Why your Canadian mortgage payment doesn't match online calculators

If you've ever run a Canadian mortgage through an online calculator and then compared it against your bank's quote, you've probably noticed the numbers don't quite match. They're close — usually within $10/month — but never identical. This isn't a rounding error or a bug. It's because almost every free online mortgage calculator is American, and Canadian mortgages are compounded differently by law.

Here's exactly what's happening, with verified numbers.

The rule

Under Section 6 of the Interest Act, Canadian residential mortgage rates must be expressed using either annual or semi-annual compounding, not in advance. Every major Canadian lender — RBC, TD, BMO, Scotiabank, CIBC, National Bank, plus credit unions and digital lenders — uses semi-annual.

American mortgages compound monthly. So when an American calculator takes your 4.79% annual rate and divides it by 12 to get a monthly rate of 0.39917%, that's wrong for a Canadian mortgage. The correct Canadian monthly rate is:

i_monthly = (1 + annualRate / 2) ^ (2 / 12) − 1
= (1 + 0.0479 / 2) ^ (1/6) − 1
≈ 0.39488%

Tiny difference — 0.00429 percentage points per month — but over 300 monthly payments on a 25-year mortgage, it adds up to real money.

A worked example

Take a typical 2026 first-time buyer scenario: $500,000 mortgage, 5.00% rate, 25-year amortization, monthly payments.

American (monthly compounding): $2,922.95/month Canadian (semi-annual compounding): $2,908.02/month

That's a $14.93/month difference. Doesn't sound like much. But over the full 25-year term:

• American math: $876,886 total paid → $376,886 in interest
• Canadian math: $872,406 total paid → $372,406 in interest

The Canadian formula saves you $4,480 over 25 years on this mortgage vs. what a US calculator quotes. Not life-changing, but enough to be wrong about.

If you want to verify this yourself, the Canadian mortgage payment calculator on Robinn uses the semi-annual formula and matches RBC's published calculator to the cent on the $500K / 5% / 25yr example. Plug those numbers in.

Why the difference exists

The Interest Act dates to 1897. The semi-annual-compounding rule was a consumer protection measure: it prevents lenders from advertising one rate and effectively charging a higher one through more aggressive compounding. By statutorily requiring semi-annual compounding, Canadian law makes it harder to hide the true cost of credit in compounding fine print.

In practice, the regulator-mandated semi-annual compounding works out slightly in the borrower's favour vs. American-style monthly compounding at the same nominal rate. It's not the difference that should drive your mortgage decisions — but it does mean American calculators consistently overstate your Canadian payment by 0.3-0.6%, and you should know that's what's happening.

What this means for you in 2026

Three practical implications:

1. Use a Canadian calculator for any Canadian mortgage math. Bankrate, NerdWallet (US), and the dozens of generic "mortgage payment calculator" sites that dominate Google results are all built on the American formula. Our calculator uses the Canadian formula and is verified against RBC's published numbers.

2. The qualifying rate uses the same Canadian compounding. When your lender stress-tests you at contract rate + 2%, that test also uses semi-annual compounding. Calculators that get the contract math wrong also get the stress-test math wrong. See our stress test guide for how the qualification process actually runs.

3. Variable-rate mortgages compute interest daily but still express rates semi-annually. The interest you accrue each day is based on the semi-annual effective rate, recomputed each day. Your statement will look like daily compounding but the math underneath is still Interest Act compliant.

What it doesn't change

• Your contract rate is still your contract rate. A 4.79% mortgage is 4.79% under either system; only the effective monthly conversion differs.
• Affordability is mostly unchanged. GDS/TDS ratios use the qualifying rate payment, which is also semi-annually compounded. The relative numbers — what you qualify for vs. what you pay — track the same way.
• The stress test doesn't disappear. Some borrowers hear "Canadian math is friendlier" and expect this to help them qualify. The stress test (contract rate + 2% or 5.25%, whichever is greater) applies regardless. See our affordability + stress test calculator for the real qualifying-rate math.

FAQ

Is semi-annual compounding actually better for the borrower?

Yes, very slightly, at the same nominal rate. A 5% Canadian mortgage and a 5% American mortgage with identical principal and amortization produce a Canadian monthly payment that's about 0.3-0.6% lower. Over decades, the savings are real but modest — typically a few thousand dollars on a typical first-home purchase.

Why don't lenders just advertise the effective annual rate?

Some sophisticated comparison tables and disclosure documents do show APR (effective annual rate), but most marketing uses the nominal rate because borrowers shop on it. Canadians intuitively compare "RBC 4.79% vs. TD 4.69%" without converting to effective terms. The semi-annual compounding rule was specifically designed to prevent a race-to-the-bottom on compounding aggressiveness while letting borrowers shop on the nominal number.

Are credit unions and digital lenders held to the same rule?

Yes, the Interest Act applies to all loans secured against Canadian real property, regardless of whether the lender is federally regulated or provincially regulated. Tangerine, EQ Bank, motusbank — all use the same semi-annual compounding.

What about commercial mortgages or HELOCs?

The Interest Act applies broadly, but commercial mortgages, lines of credit, and demand loans sometimes use different terms. HELOCs in particular are variable-rate, demand-style instruments where the daily compounding feels more direct. The semi-annual-not-in-advance rule still applies to the rate quotation; the practical effect is smaller because HELOC balances change daily anyway.

Where can I see the formula my bank is using?

Most major Canadian banks publish their math in PDF disclosure documents available on their websites — RBC's "How Your Mortgage Interest Is Calculated" document is the most cited example. The Bank of Canada also publishes academic-level explanations of mortgage math in its working paper series. Both will confirm the formula matches what's in this article.

Verify against real numbers

Open the calculator, enter $500,000 home price, $0 down (to make the principal $500,000 even), 5.00% rate, 25-year amortization, monthly payments. You should see $2,908.02/month. That's the Canadian-correct number, and it matches what your bank's amortization schedule will say to the cent.

If the calculator you've been using shows something different — usually $2,920-something — you're looking at the American formula. Time to switch.

Educational only — not financial, tax, or legal advice. Rates change; the math here is current as of publication date. Talk to a licensed Canadian mortgage broker or your lender before making decisions based on this content.