What makes a transaction bigger
Every coin you spend is an input, and inputs are the expensive part. A wallet that received many small payments has to combine them, so a single payment can carry five or ten inputs and cost several times more than one built from a single coin. Outputs are cheap: usually two โ the payment and your change.
Address type matters because SegWit and Taproot move signature data into the cheaper witness part of the transaction. Spending a native SegWit (bc1q) input takes about 68 vB, Taproot (bc1p) about 57.5 vB, nested SegWit (3โฆ) about 91 vB and legacy (1โฆ) about 148 vB.
size (vB) โ 10.5 + inputs ร input size + outputs ร output size (10 instead of 10.5 for all-legacy)
fee (sat) = size ร fee rate (sat/vB), rounded up
fee (BTC) = fee (sat) รท 100,000,000Picking a fee rate
The rate is set by demand for block space and changes during the day. Public mempool explorers such as mempool.space show what rate is likely to confirm in the next block, within an hour or within a day. When the mempool is quiet, 1โ5 sat/vB can confirm quickly; during busy periods the next-block rate can be ten times higher.
If you send BTC to a swap and the fee was too low, the order simply waits for the first confirmation. A wallet with Replace-By-Fee can raise the fee on the same transaction; do not send a second payment to the same deposit address.
Typical sizes
- 1 input, 2 outputs, native SegWit: about 141 vB โ at 5 sat/vB, 705 sats.
- 1 input, 2 outputs, Taproot: about 154 vB โ Taproot outputs are larger, inputs smaller.
- 1 input, 2 outputs, legacy: about 226 vB โ 60% more than native SegWit.
- 10 native SegWit inputs, 2 outputs: about 753 vB โ consolidating small coins when fees are low saves money later.
Sizes assume standard single-signature wallets. Multisig and complex scripts are larger; your wallet's own estimate is always the reference.
Frequently asked questions
+Why does my fee not depend on the amount?
Bitcoin charges for transaction size in virtual bytes, not for value. The number of inputs and the address types decide the size.
+What is sat/vB?
Satoshis per virtual byte. 1 BTC = 100,000,000 sats, and a vB is a weight unit divided by four, the measure used for block space since SegWit.
+Is Taproot cheaper than SegWit?
Spending a Taproot input is cheaper (about 57.5 vB vs 68 vB), but a Taproot output is larger (43 vB vs 31 vB). For a typical payment with change they come out close.
+How many confirmations does a swap wait for?
The liquidity provider decides; for BTC it is usually 1โ2. The order page shows the live count, and the confirmations lookup lists the number per coin.
+Can I lower the fee of a sent transaction?
No. You can only raise it with Replace-By-Fee or Child-Pays-For-Parent if your wallet supports them.
Calculations run entirely in your browser; nothing you type is sent to KOT.CASH. Results are estimates for information only and are not financial, tax or legal advice.