, multiply the capacity by 0.7 before entering it.

By adjusting your inputs with these correction factors, you transform the calculator from a theoretical tool into a practical engineering aid. The results will match field measurements within 5–10% instead of the 30–50% error common from raw formula misuse.

Frequently Asked Questions

How do I measure the current consumption of my device?

The most reliable method is to use a digital multimeter placed in series with the battery. Set the meter to measure milliamps (mA) and break the positive wire between the battery and device. The meter will show the instantaneous draw. For variable loads, take measurements every 30 seconds over a 10-minute normal-use period and calculate the average. Alternatively, use a USB power meter that plugs between the charger and your device; it will log voltage, current, and cumulative charge in mAh. From that, divide the total mAh consumed by the total hours tested to get your average current. Many hobbyists do not own these tools, so an easier alternative is to check the device’s documentation: most datasheets list “typical operating current” and “peak current.” Use the typical figure for the calculator, not the peak.

What is the difference between mAh and Wh, and which should I use?

mAh (milliampere-hour) is a unit of electrical charge, while Wh (watt-hour) is a unit of energy. The relationship is: Wh = (mAh × Voltage) ÷ 1,000. For this calculator, you must use mAh because the tool divides capacity by current draw in mA. If you only know the Wh rating of your battery, convert it to mAh first using this formula: mAh = (Wh × 1,000) ÷ Voltage. For example, a 12Wh battery at 3.7V has a capacity of (12 × 1,000) ÷ 3.7 ≈ 3,243 mAh. Use 3,243 mAh in the input. The mAh unit is preferred because it directly relates to current draw, which is how circuits actually consume power. Wh is better for comparing batteries of different voltages, but it will not work directly with this calculator without the conversion step.

Why does my actual battery life always seem shorter than the calculator result?

This is the most common frustration. The gap between the formula and reality stems from four factors. First, the constant current assumption: the calculator assumes your device draws exactly the current you input for every single second. Real devices fluctuate; during peak processing, a phone might draw 1,500 mA, which drains the battery faster than your average 400 mA input suggests. Second, battery internal resistance: as the battery discharges, its voltage sags, and the device compensates by drawing more current to maintain the same power (Watts). This extra draw shortens life. Third, the cutoff voltage: your device shuts off while there is still 5–15% charge left because the voltage falls below the minimum operating threshold. The calculator counts 100% of the mAh; your device only uses 85–95% of it. Fourth, temperature and age: heat accelerates self-discharge, and aging reduces effective capacity. To reconcile the numbers, apply the correction factors from the Tips section above: reduce capacity by 10% for cutoff, another 10–20% for age, and use average current rather than rated current. After these adjustments, calculated values typically match real-world runtimes within a few percent.