Bitcoin Fees & Mempool
What does sat/vB mean?
Bitcoin fees are not a percentage of the amount sent — they are a price per unit of block space. Wallets quote that price in satoshis per virtual byte (sat/vB), where 1 sat = 10⁻⁸ BTC. Your total fee is the rate times your transaction's virtual size, whether you move $10 or $10 million.
· typical 1-input, 2-output transaction ≈ 140 vB → at 5 sat/vB ≈ 700 sats
Virtual size (vB) is the post-SegWit measure of how much block space a transaction consumes — witness data is discounted 4×, which is why vB differs from raw bytes. More inputs make a bigger transaction, and a bigger transaction costs more at the same rate.
What is the mempool?
Every node keeps a waiting room of valid but unconfirmed transactions — the mempool. Miners fill each roughly 1 MvB block from it, highest fee rate first, so the mempool is a continuous auction for block space. The tiles above show how deep the queue is right now.
time_to_clear ≈ blocks_to_clear × 10 min · assumes full blocks and no new transactions
The clearing estimate is an upper-bound sketch: fresh transactions arrive continuously, so a deep mempool can persist for days when demand stays high — and drain in an hour when it doesn't.
Why miners care about fees
Fees are the second half of miner revenue, on top of the fixed 3.125 BTC subsidy per block. Over the last 10 blocks, fees averaged --. Each halving cuts the subsidy in half while fees depend only on demand, so the fee share of mining income structurally grows over time — track the longer-run trend on the Fee Share tile on the dashboard.
Why do fees spike?
Supply of block space is fixed: about 1 MvB every 10 minutes, roughly 144 blocks a day, no matter what. Demand is not — price volatility, exchange flows and inscription or token-minting waves can multiply the number of pending transactions within hours. When demand exceeds one block's worth, the auction clears at whatever the marginal bidder pays: that is a fee spike. When the wave passes, rates drift back toward the 1–2 sat/vB floor.
FAQ
- How do I pick a Bitcoin transaction fee?
- Match the tier to your urgency: the fastest tier targets the next block (~10 minutes), the half-hour tier about 30 minutes, and the economy tier settles when the mempool thins out — often hours. Your total cost is the rate times your transaction's virtual size; a typical 1-input, 2-output transaction is about 140 vB, so a 5 sat/vB fee costs roughly 700 sats regardless of the amount you send.
- What happens to an unconfirmed Bitcoin transaction?
- It waits in the mempool until a miner includes it or nodes drop it — typically after about two weeks, or sooner if the mempool fills and the lowest-fee transactions are purged. If it is stuck you can simply wait, bump the fee with Replace-By-Fee (RBF) if your wallet enabled it, or spend the unconfirmed output with a higher-fee child transaction (CPFP) so miners take both together.
- Do higher fees mean more miner revenue?
- Yes — every fee in a block goes to the miner who mines it, on top of the fixed 3.125 BTC subsidy. In calm periods fees are a few percent of the block reward; during congestion spikes they have briefly exceeded the subsidy itself. Each halving cuts the subsidy in half, so fees become a structurally growing share of mining income.