How many phone charges will my powerbank really give me?
You buy a 20,000 mAh powerbank, your phone has 4,000 mAh: sounds like 5 full charges. Reality: 3 to 4. Why? The powerbank loses energy on the way out (heat, cable, stepping voltage up from 3.7 V to 5 V at the USB port, that costs 15-30%, more if conditions are bad).
Manufacturers print the box number: our calculator shows you the real-life number. Type your powerbank capacity, pick your phone from the list (iPhones, Galaxy, Pixel, OnePlus, Xiaomi), see exactly how many top-ups you'll get.
Plus: a realistic mode (old powerbank, cheap cable, charging in cold), a USB-PD mode (modern laptop charging, fewer losses), comparison in Wh (the honest unit) and an airline-limit warning (above 27,000 mAh you cannot bring it on board).
How to use it
- Type powerbank capacity in mAh, read it off the box or the case. Quick 5k / 10k / 20k / 26.8k / 30k buttons cover the popular sizes.
- Pick a phone from the list (20+ models, iPhone 13 through 16 Pro Max, Galaxy S23-S24 Ultra, Pixel 8, OnePlus 12, Xiaomi 14, Redmi Note 13, plus tablets). Capacity fills in for you. You can also type a custom number.
- Pick a mode: Marketing (the box number, the manufacturer's lie), Standard (typical powerbank, room temperature, the most common case), Realistic (old powerbank, cold weather, cheap cable, worst case).
- Read the result: *"About 3.6 charges"*. That's the real number, not the marketing one.
- Wh / watt-hours: three tiles show the honest capacity unit (mAh lies because it ignores voltage). A 10,000 mAh powerbank at 3.7 V = 37 Wh: that's the real capacity.
- Advanced section: manual efficiency slider (50-100%), USB-PD mode (laptop charging, 85% efficiency without stepping voltage up), cell voltage override (3.6 / 3.7 / 3.8 V) and phone voltage (3.85 V default).
- If you type above 27,000 mAh the calculator warns you: this won't fly with you. Airlines block anything above 100 Wh (≈ 27,000 mAh).
When this is useful
The most common uses:
- Buying a new powerbank. The box claims *"charges your iPhone 6 times!"*. You type: 20,000 mAh + iPhone 15. Marketing mode shows 6, realistic mode shows 3-4. Now you know what you're actually getting.
- Planning a trip. 3 days off-grid, your phone dies once a day, two people. That's 6 charges. You check whether a 10,000 powerbank is enough (it's not, 4-5 max), or whether 20,000 is safer (probably yes).
- Choosing between models. A $40 Anker 10,000 Pro vs a $35 Xiaomi 20,000. The first has better efficiency, the second more capacity. You check both in realistic mode and see the concrete difference in number of charges.
- Flying. You check whether your powerbank passes airport security. You type the capacity, the calculator shows Wh (aviation rules are in Wh, not mAh) and warns you if you cross the 100 Wh ceiling.
- Charging a laptop from a powerbank. You have a USB-PD powerbank (Power Delivery, 65W or 100W). Tick PD mode in advanced, efficiency jumps to 85%, because PD outputs directly at the voltage your laptop wants (15-20 V), no stepping up from 3.7 V.
- Sanity-checking the marketing. *"30,000 mAh!"*: the number is technically true, but measured at 3.7 V inside the powerbank. What actually flows into your phone is the equivalent of 20,000-22,000 mAh (after step-up losses). The calculator makes this visible.
- Old powerbank. You've had it 3 years, it charges worse than it used to. You pick realistic mode: the default 65% efficiency accounts for worn cells, tired cables and cold weather.
Related: spec a full DIY pack in the battery pack calculator, size a wall UPS in the UPS runtime calculator, and turn watt-hours into a money figure with the electricity cost calculator.