Interest rates are given on an annual basis so you just need to divide the interest rate by how small fo a time period your looking for and multiply by the current balance
Current Balance X interest rate / period of time
For daily interest charges on her loans:
$60,000 x .068/365 = $11.18
I use this method too because that number right there makes me want to get rid of all my debt ASAP
Went to type this, saw Maigahane had done it. This is correct.
This is an excellent formula to know offhand and use in your daily life. For us, we use it to see how we're doing as we continue punching my fiancee's student loans IN THE FRIGGIN' FACE.
When we started out, loans where accruing interest at:
$23.63/day OR $708.90/month OR $8,507/year
Through massive focus and use of proper snowballing techniques, loans are currently accruing interest at:
$14.45/day OR $433.50/month OR $5,202/year.
Not bad, for 10 months work! Granted, we are two people who are putting massive amounts towards these loans.
I strongly suggest you commit this formula to memory and use it in conjunction with snowballing. Moving the needle even a little bit (i.e. $1 less in accrued interest per day) is a tremendous accomplishment, and you should feel awesome when you see progress.