Getting the txn size before sending?

[Anonymous]

I was wondering if there was a way for me to calculate the transaction fee of a transaction before sending it. I know that I pay per KB, so how can I calculate how large a transaction will be before sending it.
Thanks,
macintosh264

[jackjack]

You need to know what transactions your new transaction will use
Not easy but possible if you know how the standard client choses them