Battery Capacity - Amp-Hour, Watt-Hour, and C-Rate

A battery capacity calculator turns the three numbers behind every rechargeable pack into the answers you actually need: stored energy in watt-hours, discharge current in amperes, and runtime in hours. Use it when a label lists mAh but the spec sheet asks for Wh.

• • Compare phone and drone packs in the same units: Convert a 5,000 mAh phone battery and a 4,000 mAh drone battery into watt-hours to see which stores more energy.
• • Plan solar or backup runtimes: Enter a 12 V 50 Ah battery and try different C-rates to see how long the pack will power a fridge during an outage.
• • Size a charger or solar regulator: Pick the C-rate that matches your charger and read the discharge current directly to confirm cells stay within the safe charge rate.
• • Audit a battery label: Compare the printed mAh rating against the calculated watt-hours from the manufacturer voltage to spot under-rated third-party packs.

Capacity in amp-hours tells you how long a current can flow, voltage tells you how hard it pushes, and stored energy combines both. E equals voltage times capacity is the standard line Battery University and the IEC use to describe any pack.

Pilots who want to turn the pack capacity into actual minutes in the air can move straight to the drone flight time calculator once they know the amp-hour rating.

The battery capacity calculator is built on E = V x Q, with the C-rate added to derive discharge current and runtime.

• Voltage (V): Nominal battery voltage in volts. Lead-acid car batteries are about 12 V, a single Li-ion cell is about 3.6 V, and NiMH AA cells are 1.2 V.
• Capacity (Q): Battery charge in amp-hours when the unit is set to Ah, or milliamp-hours when the unit is set to mAh. One amp-hour equals 3,600 coulombs.
• C-rate: Charge or discharge rate relative to capacity: 1C empties a full pack in one hour, 0.2C takes five hours, and 5C empties it in twelve minutes.

The form treats the milliamp-hour and amp-hour fields as one input with a unit selector so a 2,500 mAh phone cell is read as 2.5 Ah without you having to divide by 1,000 manually.

According to Omni Calculator, the battery capacity formula is E = V x Q and a 12 V pack storing 26.4 Wh has a capacity of 2.2 Ah

According to Battery University, battery capacity is commonly rated at 1C, meaning a fully charged 1 Ah cell provides 1 A for one hour, while a 0.2C (C/5) discharge corresponds to a five-hour service time

Once you know the capacity, the next question is usually how long the pack will run a real device, and the battery life calculator handles that follow-up from the same inputs.

Four concepts come up every time battery capacity is discussed. Read these once and the rest of the page reads itself.

Manufacturers often label a pack with a C-rate as well, such as 1C continuous or 10C burst. The continuous rating is the discharge current the pack can sustain without overheating, while the burst rating covers short peaks for drone or starter use.

The same C-rate convention is what an electric vehicle range calculator uses to translate battery capacity into driving distance, so reading the EV page after this one connects the abstract C-rate to a real-world outcome.

The form takes four inputs and returns four answers. The flow below covers the most common battery capacity question and a reverse-lookup variant.

1. 1 Enter the nominal voltage: Type the printed voltage in volts. Car lead-acid is usually 12 V, Li-ion is per-cell voltage times cell count, and USB power banks are typically 3.7 V.
2. 2 Enter the amp-hour or milliamp-hour rating: Type the printed capacity and pick the matching unit. Choose Ah for car, leisure, and EV packs; mAh for phones and small cells.
3. 3 Set the C-rate: Enter the discharge rate you plan to use. Overnight discharges are 0.05C to 0.2C, inverter loads are 0.2C to 0.5C, drone or power-tool use climbs to 1C and above.
4. 4 Read the stored energy, discharge current, and runtime: Stored energy is in watt-hours, capacity in mAh is the same value in the smaller unit, discharge current is C-rate times capacity, and runtime is 1 divided by C-rate.
5. 5 Reverse-lookup when you know voltage and watt-hours: If your pack lists watt-hours instead of amp-hours, set capacity to a known cell and adjust the unit so the watt-hour reading matches, then read the implied amp-hour back from the form.

A 12 V 50 Ah leisure battery powering a 10 A camping fridge: enter 12 V, 50 Ah, and 0.2C. The battery capacity calculator returns 600 Wh of stored energy, 10 A discharge current, and 5 hours of runtime.

Once you have the pack capacity in amp-hours, the car battery life calculator takes that rating alongside climate, driving pattern, and battery age to estimate how many years of service remain before the starter battery needs replacement.

Using one battery capacity calculator for capacity, energy, C-rate, and runtime avoids four separate lookups.

• • Compare packs in watt-hours: Phone labels list mAh, car labels list Ah, and EV labels list kWh, but the battery capacity calculator turns all three into watt-hours so you can rank them on stored energy.
• • Skip the manual Ah to Wh math: The form does the E = V x Q multiplication and handles the 1,000x jump between Ah and mAh, so there is no risk of missing a decimal place.
• • Read C-rate discharge current instantly: Enter a C-rate and the form returns the discharge current in amperes, which is the number you need when sizing fuses, wiring, and charge controllers.
• • Estimate runtime without a separate formula: The runtime output is just 1 divided by the C-rate, but having it on the same panel as capacity keeps the answer tied to the chemistry and load you entered.
• • Convert mAh to Ah and back: The unit dropdown lets you enter values in either common unit and shows the result in both, avoiding the divide-by-1,000 step that catches out first-time users.
• • Verify a battery label: Enter the printed voltage and amp-hour rating to read the watt-hour figure, then compare to the box to spot third-party packs that under-rate their cells.

Most battery questions come back to two numbers: how much energy is in the pack and how fast you can pull it out. Using one battery capacity calculator for both is faster than opening a spreadsheet.

For starter batteries the capacity output feeds straight into a group-size lookup, which is exactly what the car battery sizing calculator does once you have the cold cranking amps and amp-hour rating lined up.

The battery capacity calculator assumes an ideal pack. Real batteries deliver less than the label claims.

• • The battery capacity calculator uses the nominal voltage printed on the pack. Real voltage sag during discharge means the watt-hour figure is an approximation, not a true integration of power over time.
• • Capacity labels are quoted at the manufacturer's chosen C-rate, often a slow 0.05C for lead acid and 0.2C or 0.5C for Li-ion. Drawing more current than the rating delivers less than the label.
• • The runtime output assumes a constant current draw, but real devices cycle between active and idle states, so actual on-time is usually shorter than the steady-state estimate.

Treat the calculator output as the upper bound at room temperature and rated C-rate. Derate the calculated capacity by 10 to 20 percent for critical applications.

According to NIST Special Publication 811, 1 ampere-hour equals exactly 3,600 coulombs and the milliampere-hour is one-thousandth of an ampere-hour, the same value the IEC publishes in Electropedia entry IEV 113-02-10