Rates & comparisons
The Interest Rate Differential (IRD) Trap: Why Breaking a Fixed Mortgage at a Big Bank Costs More
Two lenders can charge very different penalties on the same balance, rate and remaining term. The difference is one line in your mortgage commitment: how the comparison rate is chosen. Here is the arithmetic, step by step.
Reviewed November 13, 2026 · Indicative figures, OAC
The rule every fixed mortgage follows
Break a variable mortgage before maturity and the penalty is almost always three months' interest. Break a fixed mortgage and you pay the greater of three months' interest or the interest rate differential (IRD). The IRD is meant to compensate the lender for re-lending your money at a lower rate for the rest of your term. In principle that is fair. In practice, the way the lower rate is chosen can multiply the bill.
- 3 months' interest ($420,000 at 4.89%)
- ≈ $5,135
- Monoline IRD, contract-rate method
- ≈ $5,250
- Big-bank IRD, posted-rate method
- ≈ $7,350
- Big 6 posted 5-yr fixed this week
- 6.09%
The basic IRD formula
Most IRD calculations reduce to three numbers:
- Your contract rate. The rate on your commitment, say 4.89%.
- The comparison rate. The rate the lender says it could re-lend at today for a term close to what you have left. This is the number that changes everything.
- Balance × rate gap × years remaining. IRD ≈ (contract rate − comparison rate) × balance × remaining years.
Lenders refine this (some work month by month or discount future interest back to today), so your lender's exact figure can differ by a few hundred dollars from the simple formula. The simple version is accurate enough to compare lenders and to decide whether breaking makes sense. The prepayment clause in your commitment, and your lender's online penalty calculator or a written payout statement, are the final word.
Two ways to choose the comparison rate
| Contract-rate method | Posted-rate (discounted) method | |
|---|---|---|
| Typically used by | Many monolines and some other lenders | Big banks (RBC, TD, Scotiabank, BMO, CIBC, National Bank are examples of the category) and some others |
| Comparison rate | The lender's current rate for the remaining term | Today's posted rate for the remaining term minus the discount you received at signing |
| Effect | Gap reflects real market movement | Gap is widened by your original discount |
| When it hurts most | When rates have fallen | When rates have fallen and you negotiated a big discount |
The posted-rate method feels backwards: the better the deal you negotiated on day one, the bigger the discount that gets subtracted later, and the higher the penalty. That is the trap.
The same mortgage, two penalties
Worked example: $420,000 with 2.5 years left at 4.89%
Three months' interest: $420,000 × 4.89% ÷ 4 = $5,134.50, about $5,135.
Monoline, contract-rate method: the lender's current 3-year rate (the closest term to 2.5 years) is 4.39%. Gap 4.89% − 4.39% = 0.50%. IRD = 0.50% × $420,000 × 2.5 = $5,250. That is greater than three months' interest, so the penalty is $5,250.
Big bank, posted-rate method: the original 5-year posted rate was 6.49%, so the borrower's discount was 6.49% − 4.89% = 1.60%. Today's 3-year posted rate is 5.79%; minus the 1.60% discount gives a comparison rate of 4.19%. Gap 4.89% − 4.19% = 0.70%. IRD = 0.70% × $420,000 × 2.5 = $7,350.
Same balance, same rate, same time left: the posted-rate method costs $2,100 more, or 40% higher.
Worked example: a deeper discount
Now a hypothetical borrower who negotiated harder: same $420,000 and 2.5 years left, contract 4.49% against an original posted rate of 6.79%, a 2.30% discount.
| Calculation | Comparison rate | Result |
|---|---|---|
| Three months' interest: $420,000 × 4.49% ÷ 4 | n/a | $4,714.50 |
| Contract-rate IRD: 0.10% × $420,000 × 2.5 | 4.39% | $1,050 |
| Posted-rate IRD: (5.79% − 2.30% = 3.49%) → 1.00% × $420,000 × 2.5 | 3.49% | $10,500 |
At the monoline the IRD is smaller than three months' interest, so the penalty is $4,715. At the bank it is $10,500, about 2.2 times as much. Larger balances, longer remaining terms and bigger falls in rates push the multiple higher still.
Does breaking for a lower rate ever pay?
Rarely on rate alone. Using the first example, compare the interest you would pay over the remaining 30 months at 4.89% with what you would pay at 4.39%, on a 22.5-year remaining amortization with semi-annual compounding.
| Item | Amount |
|---|---|
| Interest at 4.89% (payment $2,556.39) | $49,258 |
| Interest at 4.39% (payment $2,441.74) | $44,179 |
| Interest saved | $5,079 |
| Net after a $7,350 posted-rate penalty | −$2,271 |
| Net after a $5,250 contract-rate penalty | −$171 |
Before legal and discharge costs, breaking loses money in both cases. Where breaking does pay is when the new mortgage does more than lower the rate: consolidating credit-card debt at 20% into a mortgage, freeing equity for a secondary suite, or a sale where the penalty is simply a cost of moving. Run your own numbers in the renewal and penalty calculator and see the refinance and debt consolidation options.
Six ways to shrink or avoid an IRD
- Use your prepayment privilege first. If your lender allows the annual lump sum (typically 15%–20%) to be applied before the penalty is calculated, prepaying 20% of $420,000 cuts the balance to $336,000 and the posted-rate IRD from $7,350 to 0.70% × $336,000 × 2.5 = $5,880. Confirm the order of operations with your lender.
- Port the mortgage. Moving from Welland to Fonthill? Most fixed mortgages can be carried to a new home within a set window, avoiding the penalty on the ported portion.
- Blend and extend. Your current lender blends your rate with today's rate for a new term. No penalty, but compare the blended rate with the open market.
- Let the buyer assume it. Some mortgages are assumable with lender approval, which can suit a sale.
- Time the break. The IRD shrinks as the remaining term shortens. Within the last few months, three months' interest is often the larger figure, and some lenders allow early renewal without penalty in the final 120 days.
- Choose the penalty method up front. On your next mortgage, weigh the penalty clause as heavily as the rate. See banks vs a broker.
Frequently asked questions
What is an interest rate differential (IRD) penalty?
An IRD is a penalty for breaking a fixed-rate mortgage early. It approximates the interest the lender loses by re-lending your balance at a lower rate for the rest of your term: (contract rate − comparison rate) × balance × years remaining. Fixed mortgages charge the greater of the IRD or three months' interest.
Why do big banks charge higher IRD penalties?
Big banks typically use the posted-rate method: they subtract the discount you received at signing from today's posted rate for your remaining term. That lowers the comparison rate and widens the gap. On $420,000 with 2.5 years left at 4.89%, this produces about $7,350 against about $5,250 using a monoline's contract-rate method.
Do variable-rate mortgages have an IRD?
Almost never. The standard penalty on a variable-rate mortgage is three months' interest. On a $420,000 balance at 3.65%, that would be about $3,833.
Can I reduce my penalty by making a prepayment first?
Often, if your lender permits the annual prepayment privilege to be used before the penalty is calculated. Prepaying 20% of a $420,000 balance would reduce a posted-rate IRD from about $7,350 to about $5,880 in our example. Ask your lender to confirm in writing before you send money.
Where do I find my lender's IRD method?
In the prepayment section of your mortgage commitment or charge terms. Most lenders also provide a penalty calculator or a written payout statement on request. A broker can read the clause with you and estimate the penalty before you decide.
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