Short answer: a sliding scale (or "graduated") penalty calculates the cost of breaking your mortgage based on how many months are left in your term, rather than using an interest rate differential (IRD) calculation. Depending on your lender and current rates, it can work out considerably cheaper than an IRD penalty.
How a Sliding Scale Penalty Works
Instead of comparing your rate to current rates (as an IRD does), a sliding scale penalty applies a set percentage of your remaining balance based on time left in the term, commonly structured as a higher percentage in the early years, stepping down the closer you get to the end of your term. Some lenders and credit unions use this method as their standard prepayment charge instead of an IRD.
Why It Can Be Cheaper Than an IRD
An IRD penalty grows larger the bigger the gap between your contract rate and current rates, which is exactly why breaking a mortgage when rates have dropped significantly can be so expensive with a big bank. A sliding scale penalty doesn't reference current rates at all, so in a falling-rate environment, it's often meaningfully lower than what an IRD would charge for the same remaining term.
Who Tends to Use Which Method
Most major banks default to an IRD (or three months' interest, whichever is greater) for fixed-rate mortgages. Sliding scale penalties are more commonly offered by monoline lenders and credit unions as an alternative structure, though it varies by lender and even by specific mortgage product.
How to Find Out Which One Applies to You
Your mortgage commitment or renewal documents will specify the penalty calculation method. If it isn't clear, call your lender directly and ask for the specific formula, along with an actual dollar estimate based on your current balance and rate.
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