21,000 gas
Fee estimation guide
Transaction fees on Bitcoin and Ethereum are not set by a company — they are the price of block space, cleared block by block. This guide shows how to compute an Ethereum fee from gas used, base fee and priority fee, read a Bitcoin quote in sat/vB, and judge whether a layer-2 route is cheaper than mainnet.
21,000 gas
12.5% per block
~10 min blocks
Size, not amount
Batched settlement
Paid even on failure
Fees change because block space is a scarce resource auctioned every block — demand rises and falls with activity, not with a clock.
On Ethereum, every block targets half of the 36 million gas limit. If the previous block used more than the target, the base fee rises by up to 12.5%; if it used less, the base fee falls by up to 12.5%. That is why fees can move noticeably within minutes during a surge and drift back down when activity cools.
On Bitcoin, there is no base fee. Miners pick transactions from the mempool, usually prioritising the highest sat/vB. When the mempool fills, the sat/vB needed for a next-block confirmation climbs; when it empties, the same transaction can confirm at a much lower rate. Blocks arrive about every 10 minutes on average, so a fee quoted at 09:00 can be stale by 09:30.
The fee is paid even when a transaction fails on Ethereum, because validators already spent the computation. On Bitcoin, a transaction that is not mined simply waits in the mempool until it is included or dropped.
Multiply gas used by the sum of base fee and priority fee; a plain ETH transfer always uses 21,000 gas.
Start with gas used. A simple ETH transfer is always 21,000 gas. A token transfer or a contract call uses more, and the wallet usually shows an estimate before you sign. The base fee is set by the protocol for the next block; the priority fee is what you add to make inclusion likely.
The formula is: fee = gas used × (base fee + priority fee). Worked example from ethereum.org: 21,000 gas at a base fee of 10 gwei plus a 2 gwei tip costs 252,000 gwei, which is 0.000252 ETH. At 20 gwei per unit, the same 21,000-gas transfer costs 0.00042 ETH — about $1.26 at $3,000 per ETH.
Because 1 gwei equals 0.000000001 ETH, converting is a matter of moving the decimal point nine places. Wallets usually present the total in ETH and in fiat, so you can see the cost before confirming.
Bitcoin fees are quoted in satoshis per virtual byte; the rate you pick determines how quickly miners include your transaction.
A Bitcoin transaction has a weight, measured in virtual bytes (vB). The fee you pay is the rate in sat/vB multiplied by the transaction size. A typical single-input, two-output transaction is around 140 vB, but the exact size depends on the number and type of inputs and outputs.
Wallets and block explorers show a range of sat/vB rates with estimated confirmation windows: a high rate for the next block, a middle rate for a few blocks, and a low rate for whenever the mempool clears. Because blocks average about 10 minutes, a rate that confirms in one block at 09:00 may need more time by 09:20 if new transactions arrive.
1 satoshi equals 0.00000001 BTC. If you pay 20 sat/vB on a 140 vB transaction, the fee is 2,800 satoshis, or 0.000028 BTC. Always confirm the total in BTC before broadcasting.
Numbers below are protocol constants or current market quotes; they are not predictions.
Layer-2 networks settle many operations off the main chain and post compressed proofs or batches back, so the per-transaction fee is usually a fraction of mainnet.
Rollups and sidechains can cut fees because they do not require every operation to be a separate mainnet transaction. A rollup batches many transactions and posts a single compressed proof or data blob to Ethereum, so the mainnet cost is shared across the batch. Sidechains run their own consensus and typically have lower fees but different security assumptions.
The trade-off is not only price. Moving funds to a layer-2 network usually means bridging, which has its own fee and a delay. Withdrawing back to mainnet can take minutes to days depending on the design. Before bridging, check the bridge's fee, the withdrawal time, and whether the layer-2 network is a rollup or a sidechain.
For a plain ETH transfer, mainnet at a low base fee can already be cheap. For frequent token swaps or contract interactions, a layer-2 route is often cheaper per operation even after accounting for the bridge.
| Network | Unit | Typical value | What it means |
|---|---|---|---|
| Ethereum | Gas per plain transfer | 21,000 | Computation cost of sending ETH |
| Ethereum | Base fee movement | up to 12.5% per block | Protocol adjustment, burned |
| Ethereum | Block gas limit | about 36 million | Targets half per block |
| Ethereum | Worked example | 0.000252 ETH | 21,000 gas at 10 gwei + 2 gwei tip |
| Bitcoin | Fee unit | sat/vB | Bid for block space |
| Bitcoin | Satoshi | 0.00000001 BTC | Smallest Bitcoin unit |
| Bitcoin | Block interval | about 10 minutes | Average time between blocks |
| Both | Fee on failure | paid (Ethereum) | Validators spent the computation |
Sources are limited to protocol documentation and public block-explorer conventions; no promotional or affiliate material is included.