How to Convert Wh to mAh (And Why It Actually Matters)

Your power bank says 20,000mAh. The airline limit is 100Wh. Are you good to fly, or about to have your charger confiscated at security?

If you’ve ever stared at battery specs wondering how these numbers relate to each other, you’re not alone. The conversion between watt-hours (Wh) and milliamp-hours (mAh) trips up a lot of people, but once you understand it, you’ll never be confused by battery specs again.

Key Takeaways

• Watt-hours (Wh) measure total energy and account for voltage, while milliamp-hours (mAh) measure charge capacity and cannot indicate energy without knowing the voltage.
• For lithium-ion power banks, the standard conversion uses the internal cell voltage of 3.7V, not the 5V USB output voltage.
• A 20,000mAh power bank at 3.7V equals 74Wh, which is safely below the 100Wh airline carry-on limit.
• Batteries with the same mAh rating can store very different amounts of energy if their operating voltages differ, making Wh the correct unit for comparison and airline safety rules.
• Real-world usable capacity is lower than the calculated value due to voltage conversion losses and heat, so rated charging output is typically around 60–70% of nominal capacity.

Infographic by UGREEN. Please credit and link to the original article when used. Original article: https://uk.ugreen.com/blogs/power-bank/how-to-convert-wh-to-mah

What’s the Difference Between Wh and mAh?

Wh (watt-hours) measures total energy, or how much work a battery can do over time. Think of it as the size of a fuel tank in terms of actual usable energy.

mAh (milliamp-hours) measures electrical charge capacity, or how much current a battery can deliver over time. It tells you capacity, but not energy, without knowing the voltage.

The key difference: Wh accounts for voltage, mAh doesn’t. Two batteries with identical mAh ratings can store very different amounts of energy if they operate at different voltages.

This is why you can’t directly compare a 10,000mAh power bank to a 10,000mAh laptop battery. The laptop battery operates at a higher voltage (typically 10.8V-14.4V versus 3.7V), so it stores roughly three times more total energy despite the same mAh number.

What is the Wh to mAh Conversion Formula?

The formula is straightforward once you have the voltage:

mAh = (Wh × 1000) ÷ V

Where:

• Wh = watt-hours (energy)
• V = voltage (in volts)
• mAh = milliamp-hours (result)

Example: Convert 100Wh to mAh at 3.7V

mAh = (100 × 1000) ÷ 3.7 = 27,027mAh

This is precisely why the airline limit of 100 Wh translates to approximately 27,000 mAh for standard lithium-ion power banks.

What is the mAh to Wh Conversion Formula?

Going the other direction:

Wh = (mAh × V) ÷ 1000

Example: Convert 20,000mAh to Wh at 3.7V

Wh = (20,000 × 3.7) ÷ 1000 = 74Wh

This confirms that a 20,000 mAh power bank is well under the 100 Wh airline limit—no approval needed.

Which Voltage Should You Use?

This is where most people get stuck. The voltage depends on the battery type and chemistry.

Quick Reference Table:

For power banks specifically: Use 3.7V (the internal cell voltage), not 5V (the USB output voltage). This is the standard used by airlines and manufacturers for capacity calculations.

Why does this matter? Laptop batteries use multiple cells in series—three 3.7V cells create 11.1V, four cells create 14.8V. That’s why laptop battery voltages are higher, and why the same mAh rating stores more energy in a laptop battery than a single-cell power bank.

Wh to mAh Conversion Table (at 3.7V)

For quick reference when dealing with lithium-ion batteries and power banks:

mAh to Wh Conversion Table (at 3.7V)

When You’ll Actually Need This Conversion

1. Checking airline limits

Airlines restrict lithium batteries to 100Wh (no approval needed) or 160Wh (with approval). Most power banks list capacity in mAh, so you need to convert to confirm compliance. A 26,800mAh power bank? That’s 99.16Wh—just under the limit.

2. Comparing batteries with different specs

One power bank lists 20,000mAh, another lists 74Wh. Are they equivalent? At 3.7V, yes—they store the same amount of energy. Without converting, you can’t make a fair comparison.

3. Estimating device runtime

If your device uses 10W continuously and your battery stores 74Wh, you can estimate roughly 7.4 hours of runtime (74 ÷ 10 = 7.4). This is useful for planning how much backup power you actually need.

4. Understanding “rated capacity” vs “nominal capacity”

Some manufacturers list two different mAh numbers. The larger “nominal capacity” is measured at 3.7V (internal cells). The smaller “rated capacity” is measured at 5V (USB output). A 10,000mAh power bank might show a rated capacity of only 6,500-7,400mAh because of this voltage difference, and that’s before accounting for efficiency losses.

Common Mistakes to Avoid

Using the wrong voltage

The most common error. For power banks, always use 3.7V (internal cell voltage), not 5V (USB output), unless the manufacturer specifically states the capacity is rated at 5V. Using 5V will give you a higher mAh number that doesn’t reflect reality.

Comparing mAh across different voltages

A 10,000mAh power bank (3.7V = 37Wh) stores far less energy than a 10,000mAh laptop battery (11.1V = 111Wh). The laptop battery holds three times more energy despite the identical mAh rating. Always convert to Wh when comparing batteries with different voltages.

Forgetting efficiency losses

Even after converting, real-world usable capacity is typically 60-70% of the calculated value due to voltage conversion losses and heat. A 10,000mAh power bank won’t deliver 10,000mAh to your phone—expect closer to 6,000-7,000mAh of actual charging capacity.

Assuming Wh and mAh are interchangeable

They measure different things. Wh is energy (accounts for voltage), mAh is charge capacity (doesn’t account for voltage). You need voltage to convert between them, there’s no shortcut.

FAQs about Wh and mAh

How many mAh is 100Wh?

At 3.7V (standard for lithium-ion power banks): 100Wh = 27,027mAh. This is the airline limit for carry-on batteries without special approval.

Is a 20,000mAh power bank under 100Wh?

Yes. At 3.7V, 20,000mAh = 74Wh, well under the 100Wh limit. You can fly with it in your carry-on without any issues.

Why do some power banks show two different mAh numbers?

The larger number is usually the “nominal capacity” at 3.7V (internal cells). The smaller number is the “rated capacity” at 5V (USB output). Both are technically correct—they’re just measured at different voltages.

Can I convert Wh to mAh without knowing the voltage?

No. Voltage is essential for the conversion. Without it, there’s no way to accurately convert between energy (Wh) and charge capacity (mAh). If a battery only lists Wh and not voltage, check the manufacturer’s specifications.

What voltage should I use for my smartphone battery?

Most smartphone batteries operate at 3.7V to 3.85V. Use 3.7V for a conservative estimate.

How do I find my power bank’s voltage?

Check the label or specifications. Most lithium-ion power banks are 3.7V internally. If it shows “3.6V” or “3.7V” with a capacity in mAh, use that voltage. If it only shows Wh, the manufacturer has already done the conversion for you—this is the most reliable number.

Why do airlines use Wh instead of mAh?

Because Wh measures actual energy, which is what matters for safety. Two batteries with identical mAh but different voltages pose different fire risks. A 10,000mAh laptop battery (111Wh) stores three times more energy than a 10,000mAh power bank (37Wh)—and that energy difference is what airlines need to regulate.

The Bottom Line

Converting between Wh and mAh is simple once you know the voltage. For most power banks, use 3.7V. The key formulas are:

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

Remember that mAh alone doesn’t tell you how much energy a battery stores—you need voltage for the complete picture. When comparing batteries or checking airline compliance, Wh is the more useful measurement because it accounts for both capacity and voltage in a single number.

UGREEN power banks clearly display both mAh and Wh specifications, so you’ll always know exactly what you’re getting—and whether you’re good to fly.