How It Works

Kleinfeld Mortgage Calculators provides 24 free calculators covering mortgages, property and money, all updated for the 2026/27 UK tax year. Here is how they work, the rates and formulas behind them, and how to get the most accurate results.

All Calculations Happen in Your Browser

When you enter figures and press Calculate, the result is computed instantly using JavaScript running directly in your web browser. Your financial data never leaves your device — we do not receive, log or store any numbers you enter. This means your income, mortgage balance, or savings figures stay completely private.

We Use Published UK Rates

Our calculators are built around rates and thresholds published by official UK sources, including:

HMRC — Income Tax bands, Personal Allowance, National Insurance rates, and Capital Gains Tax rates and allowances
HM Treasury / HMRC — Stamp Duty Land Tax (SDLT) thresholds, including first-time buyer relief and the additional property surcharge
Scottish Government — Scottish Income Tax bands and rates
Student Loans Company — student loan repayment thresholds (Plans 1, 2, 4, 5)
Government-set allowances — the Lifetime ISA annual limit and government bonus rate

We review these figures at the start of each tax year and following Budget or Autumn Statement announcements where rates change.

Mortgage Calculators

Our repayment-mortgage calculations use the standard amortising-loan formula used by UK lenders:

Monthly payment = P × (r × (1 + r)ⁿ) ÷ ((1 + r)ⁿ − 1)

Where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (term in years × 12). Interest-only calculations simply apply the monthly rate to the outstanding balance, with the full capital still owed at the end of the term.

Our Mortgage Affordability calculator applies typical UK lender income multiples (around 4× to 5× income, depending on employment type and whether it is a joint application) and a standard stress-test rate, similar to the affordability checks lenders themselves apply. Our Buy to Let calculator applies the Interest Cover Ratio (ICR) stress test method most landlord lenders use, along with the Section 24 tax-credit rules that have applied to mortgage interest relief since 2020. Actual lending decisions always depend on a full application, credit assessment, and the specific lender’s current criteria.

Income Tax and Salary Calculators

Our Income Tax and Salary calculators apply the standard PAYE method for 2026/27:

Personal Allowance: the first £12,570 of income is tax-free. This reduces by £1 for every £2 of income above £100,000, disappearing entirely at £125,140.
Basic rate (20%): income from £12,571 to £50,270.
Higher rate (40%): income from £50,271 to £125,140.
Additional rate (45%): income above £125,140.
Scotland: Scotland uses six separate bands and rates (Starter, Basic, Intermediate, Higher, Advanced, Top) — tick “Scotland” on the relevant calculator to apply these instead.

National Insurance (Class 1, employee): no NI on the first £12,570; 8% on income between £12,570 and £50,270; 2% on income above £50,270.

Our Salary Calculator also supports student loan repayments (Plans 1, 2, 4, 5) and pension contributions, calculated on the correct basis for each deduction.

Stamp Duty, Property and Rental Calculators

Our Stamp Duty Calculator applies the current banded SDLT rates for England and Northern Ireland, including 0% first-time buyer relief up to £300,000 (with partial relief up to £500,000) and the additional-property surcharge for buy-to-let and second homes. Our Property Value calculator projects growth using regional house-price trend averages as an illustrative proxy — actual property values depend on a professional valuation. Our Rental Yield calculator works out gross and net yield, cash flow, and cash-on-cash return from the figures you enter.

Lifetime ISA, Life Insurance and Currency Converter

Our Lifetime ISA calculator applies the current £4,000 annual contribution limit and 25% government bonus, projecting growth to age 50 (the LISA deadline for the bonus). Our Life Insurance and Mortgage Protection Insurance calculators estimate premiums using published UK insurer actuarial averages by age, term and health status — actual premiums are set after full medical underwriting. Our Currency Converter fetches live, mid-market exchange rates from a third-party rate provider each time you use it.

Tips for Getting Accurate Results

Use gross figures (before tax and deductions) when a calculator asks for income or salary.
Select Scotland on the Income Tax or Salary calculator if you pay Scottish Income Tax.
Use your actual mortgage rate and term where you have an offer or existing deal, rather than a rough guess, for the most useful comparison.
Check your student loan plan type (1, 2, 4 or 5) if you are using the Salary Calculator with student loan repayments switched on.
Enter your actual pension contribution percentage rather than the statutory minimum, if you contribute more than the default.

Update Schedule

The 2026/27 UK tax year runs from 6 April 2026 to 5 April 2027. We review and update the rates behind our calculators: at the start of each new tax year; following each Budget or Autumn Statement where relevant rates change; and when we are made aware of an error via our Contact page.

Limitations

Our calculators are built for standard, common scenarios. They may not fully capture: non-standard tax codes; multiple concurrent jobs; salary sacrifice arrangements; benefits in kind (company car, private medical, etc.); self-employed income and the different NI rules that apply; or rental, dividend or investment income on top of employment income. For anything outside a standard scenario, please consult HMRC directly or a qualified accountant, and always confirm any mortgage or insurance figures with a regulated adviser or lender.

Feedback

Spotted an error or have a suggestion for a new calculator? Please let us know via our Contact page.