Why IRD penalties can be so large
When rates have dropped since you locked in, your lender stands to lose the difference in interest income if you leave early — the IRD (Interest Rate Differential) is designed to make them close to whole for that lost income over your remaining term. The bigger the gap between your rate and today's comparable rate, and the more time left in your term, the larger the IRD grows. This is exactly why breaking a fixed mortgage when rates have fallen can cost thousands more than breaking one when rates have risen, where 3 months' interest usually applies instead since there's no rate gap working against you.
Why variable-rate mortgages almost never have an IRD
IRD exists specifically because your lender committed to a fixed rate for a fixed term — breaking that commitment early costs them the interest income they were counting on for the remainder of the term. A variable-rate mortgage doesn't make that same commitment; the rate floats with the market already, so there's no fixed income stream for the lender to be compensated for losing. That's why variable-rate mortgages typically charge only 3 months' interest to break, regardless of how rates have moved — one of the genuine trade-offs of choosing variable over fixed beyond just the rate itself.
Common reasons people end up calculating this
Breaking a mortgage early usually comes up for one of a few reasons: selling the home before the term ends (see the cost of selling calculator for the rest of that picture), refinancing to access equity or consolidate debt (the refinance calculator nets this penalty against the potential savings), or switching lenders mid-term to chase a better rate rather than waiting for renewal. In every case, the penalty is a real, one-time cost that needs to be weighed against whatever benefit is driving the decision — it's rarely worth breaking a mortgage on rate alone unless the gap is substantial and you're planning to stay long enough to recoup the cost.
Why every lender's number will differ slightly
Every lender calculates IRD slightly differently — some use their own posted rate as the comparison point, others use the rate you actually received after any discount, and the specific term length they compare against (matching your remaining time, or rounding to the nearest standard term) varies by institution and isn't always disclosed clearly upfront. This calculator uses a standard, commonly cited approximation of the method, but it can't replicate your specific lender's exact formula. Treat this as a ballpark for planning purposes, and get the exact figure in writing from your lender before making a final decision — a five-minute phone call can save you from an unpleasant surprise on closing day.