Battery Capacity Converter

4 min read 3 steps 6 terms 3 examples 6 FAQs Wh = mAh × V ÷ 1,000

Battery specs get confusing fast because manufacturers mix units that measure different things: milliamp-hours and amp-hours measure electric charge, while watt-hours measure energy.

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Walk-through

How to Use This Calculator

3 steps

Pick a tab

Choose mAh ↔ Wh to convert capacity to energy, Ah for the amp-hour/milliamp-hour conversion, or Run Time to estimate hours of use under a load.

Enter capacity and voltage

Type the battery's capacity (in mAh, Wh, or Ah depending on the tab) and, where the tab needs it, the pack voltage. The result updates instantly as you type — no button to click.

Read the converted result

The result card shows the converted value along with a plain-language sentence explaining the conversion, so you can double-check it matches what's printed on the battery or spec sheet.

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Reference

Formula & Methodology

4 formulas

Watt-hours from milliamp-hours

Wh = mAh × V ÷ 1,000

Multiply the milliamp-hour rating by the pack voltage, then divide by 1,000 to convert milliamps to amps. For example, 5,000 mAh at 3.7 V = 18.5 Wh.

Milliamp-hours from watt-hours

mAh = Wh × 1,000 ÷ V

The reverse conversion: multiply watt-hours by 1,000 and divide by the voltage to get milliamp-hours.

Amp-hours from milliamp-hours

Ah = mAh ÷ 1,000

An amp-hour is simply 1,000 milliamp-hours — no voltage needed for this conversion.

Run time from load

Run time (h) = Wh ÷ Load (W)

Divide the battery's total energy in watt-hours by the constant power draw of the device to estimate hours of use.

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Glossary

Key Terms Explained

6 terms

Milliamp-hour (mAh) A unit of electric charge equal to one-thousandth of an amp-hour. It's the most common capacity rating for phone, laptop, and power-bank batteries, but by itself it doesn't tell you how much energy the battery stores — that also depends on voltage.

Watt-hour (Wh) A unit of energy equal to one watt of power sustained for one hour. Unlike mAh, watt-hours account for voltage, so it's the only unit that lets you fairly compare the energy capacity of batteries with different voltages — and it's the figure airlines and shipping regulations use.

Amp-hour (Ah) A unit of electric charge equal to 1,000 milliamp-hours. Car batteries, marine batteries, and larger battery packs are typically rated in Ah rather than mAh because the numbers would otherwise be unwieldy.

Voltage (V) The electrical potential a battery cell or pack delivers. A single lithium-ion cell is typically 3.6–3.7 V nominal, while packs built from multiple cells in series have higher voltages (e.g. a 4-cell pack is roughly 14.8 V). Voltage is required to convert between charge (mAh, Ah) and energy (Wh).

Capacity How much charge or energy a battery can store when fully charged, expressed in mAh/Ah (charge) or Wh (energy). Capacity degrades gradually over hundreds of charge cycles, so a battery's real-world capacity is usually lower than its rated, factory-fresh capacity.

C-rate A way of expressing charge or discharge current relative to a battery's capacity. A 1C rate for a 5,000 mAh battery means a 5,000 mA (5 A) current; a 2C rate would be 10 A. Higher C-rates charge or discharge faster but generate more heat and wear.

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Scenarios

Real-World Examples

3 worked examples

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Phone power bank

mAh to Wh

Capacity 5,000 mAh Voltage 3.7 V

5,000 mAh at 3.7 V = 18.5 Wh — a typical single-cell phone power bank. Since it's under the 100 Wh airline carry-on limit, it can fly without special approval.

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Laptop battery

Wh to mAh

Energy 60 Wh Voltage 11.1 V

A 60 Wh laptop battery at 11.1 V (a common 3-cell pack voltage) works out to about 5,405 mAh — the reverse conversion, useful when a spec sheet only lists Wh.

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LED flashlight

Run time from load

Capacity 3,000 mAh Voltage 3.7 V Load 1 W

3,000 mAh at 3.7 V stores 11.1 Wh. At a steady 1 W draw, that's an estimated 11.1 hours of run time — an upper bound, since real devices rarely draw a perfectly constant load.

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Deep Dive

Understanding Battery Capacity: mAh, Ah, and Wh

4 chapters · 4 min read

Battery specs get confusing fast because manufacturers mix units that measure different things: milliamp-hours and amp-hours measure electric charge, while watt-hours measure energy. This article explains how the three relate, why voltage is the missing piece that ties them together, and how to use each unit for the job it's actually good at — comparing phone batteries, sizing a power bank, or checking an airline's carry-on battery limit.

Charge vs. energy: why mAh alone isn't enough

A milliamp-hour (mAh) rating tells you how much electric charge a battery can deliver, but charge isn't energy. Two batteries can both be rated 5,000 mAh and still store very different amounts of usable energy if their voltages differ — a 5,000 mAh cell at 3.7 V stores 18.5 Wh, while a 5,000 mAh pack at 7.4 V (two cells in series) stores twice as much, 37 Wh, for the same mAh number. This is why comparing two devices purely on their mAh rating can be misleading unless you also know the voltage, and it's why watt-hours (Wh) — which already bake voltage into the number — are the more reliable way to compare energy capacity across different battery chemistries and configurations.

Converting between mAh, Ah, and Wh

The conversion between mAh and Wh always runs through voltage: Wh = mAh × V ÷ 1,000. The division by 1,000 exists purely to convert milliamps to amps — if you already have the capacity in amp-hours, the formula simplifies to Wh = Ah × V. Converting between mAh and Ah is even simpler and needs no voltage at all: 1 Ah is defined as exactly 1,000 mAh. This calculator's three tabs mirror those three conversions directly: mAh ↔ Wh (needs voltage), Ah ↔ mAh (no voltage needed), and a run-time estimate that chains the mAh → Wh conversion with a load in watts.

Estimating run time from a load

Once you know a battery's energy in watt-hours, dividing by the device's power draw in watts gives you an estimate of run time in hours: hours = Wh ÷ W. A 20 Wh battery powering a 4 W device should last roughly 5 hours. This is necessarily an estimate rather than a guarantee — it assumes the load stays constant and that the battery delivers 100% of its rated capacity, neither of which holds exactly in practice. Real devices draw variable power (a screen dimming, a radio going idle), and any battery loses some usable capacity to internal resistance, temperature effects, and simple aging over its charge-cycle life. Treat the run-time figure as a useful upper-bound planning number, not a lab-verified spec.

Why watt-hours matter for shipping and travel

Most airlines and shipping carriers regulate lithium battery packs by energy content in watt-hours, not by mAh, because Wh is voltage-independent and directly relates to fire risk. The common rule of thumb is that spare lithium-ion batteries under 100 Wh can be carried on without airline approval, batteries between 100 and 160 Wh typically need airline permission, and anything above 160 Wh is usually barred entirely from passenger aircraft. Because product labels often print only mAh and voltage (or omit Wh altogether), converting mAh × V ÷ 1,000 is frequently the only way travelers can check whether a power bank or spare camera/drone battery falls under the 100 Wh threshold before a trip.

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Questions

Frequently Asked Questions

6 questions

How do I convert mAh to Wh?+

Multiply the milliamp-hour rating by the pack voltage, then divide by 1,000. For example, 5,000 mAh at 3.7 V = 18.5 Wh.

Why do I need the voltage to convert mAh to Wh?+

mAh only measures electric charge, not energy. Energy (Wh) depends on both the charge and the voltage the cell delivers it at, so two batteries with the same mAh rating can store very different amounts of energy if their voltages differ.

What does the Wh rating on a power bank mean?+

Watt-hours is the actual energy the power bank can store and is the figure airlines and shipping regulations use, since it's voltage-independent. A power bank listing both mAh and Wh lets you compare capacity fairly against another device rated only in Wh.

What is the difference between Ah and mAh?+

An amp-hour (Ah) is 1,000 milliamp-hours (mAh). Car and marine batteries are usually rated in Ah, while phone and laptop batteries use the smaller mAh unit. No voltage is needed to convert between the two.

How accurate is the run time estimate?+

The run time estimate assumes a constant load and 100% usable capacity, so it's an upper bound. Real-world run time is usually a bit shorter due to conversion losses, battery aging, and loads that aren't perfectly constant.

Why do airlines limit batteries to 100 Wh?+

Most airlines cap carry-on lithium battery packs at 100 Wh without special approval (with some allowing up to 160 Wh with airline permission) because higher-energy cells pose a greater fire risk. Converting your battery's mAh and voltage to Wh is the only way to check if it's under that limit.

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