Mortgage Compounding Around the World: Why a 6% Loan Doesn't Cost the Same in Canada, the US and the UK

The headline rate is not the price

If you're shopping for a mortgage in a country other than the one you grew up in, the most common and most expensive mistake is to assume that a "6% mortgage" means the same thing in every market. It doesn't. The compounding convention — how often the lender mathematically applies interest to the outstanding balance — varies by country, and that single technical detail moves the effective cost of a loan by tens of thousands over a 30-year term.

This piece walks through the math from first principles, compares the four largest English-speaking mortgage markets, and gives you a small mental model you can apply to any quote.

What "compounding" actually means

Compounding describes how often interest charged earlier in a period is itself charged interest in later periods. Three conventions cover almost every developed-market mortgage:

1. Monthly compounding (US, UK, Australia, eurozone, Japan, India). Interest is calculated every month on the outstanding balance and added to it. The monthly rate is simply annual / 12.
2. Semi-annual compounding (Canada — required by federal statute). Interest is calculated twice a year. The implied monthly rate is mathematically lower than monthly compounding would yield for the same nominal annual rate.
3. Daily compounding (some New Zealand and Australian variable products, certain US lines of credit). Interest is charged on the daily balance; effectively close to but slightly higher than monthly compounding.

For the rest of this article we'll focus on monthly versus semi-annual, because that's where the largest practical difference lies and where the most cross-border confusion happens.

The formula

The textbook amortisation formula for a fixed-rate loan is:

M = P × r(1+r)^n / ((1+r)^n − 1)

where M is the monthly payment, P is the principal, n is the number of monthly payments, and r is the effective rate per month. Everything depends on what you plug in for r.

• Monthly compounding country: r = annual_rate / 12
• Semi-annual compounding country (Canada): the rate per semi-annual period is annual_rate / 2. To get the equivalent monthly rate that produces the same effective annual yield, you take the sixth root of (1 + semi-annual rate):

r = (1 + annual_rate / 2)^(1/6) − 1

That single change — the sixth root — is why a Canadian mortgage at 6% nominal is mathematically cheaper than a US mortgage at 6% nominal.

A worked example: $400,000, 30 years, 6.5%

Run the numbers through both conventions:

United States (monthly compounding). - Monthly rate r = 0.065 / 12 = 0.005417 - Effective annual rate: (1 + 0.005417)^12 − 1 = 6.697% - Monthly payment: $2,528.27 - Total interest over 30 years: $510,177

Canada (semi-annual compounding). - Semi-annual rate: 0.065 / 2 = 0.0325 - Equivalent monthly rate: (1.0325)^(1/6) − 1 = 0.005345 - Effective annual rate: (1.0325)^2 − 1 = 6.606% - Monthly payment: $2,505.61 - Total interest over 30 years: $501,999

Same headline rate. Same principal. Same term. The Canadian borrower pays $22.66 less every month and saves $8,178 in interest across the life of the loan. That is not a rounding error — that is a structural feature of the law that governs how Canadian mortgages must be quoted.

Why does Canada do this?

The federal Interest Act of Canada, Section 6, requires lenders to disclose the compounding period on any mortgage backed by real property. By long convention this is set at semi-annual, and the rate quoted to consumers is the nominal annual rate compounded semi-annually. The intention, when the statute was modernised in the 20th century, was consumer clarity: every Canadian mortgage shopper is comparing the same kind of number.

The unintended side-effect is that Canadian rates look identical to US rates on paper but are mathematically lower in cash terms, because monthly compounding accelerates compounding more aggressively than semi-annual compounding at any positive rate.

A useful rule of thumb

For typical mortgage rates (3–10%):

• The effective annual rate from monthly compounding is about 15–25 basis points higher than the nominal rate.
• The effective annual rate from semi-annual compounding is about 8–13 basis points higher than the nominal.
• That means a Canadian "6.5%" is roughly equivalent to a US "6.4%" once you normalise for compounding.

If you are shopping mortgages across borders, do not compare quoted rates. Compare effective annual rates — your calculator should expose this number, and ours does.

What does *not* differ between countries

A few things people assume vary by country actually don't, mathematically:

• Amortisation formula. Every developed market uses the same M = P·r(1+r)^n / ((1+r)^n − 1). Only r differs.
• Total interest paid as % of principal at a given effective annual rate. Identical across countries holding effective annual constant.
• Sensitivity to rate changes. A 100bps move in effective annual rate moves the monthly payment by roughly 10–12% on a 30-year mortgage in every market.

What *does* differ

These are real differences worth knowing about:

• Term lengths. The 30-year fixed is dominant in the US, rare in Canada (where 5-year fixed renewed every 5 years is standard) and the UK (where 2- and 5-year fixed are most common before reverting to a variable rate).
• Prepayment penalties. Calculated very differently. US conventional mortgages typically have no prepayment penalty; UK fixed deals usually have early-repayment charges of 1–5% during the fix; Canadian fixed mortgages use an "interest rate differential" calculation that can be punishing in falling-rate environments.
• Portability. In the UK and Canada, mortgages can often be "ported" between properties when you move; in the US this is essentially unheard of.

How to use this on your own quote

When a lender hands you a quote in a new country, ask three questions:

1. What compounding period applies to this rate?
2. What is the effective annual rate?
3. What is the cash monthly payment for my principal and term?

If the lender can't answer the first two without checking, you're not yet talking to the right person.

FAQ

Is a Canadian 6% mortgage always cheaper than a US 6% mortgage? For the same principal and term, yes — by roughly 25 basis points of effective annual rate. The difference is small in any single month but compounds across 25–30 years to several thousand dollars.

Does compounding matter for variable-rate mortgages too? Yes. The compounding rule applies regardless of whether the rate is fixed or floating. A Canadian variable mortgage still computes the equivalent monthly rate by the sixth-root formula above.

Why don't all countries use the same convention? History and law. The US locked in monthly compounding when 30-year fixed mortgages became standard mid-century; Canada codified semi-annual in the Interest Act; the UK never legislated a particular convention but the market converged on monthly. Standardisation would help cross-border comparison but is not on any regulatory agenda.

Where does my calculator slot into this? Our mortgage calculator exposes a "Country preset" dropdown. Select Canada and the math uses semi-annual compounding (converted to the equivalent monthly rate); select US/UK/EU/AU/JP/IN and the math uses monthly compounding directly. The effective annual rate is shown beneath the result so you can compare quotes head-to-head.

What about Islamic mortgages, shared-equity schemes, and offset mortgages? Those use different mechanics entirely (profit-sharing rather than interest, equity participation, or offset account structures) and are not covered by the amortisation formula. Treat them as separate products and compare on total cost over a holding period, not on rate.