On-chain · Fee · Estimator loading mempool…

Input typeYour wallet type determines the input shape.PriorityCurrent mempool recommendation.InputsOutputs

Tx size

140.5 vbytes

Fee · sats

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Fee · USD

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How it works

On-chain Bitcoin transaction fees are not fixed — they are determined by how much block space your transaction occupies and by how congested the mempool is at the moment you broadcast. This estimator fetches live fee rates from mempool.space, combines them with a precise vbyte calculation for your transaction shape, and converts the result to your local currency using the live Bitcoin price. Everything runs client-side; no transaction data or personal information is sent to any server.

What is a vbyte?

“Vbyte” (virtual byte) is the unit Bitcoin uses to measure transaction size after the SegWit upgrade introduced the witness discount. Before SegWit, transaction size was measured in raw bytes. SegWit split the transaction into two parts: the base data (inputs, outputs, amounts, version, locktime) and the witness data (signatures and scripts). Because signatures are the largest part of any transaction, and because moving them to the witness field is provably safer, the protocol rewards SegWit users with a 4x discount on witness data weight. The vbyte formula is:

vbytes = (base_weight + witness_weight / 4) / 4

In practice, this means a Taproot input (P2TR) costs roughly 57.5 vbytes, a Native SegWit input (P2WPKH) costs 68 vbytes, a Wrapped SegWit input (P2SH-P2WPKH) costs 91 vbytes, and a Legacy input (P2PKH) costs 148 vbytes. Outputs are cheaper and roughly equal across types at about 31 vbytes each, because output scripts do not carry signatures. Every transaction also has a fixed overhead of about 10.5 vbytes for the version, input count, output count, and locktime fields.

Inputs and outputs

A Bitcoin transaction consumes previous unspent outputs (UTXOs) as inputs and creates new UTXOs as outputs. A simple payment typically has one or two inputs (depending on whether your wallet needs to combine UTXOs to cover the amount) and two outputs: the payment output and a change output back to your own wallet. If you are sweeping a wallet or consolidating many small UTXOs, you might have dozens of inputs with one or two outputs. The more inputs, the larger the transaction and the higher the fee at any given fee rate.

Fee rate tiers

The estimator pulls four fee rate recommendations from mempool.space:

Fastest (~10 min): The fee rate that gets your transaction into the very next block with high probability. Useful for time-sensitive payments.
Half hour: Expected confirmation within two to three blocks. A good balance for most payments.
One hour: Expected confirmation within six blocks. Suitable when you are not in a hurry.
Economy (cheapest): The lowest fee rate that nodes in the current mempool will forward. Confirmation could take hours or days if congestion rises. Not recommended when you need reliable timing.

These are recommendations, not guarantees. The mempool is a dynamic market. If a sudden surge of transactions floods the mempool after you broadcast, a transaction with a lower fee rate may fall further back in the queue.

The SegWit discount in practice

To illustrate why format matters, consider a simple one-input, two-output transaction:

Legacy (P2PKH): 148 + (2 × 31) + 10.5 = 220.5 vbytes
Native SegWit (P2WPKH): 68 + (2 × 31) + 10.5 = 140.5 vbytes
Taproot (P2TR): 57.5 + (2 × 31) + 10.5 = 109.5 vbytes

At 10 sat/vbyte, those transactions cost 2,205 sats, 1,405 sats, and 1,095 sats respectively. Over many transactions, the savings compound. Taproot inputs are about 50% cheaper than Legacy inputs. If you are still using a Legacy wallet, migrating to Native SegWit or Taproot is the single highest-leverage change you can make to reduce your fee burden.

RBF: Replace-by-Fee

If you broadcast a transaction with a fee rate that turns out to be too low — perhaps because mempool congestion spiked after you sent it — you are not necessarily stuck waiting. Replace-by-Fee (RBF, BIP 125) allows you to rebroadcast a replacement transaction spending the same inputs but with a higher fee rate. Most modern wallets (Bitcoin Core, Sparrow, Blue Wallet, Electrum) support RBF. The replacement transaction must pay a fee at least as high as the original fee plus the minimum relay fee increment, which encourages miners to prefer the new version.

Not all transactions can use RBF. A transaction signals RBF by setting the sequence number of at least one input to a value below 0xFFFFFFFE. Wallets that do not set this signal cannot be replaced, at least not by the sender; the recipient or a miner could still use Child-Pays-for-Parent to accelerate the transaction.

CPFP: Child-Pays-for-Parent

Child-Pays-for-Parent (CPFP) is the complementary mechanism. If you receive an unconfirmed transaction with a low fee rate, you can create a new transaction spending the unconfirmed output — a “child” transaction — with a high enough fee that the combined fee of parent and child attracts a miner. Miners consider the effective fee rate of the entire package, so a sufficiently generous child can pull a stuck parent into the next block. CPFP is useful when you are the recipient (and therefore cannot use RBF) or when you want to accelerate without invalidating the original transaction.

FAQ

Why are Taproot fees lower?

Taproot (P2TR) uses Schnorr signatures, which are slightly smaller than the ECDSA DER-encoded signatures used by older formats. More importantly, Taproot key-path spends place the signature in the witness field where the 4x weight discount applies. The x-only 32-byte public key encoded in the output script is also smaller than the 33-byte compressed public keys used in P2WPKH. The combined effect is that Taproot inputs occupy only 57.5 vbytes versus 68 for P2WPKH.

Can I underpay and get stuck?

Yes. If the fee rate you choose falls below the minimum relay fee (currently 1 sat/vbyte by default for most nodes), your transaction may not propagate at all and could time out without ever being broadcast to the broader network. If it does propagate but the mempool becomes congested, it could sit unconfirmed for days or weeks. Most wallets will eventually drop the transaction from memory after a configurable timeout (default 336 hours in Bitcoin Core), at which point the funds return to your wallet as an unconfirmed UTXO. During that period, the funds are in limbo.

What is replace-by-fee and how do I use it?

RBF lets you bump the fee of an unconfirmed transaction by broadcasting a replacement. In practice, look for a “Boost” or “Replace” button in your wallet. Sparrow Wallet, Bitcoin Core, and Electrum all have explicit RBF bump features. The replacement must pay a higher absolute fee and a higher fee rate than the original. Not all wallets or exchanges accept incoming unconfirmed RBF transactions, so check with your counterparty before relying on it.

Does this tool need an internet connection?

It fetches live fee rates from mempool.space once per minute (cached in localStorage). If you are offline, it will use the cached rates from your last visit. The vbyte calculation and fee arithmetic are purely local and work without any network connection.

How accurate is this estimate?

It is an estimate based on typical input and output sizes for each address type. Real wallet transactions may differ slightly — for example, multisig scripts, time-locked outputs, or unusual script types will not match the standard sizes used here. The estimate is accurate enough for budgeting purposes but should not be treated as an exact figure. Most wallets perform a similar calculation and arrive within a few percent of this tool’s result.