Your best bet is to contact the leader directly and request what's called a "payoff amount". Ask them to calculate the amount to a date in the future, so you have time to mail a check or pay online.
Otherwise, if you'd like to calculate it yourself, you should find out what the remaining principal is and the interest rate. Than take the daily interest rate (interest rate divided by 365) and multiply the principal by that number (plus 1) and square root it for the number of days past today. (Don't forget to include accrued interest in the "principal").