Seeds
Result
Show the working
HMAC-SHA256(server seed, "client seed:nonce:round"), read 4 bytes at a time as a float from 0 to 1. This is the scheme most crypto casinos use, but each site publishes its own exact formula: if a result doesn't match, compare with their fairness page before assuming anything is wrong.
A provably fair calculator lets you check a crypto casino’s result yourself instead of taking their word for it. Paste the server seed the casino revealed, the hash they showed you before you played, your client seed and the bet’s nonce. If the hash matches the seed, the casino couldn’t have swapped it. And if the calculator gets the same roll as your bet history, say 53.46 on dice, the result was fixed before you clicked. Everything runs in your browser with the built-in Web Crypto library; nothing is sent anywhere. It’s free.
How the provably fair calculator checks a bet
The usual scheme goes like this. Before you play, the casino picks a secret server seed and shows you its SHA-256 hash. You choose a client seed. Each bet uses a nonce, a counter that goes up by one per bet. The result comes from:
HMAC-SHA256(server seed, "client seed:nonce:round")
Every four bytes of that output become a number between 0 and 1. Dice multiplies it out to 0.00–100.00, roulette to a pocket from 0 to 36, mines shuffles the 25 squares, and so on. When you rotate to a new seed pair, the casino reveals the old server seed and you can recompute every bet you made with it.
If the numbers don’t match
Don’t jump to conclusions. Each casino publishes its own exact formula on its fairness page, and small differences (a different separator, a different way to turn bytes into a number, a house edge applied in a different place) change the result. Check the nonce: some sites start at 0, others at 1. If you’ve matched their published method and still get a different result, that’s worth raising with them.
Provably fair proves the casino didn’t change the outcome after you bet. It says nothing about the house edge, which is still built into the payouts. The RTP and house edge calculator shows what that costs, and the mines calculator gives the real odds and multipliers for mines games.
Crash games use the same kind of provably fair seeds. The crash game simulator shows the real chance of reaching each multiplier and tests auto cash-out targets.
Gamble with money you can afford to lose. If it stops being fun, BeGambleAware.org is free and confidential.