wanted to break this down a bit, while still keeping it simple. the following assumes mempool.space is accurate at calculating fees.

to fee bump a transaction with CPFP, look at the virtual size of the transaction you wish to fee bump. assuming your CPFP transaction is Segwit, Taproot, RBF, that will be about 111 vB. Add each vB and we get total vB. multiply that by the effective fee you wish to your original transaction to be. subtract your original fee. divide the result by 111 and that is the sats/vB of your CPFP transaction.

Example:

orig_tx_vb = 160

orig_tx_fee = 500

next_block_fee = 15 sats/vB

orig_tx_vb + 111 = 271 (total_vb)

total_vb * next_block_fee = 4065 (total_fee)

total_fee - orig_tx_fee = 3565

3565 / 111 = ~32 sats/vB required in CPFP to target the next block

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