If you’ve ever squinted at a 10,000mAh label and thought, “Does that mean two full charges? or does it mean one and a half?” you’re not alone. Knowing how many times a portable charger (aka power bank) will charge your phone requires more than estimating with the eyeball test. There are capacity ratings, conversion losses, device batteries and plain real-world considerations that put the kibosh on promised performance. I have evaluated dozens and dozens of portable chargers over the years. Here’s a straightforward, no-nonsense guide to help you choose the best power bank for your essential needs.
How power bank capacity is calculated and why the rating on the label is not accurate
Manufacturers note capacity in milliamp-hours (mAh), but that figure applies to the battery cells contained within the power bank at their nominal voltage level of roughly 3.7 volts. Phone charging occurs at a higher USB voltage (5 volts usually or over USB-C PD with variable voltages), and energy is lost converting the voltage. Throw in cable resistance, heat and the electronics of the power bank itself, and you have less purely usable energy.
Many pros consider a rough rule of thumb that involves taking the labeled capacity and multiplying by an efficiency factor (usually 60 to 90 percent) to determine the usable capacity. A realistic compromise figure is around 70 percent efficiency for well-built banks.
The fast formula (and why it works)
To estimate full charges:
Multiply the power bank mAh by an efficiency factor (let’s use 0.7 for reference).
Divide that result by your phone’s battery capacity in mAh.
Written as an equation:
Predicted full charges = (PowerBank_mAh × 0.7) ÷ PhoneBattery_mAh
This handles voltage conversion and normal losses without getting into technical units.
Examples you can trust (digit-by-digit)
If your phone has a 3,000mAh battery and you have a 10,000mAh power bank:
10000 × 0.7 = 7000 mAh usable.
7000 ÷ 3000 = 2.333… → approximately two and a third full charges.
You have a 4,000mAh battery phone and you own a 20,000mAh power bank:
20000 × 0.7 = 14000 mAh usable.
14000 ÷ 4000 = 3.5 → approximately three and a half full charges.
If your phone has a 4,000mAh battery and you have a 5,000mAh power bank:
5000 × 0.7 = 3500 mAh usable.
3500 ÷ 4000 = 0.875 → roughly about 87.5 percent of a full charge.
Why the real-world numbers are often lower than estimates
Efficiency decreases with both age and temperature. Older power banks and aged phone batteries can retain less charge. Fast charging modes (Quick Charge, Power Delivery) may be slightly more efficient by incurring shorter time and less heating, but they also load chargers and sometimes activate protective throttling which caps the final delivered energy. If you charge and use your phone (GPS, streaming), know that the phone uses power while it is being charged, so you will probably get fewer full charges than the static estimate.
Watt-hours: a more transparent number for heavy flyers and safety-minded buyers
Even more accurately, capacity is tracked in watt-hours (Wh). To convert mAh to Wh: Wh = (mAh × Voltage) ÷ 1000. For instance, a 10,000mAh bank at 3.7V produces 37Wh. Airlines usually restrict the capacity of power banks in carry-on to 100Wh, so a 20,000mAh (≈74Wh) bank is normally okay but anything above 100Wh requires the airline’s advance approval.
How to pick the perfect portable charger for you
Look to 20,000mAh or greater if you want several full top-ups during a single trip. If light and pocketable is what you need, 5,000 to 10,000mAh may just get you through a day. Opt for power banks with USB-C Power Delivery if you have relatively new phones and laptops. PD transfers higher wattage than normal, meaning it charges devices quicker. Seek out banks with real-world tests or user reviews rather than relying on specs alone.
My quick personal test trick
When I’m shopping, I look to see how many full cycles reviewers actually got with the same phone model that I have. In field testing I’ve conducted, trustworthy 10,000mAh banks tend to provide between two and 2.5 full charges to a 3,000mAh phone; budget packs often fail to hit the two charge mark. That’s a practical reality that cuts through the marketing.
Conclusion
That labeled mAh on a power bank is an approximation, not a guarantee. Estimate full charges with a simple efficiency-adjusted formula: multiply the bank’s mAh by about 0.7, then divide that product by your phone’s mAh. Keep age, charging habits and temperature in mind whenever setting expectations. Choose the size and features that work for you, whether that is a pocket-friendly 5,000mAh power bank or a 20,000mAh travel unit.
FAQs
What does mAh tell me about a power bank’s output?
mAh indicates the interior cell capacity at nominal cell voltage (3.7V). This is not directly how many times your phone will be able to charge, due to voltage conversion and losses. For realistic estimates you should use the efficiency-adjusted formula in this article.
Bigger power bank equals better, right?
Higher capacity results in more charges but also greater weight and longer recharge time. Decide based on how many full charges you want versus how portable you need it to be.
Do power banks charge only mobiles or can they also charge laptops?
There are USB-C PD power banks that can supply enough wattage to charge laptops. Verify the bank’s max output (for example 45W or 60W) and your laptop’s required wattage before assuming compatibility.
How long does it take for a power bank to charge?
Charge time varies depending on the input rating of your bank and the charger you have. A 20,000mAh bank with only 18W input will take a lot longer to top up than one that charges at 45W. More input watts and a compatible charger equal faster refilling.
Are there any safety or airline guidelines to be aware of?
Airlines generally allow power banks only in cabin baggage and up to 100Wh without approval. Opt for banks with overcharge, short-circuit and temperature protection, and steer clear of dirt-cheap no-name units.