UK Calculator Methodology
Last updated 25 July 2026
Every UK calculator on Fiscalgrove is built from the same underlying amortising-loan formula lenders use, plus the specific statutory or lending-convention data relevant to that calculator. This page summarises the formulas and sources; each calculator's own page also explains its formula and worked example in context.
The core mortgage formula
For a standard repayment mortgage, the monthly payment is:M = P × r × (1+r)^n ÷ ((1+r)^n − 1), where P is the mortgage amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments (term in years × 12). For interest-only mortgages, the payment is simply P × r each month, with the full balance still owed at the end of the term. All calculations run on full, unrounded figures — we only round to the nearest penny for display, so amortisation schedules don't drift from cumulative rounding.
Statutory and convention-based figures
- Stamp Duty Land Tax (SDLT) bands, first-time-buyer relief, and the additional-property/non-resident surcharges are sourced directly fromHMRC's published guidance, re-checked at least every three months and after any Budget announcement.
- Income multiples (used in our Affordability calculator) reflect common high-street lending practice (typically 4-4.5x combined income, editable up to around 6x for specialist cases) rather than a single regulatory number — the Bank of England withdrew its mandatory stress-test rate in 2022, so individual lenders now set their own criteria.
- LTV tiers reflect commonly used industry pricing bands (60/75/80/85/90/95%), not one specific lender's criteria.
- Overpayment caps default to 10% of the outstanding balance per year, a common (but product-specific) penalty-free allowance — always check your own mortgage offer document for the exact figure.
Editable defaults
Every default value used in a calculation (income multiple, stress-test margin, overpayment cap, and so on) is visible and editable in the calculator itself — nothing is a hidden assumption. Where a figure is genuinely statutory (like SDLT bands), it isn't editable, since that would produce an inaccurate result.