Bitcoin fees and the mempool, explained
What you are actually paying for when you send bitcoin, how the mempool decides what confirms next, and practical ways to pay less.
Updated 14 Sept 2026 · Intermediate · 3 min read
In short
- Bitcoin fees are priced per unit of block space (sat/vB), not per amount sent: moving 0.001 BTC can cost the same as moving 100 BTC.
- The mempool is the waiting room of unconfirmed transactions; miners fill blocks from the top of it by fee rate.
- Transaction size depends on how many coins you spend and your address type, so wallet habits change what you pay.
What a fee pays for
Every block has a fixed amount of space, and a new one arrives about every ten minutes. Miners choose which transactions go in, and they choose the ones paying the most per unit of space. The fee is a bid for that space, measured in satoshis per virtual byte (sat/vB). One satoshi is 0.00000001 BTC.
Because the price is per byte, the amount you send barely matters. What matters is how large the transaction is, mostly how many inputs (coins you are spending) and outputs (recipients plus change) it has, and which address types are involved.
fee (sats) = fee rate (sat/vB) × transaction size (vB)Virtual bytes, and why SegWit is cheaper
Since the SegWit upgrade in 2017, blocks are measured in weight. Signature data (the “witness”) counts for a quarter as much as the rest of a transaction, and a virtual byte is a quarter of a weight unit, which is why wallets quote sizes in vB. Spending from a native SegWit (bc1q…) or Taproot (bc1p…) address therefore costs noticeably less than spending from a legacy address starting with 1: a one-input, two-output legacy payment is about 226 bytes, against about 141 vB for native SegWit.
The mempool
Every node keeps its own mempool: the unconfirmed transactions it has seen and considers valid. There is no single global mempool, but well-connected nodes see nearly the same set. When the mempool holds more than the next block can take, the lowest-paying transactions wait.
Our fee estimator sorts the mempool by fee rate and slices it into projected blocks: the first slice is what the next block would contain if it were mined now. The fee rate at the bottom of that slice is roughly what you need to get into the next block, until new transactions arrive and push the line up.
Reading fee estimates
- Next block / fastest: for payments that must confirm in about ten minutes.
- Half an hour / an hour: a sensible default for most payments.
- Economy: for moves that can wait. It may take hours or, in busy periods, days.
Estimates are forecasts, not guarantees. Block discovery is random (sometimes two blocks arrive a minute apart, sometimes an hour passes with none) and demand changes quickly. If you choose a low fee, make sure RBF is on so you can bump it later.
Ways to pay less
- Receive to SegWit or Taproot addresses. The saving applies every time those coins are spent.
- Consolidate small coins when fees are low. Many small inputs make every future payment larger. Merging them in a quiet period saves money later, at some cost to privacy, because it links those coins together.
- Batch payments. One transaction paying ten people is far smaller than ten separate transactions.
- Choose the speed you need. Picking the fastest option every time is the most common way to overpay.
- Avoid dust. Tiny outputs can cost more to spend than they are worth.
Fees and the future of Bitcoin
Miners earn the block subsidy (newly created bitcoin) plus the fees in each block. The subsidy halves every 210,000 blocks, roughly every four years; since the April 2024 halving it has been 3.125 BTC per block. As it shrinks, fees become a larger share of what pays for the network’s security. The Bitcoin dashboard shows the block height, the next halving and the latest difficulty adjustment.