Methodology

The core formula

Monthly principal and interest uses the standard fixed-rate amortization formula, with no adjustments or fudge factors:

P = L × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]

L = loan amount (price − down payment)
r = annual rate ÷ 100 ÷ 12   (monthly rate)
n = term in years × 12       (number of payments)

When the rate is zero the formula degenerates, so we fall back to L ÷ n. The amortization schedule shown on the calculator is built by running that payment month by month against a declining balance, then aggregating to years — it is not an approximation curve.

Your total monthly figure is then: principal and interest, plus one twelfth of the annual property levy, plus one twelfth of annual home insurance, plus any monthly HOA, strata, or service charge you enter.

Affordability

The affordability read uses the 28/36 front-end debt-to-income rule of thumb that lenders have used industry-wide for decades — not a proprietary score. If your total monthly housing payment is at or under 28% of gross monthly income we show it as comfortable, up to 36% as caution, and above that as a stretch.

If you don't enter an income, we return nothing rather than guessing at one. The salary-to-price tables on US state pages invert the same 28% rule against that state's real property-tax rate, assuming 20% down — and they exclude insurance and HOA, because those genuinely vary by property rather than by state and we won't invent a number for them.

Where the default rates come from

Every default rate is a real published figure with a named source. You can and should overwrite it with your own quote — the default exists so the calculator opens with something honest, not so you'll treat it as your rate.

US and Canadian rates are refreshed from free, keyless public data endpoints (FRED and the Bank of Canada Valet API respectively). Australian and UK figures are updated manually, because the RBA and Bank of England publish theirs as spreadsheets and form-driven database queries rather than as simple feeds. That means those two can lag — the date above each figure is the honest answer to how current it is.

Where the property-tax rates come from

US state pages start from a real effective property-tax rate for that state — 50 states in the dataset, on an effective-rate basis (tax actually paid as a share of home value on owner-occupied housing), 2024 effective rates (retrieved 2026-07-19). Source: https://www.rocketmortgage.com/learn/property-taxes-by-state.

This is the figure that makes a Texas calculation genuinely differ from a Hawaii one rather than being the same page with a different name on it. But it is a state-level average: counties, municipalities, and school districts set actual millage, so your bill can sit well either side of it. Every page that uses it says so, and the field is editable — check your county assessor and enter the real number.

Outside the US we do not apply a percentage-based property tax at all, because it would be wrong. UK Council Tax is a banded assessment and Australian council rates are largely land-value based; neither is a percentage of purchase price. Both are modeled as a flat annual amount you enter yourself.

Country-specific mechanics we actually model

The four supported countries are not the US calculator with the currency symbol swapped:

• Canada — CMHC default insurance. Calculated exactly against CMHC's published tiered premium schedule, which applies below 20% down. We also enforce the real rule that CMHC insurance is unavailable at a purchase price of $1,000,000 or more, so 20% down becomes mandatory.
• Canada — term vs amortization. Canadian mortgages default to a 5-year term inside a 25-year amortization, and the calculator reflects that rather than assuming a US-style 30-year fixed.
• Australia — Lenders Mortgage Insurance. LMI is real and usually applies below 20% deposit, but the premium is insurer-specific and genuinely too variable to compute honestly. We flag that it will likely apply instead of inventing an amount.
• UK — fixed-rate reversion. The default is a 2-year fixed that reverts to the lender's standard variable rate afterwards, which is how UK products actually behave.

What we deliberately don't estimate

A calculator can always produce a number. The question is whether that number means anything. We leave these to you rather than fabricate a default:

• Home insurance premiums — driven by the specific property, its construction, and claims history.
• HOA, strata, and service charges — set per development, with no meaningful regional average.
• Australian LMI premiums — insurer-specific, as above.
• Per-city figures of any kind. We generate pages down to the level where the numbers genuinely differ, and no further.
• Your actual rate. We are not a lender and have no pricing engine; only a lender can quote you.

Everything runs in your browser

All calculation happens client-side. The numbers you enter — price, deposit, rate, income — are never transmitted to or stored on our servers. There is no account, no lead form, and nothing to opt out of, because there is nothing collected in the first place. See our privacy policy for the full detail.

Corrections

If a figure looks wrong for your area, tell us and we'll check it against the source and fix it — see contact and our editorial policy. Our estimates are for planning and comparison only and are not financial advice; please read the disclaimer.