For loan customers:

Accrued interest = Remaining principal amount x monthly interest rate x no. of months in grace period

For credit card customers:

Regular Transaction Computation
Outstanding principal balance (as of cut-off for which grace period will apply) X
Daily Interest Rate X # of days balance remained unpaid

Installment Plan Computation
Unpaid principal balance (as of cut off for which grace period will apply) X
Annual Interest Rate X interest period duration (no. of grace period days)/360