Bitcoin mempool fees: how sat/vB works and how not to overpay

Bitcoin mempool fees are what you pay miners for space in a block. The fee does not depend on how much BTC you send: it equals a fee rate in satoshis per virtual byte (sat/vB) multiplied by the size of your transaction. While a payment waits for confirmation, it sits in the mempool, the queue of unconfirmed transactions from which miners pick the best-paying ones. As of 2 PM ET on Oct 20, 2026, mempool.space showed that 2 sat/vB was enough for the next block and 1 sat/vB for confirmation within an hour, so a typical payment cost under $0.25. Below: how the mempool works, how to calculate the fee yourself, and what to do when a transaction gets stuck.

What is the bitcoin mempool?

The mempool (short for memory pool) is the set of transactions that have been broadcast to the network but not yet included in a block. There is no single global mempool: every node keeps its own copy and decides which transactions to accept based on its own policy. That is why different block explorers show slightly different numbers. A node's mempool also has a memory cap: when the queue overflows, the cheapest transactions are evicted first and the minimum rate to get in temporarily rises.

Block space is scarce. Under BIP141, a block's weight cannot exceed 4,000,000 units, which works out to roughly 1 million virtual bytes. A block is found about every 10 minutes on average, and when demand exceeds that space, it becomes an auction: miners fill blocks with the highest fee rates first and everyone else waits.

Satoshi Nakamoto built this into the Bitcoin whitepaper (section 6): a miner's reward combines newly issued coins and transaction fees, and once issuance ends, the incentive can move entirely to fees.

How bitcoin fees are calculated: sat/vB and transaction size

There is one formula:

fee = fee rate (sat/vB) × transaction size (vB)

One satoshi is 0.00000001 BTC. Virtual size (vsize), as defined in BIP141, is the transaction's weight divided by 4. Signature data in SegWit transactions is discounted, which is why SegWit payments are cheaper than legacy ones.

Size depends on the number of inputs and outputs and the address type, not on the amount. Approximate sizes for a typical "1 input, 2 outputs" payment (recipient plus change), per the Bitcoin Optech size calculator:

Every extra input adds weight. If your wallet builds a payment from ten small deposits, the transaction will be several times larger, and pricier, than one spending a single large coin.

What a payment costs: a live example from Oct 20, 2026

Take a 141 vB SegWit payment and a BTC price of $82,823 (our data as of 2 PM ET on Oct 20, 2026; live price on the Bitcoin page):

Here is what the mempool looked like that afternoon, per the mempool.space API: 53,883 transactions waiting, about 24.8 million vB in total, or roughly 25 blocks' worth. Yet almost all of that backlog paid less than 1 sat/vB, so 2 sat/vB was enough to make the next block. We show the current fast-confirmation rate in the Numbers block on our dashboard.

Over the last 144 blocks (972,220⁠–⁠972,363, about one day), miners earned 1.99 BTC in fees out of 451.99 BTC in total rewards, or just 0.44%. Across 739,724 transactions, that is about 268 sats, roughly $0.22, per payment (mempool.space reward stats).

Why bitcoin fees spike and crash

Fee rates are pure supply and demand. Block space supply is nearly fixed, while demand swings. What pushes fees up:

1. Token and inscription manias. On halving day, April 20, 2024, the launch of the Runes protocol pushed the average fee to a record $128.45 per transaction according to YCharts, and users of block 840,000 alone paid about $2.4 million in fees (ForkLog). The next day the average fell to $34.86.
2. Sharp price moves. Crashes and rallies send coins flooding to and from exchanges.
3. Consolidation. Exchanges and large holders merge small outputs during quiet periods, soaking up cheap space.

In 2026, demand for block space is weak. On Aug 11, 2026, Glassnode co-founder Rafael Schultze-Kraft noted that fees made up only about 0.7% of miner revenue and had stayed below 1% for almost a year (ForkLog). Miners' main income is the 3.125 BTC block subsidy, which is cut in half at every halving. For how this ties into network security, see our guide to hashrate and difficulty.

Sub-1 sat/vB fees: what changed in 2025

For years, 1 sat/vB was the practical floor: Bitcoin Core nodes would not relay anything cheaper by default. Bitcoin Core 29.1, released Sept 4, 2025, lowered the default relay floor (-minrelaytxfee) to 100 sats per 1,000 vB, or 0.1 sat/vB, and the minimum block-inclusion rate (-blockmintxfee) to 1 sat per 1,000 vB. The change came in PR #33106 (Bitcoin Optech).

You could see the effect on Oct 20, 2026: in mempool.space's projected blocks, the median fee rate was around 0.14 sat/vB. Not every wallet and node has upgraded, though, so a sub-1 sat/vB transaction may propagate less reliably and wait longer.

How to speed up a stuck bitcoin transaction: RBF and CPFP

If you set the rate too low and the mempool fills up, a payment can sit unconfirmed for hours. Two standard tools fix this:

1. RBF (Replace-by-Fee). You broadcast a new version of the same transaction, spending the same inputs with a higher fee, usually by shrinking the change output. The replacement must pay both a higher fee rate and a higher absolute fee (Bitcoin Optech). Since Bitcoin Core 28.0 (Oct 2, 2024), nodes accept replacement of any unconfirmed transaction by default, even one that did not signal replaceability (full RBF).
2. CPFP (Child Pays for Parent). You spend an output of the stuck transaction in a new "child" transaction with a high fee rate. A miner can only include the child together with its parent, so it looks at the combined rate of the pair (Bitcoin Optech). The recipient can do this too: if the sender will not bump the fee, the receiver can accelerate the incoming payment.

Both features are available in wallets such as Sparrow, Electrum and many hardware wallets. Exchange withdrawals are broadcast by the exchange, so only the exchange can bump them.

Common mistakes

• Looking at the amount instead of the rate. Sending $10 or $10 million with the same number of inputs costs the same.
• Overpaying in a quiet market. Many wallets default to "high priority." Check the mempool before you send.
• Hoarding tiny deposits. Twenty $5 inputs can cost more to spend than they are worth when fees spike. Consolidate them while the network is empty.
• Using legacy addresses. An address starting with "1" makes each payment about 1.6 times larger than its SegWit equivalent.
• Confusing network fees with exchange fees. Exchanges charge a flat withdrawal fee that can be many times higher than the actual network rate.

FAQ

Do bitcoin fees depend on the amount sent? No. They depend only on the transaction's size in vB and the fee rate in sat/vB.

What happens if I set the fee too low? The transaction waits in the mempool until space frees up. If demand stays high long enough, nodes eventually drop it and the coins remain at your address. You can speed it up with RBF or CPFP.

Where can I check current bitcoin fees? In a mempool explorer such as mempool.space, or in the Numbers block on the CoinLab dashboard, which updates automatically.

Why are bitcoin fees so low in 2026? Demand for block space dropped after the Ordinals and Runes craze faded, and since 2025 Bitcoin Core nodes relay transactions below 1 sat/vB by default.

This is not investment advice.