How we calculate

Every figure comes from GOV.UK, HMRC or another official source, and each one links to the page it came from. We check the rates before each tax year starts on 6 April and after every Budget, and each calculator says when it was last checked (currently 18 October 2026). Every calculator is tested against worked examples done by hand. If we get something wrong, we fix it and record it below.

Our sources

Every rate, threshold and band is kept in one file per tax year, with the official page it came from. Nothing is typed into a calculator by hand.

Income Tax, National Insurance and student loans

• GOV.UK: Rates and thresholds for employers 2026 to 2027
• GOV.UK: Income Tax rates and Personal Allowances
• GOV.UK: Scottish Income Tax
• GOV.UK: National Insurance rates and categories
• GOV.UK: Repaying your student loan

Savings interest and dividends

• GOV.UK: Income Tax rates and allowances, including savings and dividends

Child Benefit charge

• GOV.UK: High Income Child Benefit Charge

Maternity and sick pay

• GOV.UK: Maternity pay and leave
• GOV.UK: Statutory Sick Pay

Capital Gains Tax and VAT

• GOV.UK: Capital Gains Tax rates and allowances
• GOV.UK: VAT rates

Stamp duty, LBTT and LTT

• GOV.UK: Stamp Duty Land Tax residential rates
• Revenue Scotland: LBTT residential rates
• Revenue Scotland: Additional Dwelling Supplement
• Welsh Government: Land Transaction Tax rates

Vehicle tax and company cars

• GOV.UK: Vehicle tax rate tables
• GOV.UK: Rates for cars registered 2001 to 2017
• HMRC: Company car benefit appropriate percentages

Pensions

• GOV.UK: Workplace pensions contributions
• GOV.UK: Tax on your private pension
• GOV.UK: The new State Pension
• FCA: standard projection rates (COBS 13 Annex 2)

Imports: duty, excise and VAT

• GOV.UK: UK Trade Tariff (duty rates by commodity code)
• HMRC: monthly exchange rates
• GOV.UK: VAT on imports
• GOV.UK: Tax and duty on goods sent from abroad
• GOV.UK: Alcohol Duty rates
• GOV.UK: Tobacco Duty rates

Interest rates, inflation, pay, house prices and rents

• Bank of England: quoted household interest rates (September 2026)
• ONS: CPI index, all items (D7BT)
• Bank of England: inflation target and Bank Rate
• ONS: Average weekly earnings growth
• ONS: Private rent and house prices

Figures we fill in for you

Where a calculator needs a figure that is published officially, we use it as the starting value so you don't have to look it up. You can always change it.

• Mortgage rates: 5.13% for a 2-year fix at 75% loan-to-value, 4.98% for a 5-year fix and 6.58% for revert-to rates (Bank of England, September 2026).
• Savings 2.06%, credit cards 24.71% and £10,000 personal loans 6.98% (Bank of England, September 2026).
• Inflation: 3.5% now (ONS CPI, September 2026), the Bank of England's 2% target for the years ahead, and the full ONS CPI index back to 1988 for past values.
• Pay rises 3.7% (ONS, August 2026); house prices 0.9% and rents 3.8% a year (ONS).
• Investment and pension growth: 5%, the FCA's middle projection rate.
• Import duty rates from the UK Trade Tariff and currency conversion at HMRC's monthly exchange rates.

How we work out interest

Loans, credit cards and car finance are quoted as an APR, so we turn it into a monthly rate as (1 + APR) to the power of 1/12, minus 1. Savings and investments use the AER the same way, with deposits at the end of each month. Mortgages use the nominal yearly rate divided by 12, which is how UK lenders work out monthly payments.

How we check

Each calculator is tested against worked examples calculated by hand from the official rules, and every rate change must pass those tests before it goes live. We review all rates before each new tax year on 6 April, after every Budget, and when HMRC, the Bank of England or the ONS publish new figures.

Rounding

We calculate with exact figures and round only the final amounts to the nearest penny, so totals can differ from your payslip by a penny or two.

What we don't cover yet

• Self-employed National Insurance (Class 2 and 4) and tax on investments held outside an ISA or pension.
• Student loan balances, interest and write-off dates.
• Northern Ireland's import rules for goods that could enter the EU, and Small Producer Relief on alcohol.

Changes and corrections

• 18 October 2026: Added the tax code checker and 12 new guides; every figure in them is worked out from the same rates and calculations as the calculators.
• 18 October 2026: Bank of England average rates (September 2026), ONS earnings growth and FCA projection rates are now the starting values in the mortgage, savings, loan, card and pension calculators.
• 18 October 2026: Import calculators look up duty rates in the UK Trade Tariff (19 October 2026) and convert currency at HMRC's rates for 2026-10.
• 18 October 2026: Tobacco Duty uses the rates in force from 1 October 2026; Alcohol Duty the rates from 1 February 2026.
• 18 October 2026: Stamp duty calculator extended to Scotland (LBTT and the Additional Dwelling Supplement) and Wales (LTT).
• 18 October 2026: All 2026/27 tax rates and thresholds checked against GOV.UK and HMRC.

Spotted a mistake? Tell us with the contact form. We fix confirmed errors quickly and record them here.