Bitcoin Transaction Size Estimator in Bytes

How Bitcoin transaction size is estimated

Fees on Bitcoin depend on virtual size, not on the BRL amount you're sending: fee = sat/vByte · vSize. A standard SegWit transaction with 2 inputs and 2 outputs comes in around 140 vBytes. Build the same thing with legacy P2PKH addresses and it balloons to roughly 250 vBytes, nearly twice the cost. The reason is BIP-141: SegWit, activated in 2017, pulls the witness data out and weights it at a quarter of its raw size. That discount is why wallets such as Electrum or Sparrow ship with bech32 (bc1q...) addresses as the default. To put numbers on it, at 30 sat/vByte a 140 vByte SegWit tx runs 4,200 sats, while a legacy 250 vByte tx costs 7,500 sats at the same rate.

When the mempool gets congested, sat/vByte climbs; at the peak of bull cycles, fees have shot past 300 sat/vByte. If a transaction gets stuck, RBF (Replace-By-Fee, BIP-125) lets you rebroadcast the same one with a higher fee. Micro-payments are better handled by the Lightning Network, which keeps transfers off-chain and only settles on-chain when channels open or close, so the per-vByte charge never applies in between.

Applications

A HODLer might wait for a low-fee window before consolidating coins. Exchanges like Mercado Bitcoin, Foxbit and Binance batch many withdrawals into one tx with lots of outputs so the fee spreads across users. Wallet developers behind Electrum, Sparrow and BlueWallet surface the estimated cost before you hit send. And on a smaller scale, it's whatever lets you decide between broadcasting now or holding off until the mempool clears.

FAQ

What is the difference between bytes and vBytes? vBytes count witness data at 1/4 the weight of ordinary bytes. On a legacy tx, bytes and vBytes are the same number. On a SegWit tx, the vByte count is lower than the raw byte count, so it pays less.

Why does adding inputs cost more than outputs? An input has to carry a signature, roughly 68-148 bytes depending on the address type, whereas an output is little more than a script plus a value at about 31 vBytes. So consolidating UTXOs costs you up front, but it trims the fees you'll pay later.

Does Taproot reduce size further? It does. Taproot (bc1p..., activated 2021) is a touch smaller for single-sig and far smaller for multisig and scripts, because a complex spending path only ever reveals the branch you actually used.