Power Bank Calculator (mAh to Wh, real charges, flight limit)
Type the number on your power bank, pick what you're charging, and see the honest number of charges, with the flight check alongside.
Sleepover or trip planner: will the bank last the night?
Add everything that needs charging from the bank above, with the charge each one starts at. In list order, the first gadget gets charged first; shared evenly, everyone gets the same share.
Nothing on the list yet. Add a gadget above.
charge going into gadgets lost as heat left in the bank
How it works
Enter the capacity from the label in mAh (or Wh if that's what's printed), and the cell voltage it lists, which is 3.7 V on most banks. Pick a device or type your own battery size. Everything updates as you type, and nothing is sent anywhere.
The calculator turns both batteries into watt-hours (mAh × V ÷ 1000), because mAh at different voltages aren't comparable. A 10,000 mAh bank at 3.7 V holds 37 Wh; a typical phone's 4,500 mAh at 3.85 V is about 17 Wh.
Then it applies a real-world efficiency, 65% by default, for the losses in boosting to USB voltage, the phone's own charging circuit and heat. Typical banks land between 60% and 70%; slide it if you've measured your own. Battery health shrinks the device battery (an 80% health phone fills faster), and charge from/to covers top-ups like 20% to 80%.
The flight check uses the common IATA limits: up to 100 Wh in hand luggage, 100–160 Wh with airline approval, over 160 Wh not allowed, and never in checked bags. Airlines can be stricter, so check yours. Charges I want sizes a bank for you, and the charger wattage gives a rough refill time (an estimate; the last part of charging is slower).
The sleepover or trip planner charges several gadgets from the same bank, for example two phones, a tablet and a torch overnight. Each gadget gets its own starting charge and target. In list order fills the first one before moving on, like plugging them in one after another, so put the one that matters most at the top. Share evenly gives every gadget the same fraction, like taking turns. The bar shows where the bank's energy goes: into gadgets, lost as heat, or left over. It also tells you the bank size the whole list needs.
The little orange heat puffs are the conversion losses made visible: roughly one puff per watt-hour that warms the bank, cable and phone instead of charging it. That's why a power bank and a charging phone both feel warm.
Underneath, the sleepover tent shows how everyone ends the night. Gadgets that reach their target are wide awake, part-charged ones yawn, and any gadget the bank couldn't reach falls asleep in a nightcap with little Zzz bubbles. If a torch runs out, the whole tent goes dark, which is the cue to move it up the list or pack a bigger bank.
Every device size in the list is a typical figure, not a measurement of a particular model. For exact numbers, look up your device's battery in Wh. Background: power bank mAh vs Wh, and why batteries give up in the cold. Need the same maths from a script or shop page? Anvil's free power bank API returns it as JSON, trip plans included, with no key.
Questions
How many charges does a 10,000 mAh power bank give?
About 1.4 full charges of a typical 4,500 mAh phone at 65% efficiency, or about 2 for a 3,000 mAh phone.
Why not just divide the mAh numbers?
The bank's mAh is counted at about 3.7 V and the phone's at about 3.85 V, and energy is lost in conversion, so dividing mAh overestimates by roughly half.
Is a 20,000 mAh power bank allowed on a plane?
Usually yes, in hand luggage. At 3.7 V it's 74 Wh, under the common 100 Wh limit. Never in checked bags.
Can one power bank charge two phones and a tablet overnight?
Add them to the planner. A 10,000 mAh bank gives about 24 Wh to devices, which covers one phone from empty and part of a second. Two phones from 20% plus a tablet from 30% need around 49 Wh into the gadgets, so you'd want a bank of about 75 Wh (20,000 mAh or more).
Where does the lost energy go?
Mostly into heat in the bank's voltage converter, the cable and the phone's charging circuit. The planner shows it as orange puffs and a striped part of the bar.
Which gadget runs out first overnight?
In list order, the last gadgets on the list run out first, because the bank fills each one before moving on. The tent picture shows them asleep in nightcaps. Put the one you need in the morning, like a phone alarm or a torch, at the top.
Is anything uploaded?
No. It's plain JavaScript running in your browser, and the share link keeps your numbers in the address bar only.