UTXO Puzzle: Bitcoin coin selection game
Pay exactly with the coins in your wallet and waste the least.
How to play
- Three rounds a day, the same for everyone: low fees, high fees and a tricky one. Each has a wallet of 8–12 coins, a payment and a fee rate in sat/vB.
- Tap the coins you want to spend. Size = 10.5 vB overhead + 68 vB per P2WPKH input + 57.5 vB per P2TR input + 31 vB for the payment + 31 vB for change (if any).
- Fee = size × fee rate, rounded up. Change = inputs − payment − fee.
- Change under 546 sats is dust: the transaction is rebuilt without a change output and the leftover goes to the miner.
- Cost = fee + anything left to the miner. You submit once per round; the preview shows exactly what the server will compute.
- Round score = 1,000 × cheapest possible cost ÷ your cost. Total 0–3,000. Season points: 2,900+ → 3, 2,600+ → 2, 2,000+ → 1. Ties are broken by time.
About this game
A Bitcoin wallet does not hold a balance; it holds coins, called UTXOs (unspent transaction outputs). To pay someone, your wallet picks some of those coins as inputs, sends the payment and returns the rest to you as change. Every input and output takes space, and space is what you pay fees for.
Each day brings three rounds: one with low fees, one with high fees and one with a twist. Pick the coins, watch the transaction build itself live with its size, fee and change, and try to pay as little as possible. Afterwards you see the cheapest possible transaction and why it wins.
Is the UTXO Puzzle free?
Yes. Like every FreeBTC game it costs nothing to play. Good results add season points, and season leaders share pots funded by the site’s ad revenue.
What is a UTXO?
An unspent transaction output: a coin your wallet can spend. Your balance is just the sum of your UTXOs, and a payment always spends whole coins, returning any excess as change.
Why does spending more coins cost more?
Fees are paid per virtual byte, and every input adds about 57.5–68 vB. At 100 sat/vB one extra input costs several thousand sats, which is why wallets consolidate small coins when fees are low.
Why 546 sats?
It is the classic dust limit for legacy outputs. Real limits depend on the output type (294 sats for P2WPKH, 330 for P2TR at default settings), but 546 is a common safe floor, and it keeps the puzzle simple.