Methodology & data sources
Everything on this site runs in your browser. Nothing you type is sent anywhere. This page explains exactly how the numbers are produced so you can check them.
Monthly payment
For an equal-payment (annuity) loan the payment is P × i × (1+i)^n ÷ ((1+i)^n − 1), where P is the loan amount, i the monthly rate and n the number of months. For equal-principal (linear) loans, each month repays P ÷ n plus that month's interest.
The monthly rate is the annual rate ÷ 12, except in Canada, where fixed-rate mortgages compound semi-annually by law: i = (1 + r/2)^(1/6) − 1.
Extra payments reduce the balance immediately and shorten the loan; the scheduled payment stays the same.
Taxes, insurance and upfront costs
- US private mortgage insurance (PMI) is charged while the balance is above 78% of the original home value, the point at which US law requires automatic cancellation.
- Canadian mortgage default insurance premiums (4.00%, 3.10%, 2.80%) are added to the loan when the down payment is under 20%.
- UK Stamp Duty Land Tax (England & Northern Ireland), Irish stamp duty and Singapore Buyer's Stamp Duty are calculated from their official bands. Other purchase costs are shown as editable percentages based on typical published ranges.
- US state property tax defaults are approximate statewide effective rates; your county's rate may differ substantially.
Interest-rate data
- United States: Freddie Mac Primary Mortgage Market Survey (30- and 15-year fixed), via FRED.
- Euro area countries: European Central Bank MFI interest rate statistics — average agreed rate on new loans to households for house purchase.
- United Kingdom: Bank of England quoted rates for 2- and 5-year fixed mortgages at 75% loan-to-value.
- Canada: Bank of Canada, 5-year conventional mortgage (posted rate — most borrowers negotiate lower).
- Other countries: central-bank policy rates from the Bank for International Settlements. Where no mortgage-rate series exists, the calculator starts from the policy rate plus a typical lender margin and labels it as an estimate.
Data is refreshed automatically every week. Averages describe the market, not the rate you will be offered.
Rate scenarios
The "What if interest rates change?" tool only applies when the rate can change: variable loans reset once a year, fixed-period loans reset yearly after the fixed period ends, and fully fixed loans never reset.
You choose a band around today's rate — either a percentage of the rate (±5% of 4.00% is 3.80–4.20%) or percentage points (±5 points of 4.00% is 0–9.00%). Six named scenarios walk the rate to the edges of that band. The likely range comes from 600 simulated paths where the rate takes a random step at each reset, is pulled gently back toward today's rate, and never leaves the band. The payment is recalculated on the remaining balance and remaining term at every reset.
This is an illustration of risk, not a forecast. Nobody can reliably predict interest rates.
Reviews
Country rules (tax bands, lending limits) are reviewed at least once a year and each country lists its sources. If you spot something out of date, please tell us at pentasoul05@gmail.com.