UTXO consolidation:
when does it pay off?

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Transaction size in virtual bytes (vB) = overhead + inputs × input size + outputs × output size, rounded up to a whole vB. Virtual size is transaction weight ÷ 4, rounded up, where witness bytes count once and all other bytes four times^[2]. Overhead is version, locktime and the input and output counts (10 bytes; the input count grows from 1 to 3 bytes at 253 or more inputs, adding 2 vB); SegWit and Taproot transactions add a 2-byte marker and flag counted as witness data, which makes 10.5 vB^[2]. Sizes used:

• SegWit (P2WPKH): 10.5 vB overhead, 68 vB per input, 31 vB per output.
• Taproot (P2TR): 10.5 vB overhead, 57.5 vB per input (a key-path spend carrying one 64-byte signature^[3]), 43 vB per output.
• Legacy (P2PKH): 10 vB overhead, 148 vB per input, 34 vB per output.

With N UTXOs:

• Consolidate now = size(N inputs, 1 output) × current rate.
• Spend individually later = N × size(1 input, 2 outputs) × future rate.
• Savings = the later cost minus (consolidation fee + one later payment of size(1 input, 2 outputs) × future rate), in sats. Positive savings shows Consolidate now, negative shows Wait, zero shows Break-even. USD = sats ÷ 100,000,000 × price; with no price (blank or 0), amounts show in sats.
• Break-even current rate = (N - 1) × size(1 input, 2 outputs) × future rate ÷ size(N inputs, 1 output). Consolidating now wins at any current rate below it.
• With an average UTXO size entered: consolidation fee as a share of N × size.
• With an average UTXO size entered, two checks come before the comparison. If the consolidation fee is at least N × size, the merged output would be zero or negative, so the verdict says consolidating at this rate costs at least what the UTXOs hold and gives the highest current rate that leaves something: N × size ÷ size(N inputs, 1 output). Otherwise, if size is at or below size(1 input, 2 outputs) × future rate, one-by-one payments cannot be built at the future rate, so the verdict says so and shows what consolidating leaves and what spending it later costs.

Without an average UTXO size, the verdict is the fee comparison alone. The count must be a whole number from 2 to 500; above 675 Legacy inputs a single consolidation passes the 400,000 weight-unit limit on a standard transaction^[4], and 500 keeps every address type under it. A count outside 2 to 500, a fractional count, a fee rate of 0 or less, a negative price, or a size of 0 or less shows an input message instead of results. The later path assumes one transaction per UTXO; a wallet that combines inputs in one payment pays less. The privacy cost of linking UTXOs is not priced.

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