Fee Estimation

Fee estimation is the wallet's job of predicting how many satoshis per virtual byte (sat/vB) you need to pay to get your transaction confirmed within some target number of blocks. Pay too little and you wait; pay too much and you overpay.

Estimators work from one or both of two signals:

• Current mempool state. How much unconfirmed transaction data is queued up at each fee rate? mempool.space, for example, sorts the waiting transactions into the blocks it expects miners to build next and recommends fee rates from those projected blocks. The fuller the mempool, the higher the floor.
• Recent confirmation history. How long did transactions at each fee rate take to get mined? Bitcoin Core's estimator sorts the transactions its node sees into fee-rate buckets and tracks how many blocks each one took to confirm.

Bitcoin Core exposes its estimator through the estimatesmartfee RPC, which takes a target from 1 to 1,008 blocks and treats a target of 1 as 2. Fee sites and wallets usually offer a few tiers instead. mempool.space's fee API, for example, names its rates fastest, half hour, hour and economy. Lower urgency means a lower fee.

When blocks have room to spare, estimates for every target can fall to the floor. Bitcoin Core's estimator bottomed out at 1 sat/vB until version 31.0 (April 2026) lowered its minimum to 0.1 sat/vB, matching the default relay minimum that version 29.1 had set in September 2025. When Core's own wallet picks a fee from its estimates, it still won't go below 1 sat/vB unless its -mintxfee setting is lowered.

When demand jumps, estimates jump with it. On April 20, 2024, when the Runes token protocol switched on at the fourth halving, the median fee rate in block 840,000 was about 200 sat/vB.

Estimates are guesses. If you underpay and get stuck, see Fee Bumping. The live fee market is visible in the Mining rabbit hole section 6 and on the Node page.