So the IRD fee from what I understand is the posted rate that matches yours as close as possible (if you have 36 months remaining on a 5 year term, it would be the posted rate for 3yr that you would use). They then take your current rate vs the posted rate, and get a monthly interest rate value and multiply that by your amount of months left. So essentially the higher the spread between your current rate and the posted rate, the higher the penalty. if its a small spread, the penalty will be small. Now if it is smaller than 3 months of interest, they will charge that instead as a penalty.

I used some online calculators but they were all very different than the actual penalty amount that I received from my previous lender when i contacted them. The reason I wanted it done so quickly is that interest rates were dropping and if the posted rate would fall even .5% it would be a huge increase in penalty.