Block Builder: Build a Bitcoin block with real transactions
Be the miner: fill a block with real transactions and maximise the fees.
How to play
- Every day everyone gets the same 16–20 transactions, taken from a real recent Bitcoin block (each one links to mempool.space).
- Each transaction shows its fee in sats, its size in vB (virtual bytes) and its fee rate in sat/vB. Sort the list however you like.
- Your block has a fixed capacity of about 40–50% of the transactions’ total size, so not everything fits. Tap a transaction to add or remove it.
- When you are happy, press “Mine this block”. You submit once; a block over capacity is rejected and you can fix it.
- Score = your fees ÷ the best possible fees, computed exactly. 100% is only for a truly optimal block.
- Season points: 99% or more → 3, 95% or more → 2, 85% or more → 1. Ties are broken by time.
About this game
Every block has limited space and far more transactions want in than fit. Miners choose, and they choose the ones that pay best. In Block Builder you make that choice: today’s transactions come from a real recent block, with their real fees and sizes, and your block only has room for about half of them.
The trick is the one miners live by: what matters is the fee per virtual byte, not the total fee. Then comes the last stretch, where the leftover space turns into a small knapsack puzzle. When you finish you see the optimal block and how many sats you left on the table.
Is Block Builder 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.
Are the transactions real?
Yes. They come from a real block mined recently, fetched from mempool.space, and every txid links to it. If mempool.space cannot be reached, a realistic simulated set is used that day and clearly marked as such.
Why does the fee rate matter more than the total fee?
Because space is what a miner is short of. A transaction paying 20,000 sats for 2,000 vB earns 10 sat/vB; ten small ones paying 30 sat/vB in the same space earn three times as much.
Do real miners build blocks like this?
Broadly, yes: node software ranks transactions (together with any unconfirmed parents) by fee rate and fills the block from the top. A real block holds up to 1,000,000 vB, so the leftover gap at the end matters much less than in this small block.