Unfortunately I don't know of a tool that calculates it well how much you should pay for that new tx.
It's not difficult to do the calculation.
S
A = Size of the unconfirmed transaction
S
B = Size of the new transaction (the transaction you will make for doing CPFP)
f
A = The fee rate used for the unconfirmed transaction.
f
B = The fee rate you should use for the new transaction.
f = the fee rate required for a fast confirmation.
f
B = (f*(S
A+S
B) - S
A*f
A) / S
BI'm sorry hosseinimr93 but I don't really understand your formula.
CMIIW but
SA*fA is the fee paid for the first transaction.
f*(SA+SB) is the fee to be paid for a fast transaction with a length of the 2 ones.
So
f*(SA+SB) - SA*fAmeans you won't pay "again" the fee of the first transaction but only the fee for a fast transaction with a length of the 2(ie the same price of the 2 tx sent quickly), when they'll be mined :
SA*fA + f*(SA+SB) - SA*fA = f*(SA+SB)For me, this fee is OK, it's the fair price IMO. I don't understand why you want to reduce it and you're dividing
f*(SA+SB) - SA*fA by the length of the new transaction.