How we calculate
Every figure on this site comes from the same tested calculation engine, and every assumption is stated next to the result it affects.
The approach
Each calculator compares two choices with the same money over the same period. Whichever choice costs less at any point is assumed to invest the difference, so neither side is given money the other does not have. The result is expressed as a break-even: the value of the one unknown, such as price growth or investment return, at which both choices finish exactly level.
Break-evens are found by searching for that value and then checked by putting it back into the calculation. Where no realistic value makes one choice win, the calculator says so in words rather than printing an implausible number.
Standard formulas
- Loans and mortgages use standard amortisation at a fixed rate, repaid monthly.
- Quoted loan rates are treated as nominal annual rates compounded monthly, which is how lenders quote them. Investment returns are treated as the return actually earned over a year.
- All figures are nominal, not adjusted for inflation.
Each calculator page sets out its own assumptions in full, under "How it is calculated".
Testing
The calculation engine has an automated test suite that runs before every change goes live. It covers edge cases such as zero interest, zero deposits, loans that outlast the comparison and every input at its minimum and maximum, and it checks identities that must always hold: that every breakdown adds up to its total, that both choices start level, and that each break-even really does make the two choices equal. A separate set of browser checks confirms that what is shown on screen matches the engine.
Figures quoted in the explanations and guides are not typed in by hand. They are calculated by the same engine when the site is built, so the text cannot drift from the calculators.
What is left out
Tax, transaction costs such as stamp duty and agent fees, insurance and lender fees are not included, because they differ so much between countries, states and individuals that any one set of rules would make the results wrong for most readers. Risk is not modelled either. Each calculator states what it leaves out and which way that usually moves the answer.
Independence
The calculators do not recommend products, and no lender, broker or agent pays for placement or influences the results. If the site carries advertising, it is shown separately from the calculators and does not change any figure.
Reviews and corrections
Each calculator shows the date its figures and explanation were last reviewed. If you think a result is wrong, please send the link from your address bar through the feedback form: it contains every input, so the exact scenario can be checked. Confirmed errors are fixed and the review date updated.
Not advice
The calculators do arithmetic with the assumptions you choose. They do not know your circumstances and are not financial, tax or legal advice. See the terms of use.