How to Convert Battery Capacity Between mAh, Wh, and Ah - Complete Guide with Formula & Examples

Battery capacity conversion is the process of translating battery specifications between different units of measurement. The three most common units are milliampere-hours (mAh), watt-hours (Wh), and ampere-hours (Ah). Understanding these conversions is essential when comparing batteries across devices like smartphones, laptops, power banks, and electric vehicles.

Manufacturers use different units depending on the device type. Smartphone batteries are typically rated in mAh, while laptop and aviation batteries often use Wh. This inconsistency makes it difficult to compare battery life or determine compatibility. For example, a 10,000mAh power bank and a 37Wh laptop battery can't be directly compared without conversion.

Battery capacity conversion is critical for travelers (airlines limit power banks to 100Wh), engineers designing power systems, and consumers comparing devices. Knowing how to convert between units helps you make informed decisions about battery purchases, understand actual energy storage, and ensure compliance with transportation regulations.

The key to converting between mAh and Wh is understanding that Wh = (mAh × Voltage) ÷ 1000. This formula accounts for voltage, which is the missing piece when comparing mAh values alone. A 3,700mAh battery at 3.7V stores 13.69Wh, while the same 3,700mAh at 12V stores 44.4Wh—over three times more energy.

Core Formulas:

• mAh to Wh: Wh = (mAh × V) ÷ 1000
• Wh to mAh: mAh = (Wh × 1000) ÷ V
• mAh to Ah: Ah = mAh ÷ 1000
• Ah to mAh: mAh = Ah × 1000

Voltage is critical: Without knowing the battery voltage, you cannot accurately convert between mAh and Wh. Common voltages include 3.7V for lithium-ion smartphone cells, 11.4V-12.6V for laptop batteries, and 3.2V for LiFePO4 cells.

Example 1: Smartphone Battery
A typical smartphone battery is rated at 4,000mAh with a nominal voltage of 3.7V. To find its energy capacity in Wh:
Wh = (4,000 × 3.7) ÷ 1000 = 14.8Wh
This means the battery stores 14.8 watt-hours of energy.

Example 2: Airline Travel Compliance
You have a power bank labeled 20,000mAh at 3.7V. Airlines allow carry-on power banks up to 100Wh. Is yours compliant?
Wh = (20,000 × 3.7) ÷ 1000 = 74Wh
Yes, 74Wh is under the 100Wh limit, so it's allowed in carry-on luggage.

Example 3: Laptop Battery Comparison
Two laptops advertise different battery specs: Laptop A has a 60Wh battery, while Laptop B has a 5,000mAh battery at 11.4V. Which has more capacity?
Laptop B: Wh = (5,000 × 11.4) ÷ 1000 = 57Wh
Laptop A's 60Wh is greater than Laptop B's 57Wh, despite the seemingly higher mAh on Laptop B.

1. Ignoring voltage: The most common mistake is assuming mAh alone indicates total energy. A 5,000mAh 12V battery stores 60Wh, while a 5,000mAh 3.7V battery stores only 18.5Wh—over three times less.

2. Confusing nominal vs. maximum voltage: Lithium-ion batteries have a nominal voltage (typically 3.7V) but charge to 4.2V. Using 4.2V instead of 3.7V will overestimate capacity by about 14%.

3. Mixing up mAh and Ah: A 2.4Ah battery equals 2,400mAh, not 240mAh or 24,000mAh. Always multiply or divide by 1,000 correctly.

4. Assuming all power banks are 3.7V: USB output is 5V, but internal cells are 3.7V. When calculating capacity, use the internal cell voltage, not the output voltage.

5. Not accounting for efficiency losses: Real-world conversion efficiency is 85-95%. A 10,000mAh power bank at 3.7V (37Wh) delivers only ~31-35Wh at 5V USB output due to conversion losses.