First we get the interest rate applicable to you. For Plan 1 and Postgraduate loans that is a set value and for Plan 2 loans we calculate the variable amount from the 3% and add it to the base RPI percentage. Let's call that x

Then we get the daily interest rate (remember interest is calculated daily and applied to your balance each month): y = x / 100 / 365

and finally we get the interest amount: amount = (((y + 1)^daysInTheMonth) - 1) * currentOutstandingDebt

As an example, let's say x is 5.6%:

y = 5.6 / 100 / 365 which is y = 0.00015342465, our daily percentage.

If the month we are calculating the interest to apply for has 30 days and the current outstanding debt is £40,000, the amount to be added is amount = (((0.00015342465 + 1)^30) - 1) * 40000 which is amount = 184.519747857, so about £184.