Methodology

How every figure is sourced, computed, and kept current.

This page brings together the formulas, official sources, assumptions and review dates used by the UK calculators.

Calculation basis

Tax bands, thresholds and statutory rates come from the public-authority publications below. Each calculator links its main source beside the method.

Source	Scope	URL
gov.uk — Stamp Duty Land Tax	SDLT rates England & Northern Ireland	https://www.gov.uk/stamp-duty-land-tax/residential-property-rates
Revenue Scotland — LBTT	LBTT rates Scotland	https://revenue.scot/taxes/land-buildings-transaction-tax
GOV.WALES — LTT	LTT rates Wales	https://www.gov.wales/land-transaction-tax-rates-and-bands
HMRC — Income Tax rates	Income Tax bands England, Wales, NI	https://www.gov.uk/government/publications/rates-and-allowances-income-tax/income-tax-rates-and-allowances-current-and-past
Scottish Government — Income Tax	Scottish Income Tax 6-band	https://www.gov.scot/publications/scottish-income-tax-rates-and-bands/
HMRC — National Insurance	NI Primary Threshold, Class 1 rates	https://www.gov.uk/government/publications/rates-and-allowances-national-insurance-contributions
HMRC — Pension tax relief	Relief at source, annual allowance	https://www.gov.uk/tax-on-your-private-pension/pension-tax-relief
Bank of England — Bank Rate	BoE base rate for mortgage reference	https://www.bankofengland.co.uk/monetary-policy/interest-rates-and-bank-rate
Bank of England — Borrowing calculator	Personal-loan formula benchmark	https://www.bankofengland.co.uk/education/education-resources/borrowing-calculator
FCA — Credit-card repayment research	Minimum-payment assumptions and persistent debt	https://www.fca.org.uk/publication/research/research-note-helping-credit-card-users-repay-their-debt-summary-experimental-research.pdf
GOV.UK — VAT rates	UK standard, reduced and zero VAT rates	https://www.gov.uk/vat-rates
PRA / Bank of England — FSCS	£120,000 deposit protection limit	https://www.bankofengland.co.uk/explainers/what-is-the-financial-services-compensation-scheme

Calculator methods and sources

Stamp Duty (SDLT/LBTT/LTT)

Progressive band structure: for price P and bands (b₁, r₁), …, tax = Σ of min(P, bᵢ₊₁) − bᵢ × rᵢ (clamped at 0). First-time buyer relief changes the nil-rate treatment. Additional-property rules are modelled separately for each nation: higher SDLT rates, Scotland's flat-rate ADS, and Wales's separate higher residential LTT bands.

Mortgage monthly payment

M = P × r(1+r)ⁿ / ((1+r)ⁿ − 1), where P = principal,r = monthly rate, n = months. Monthly compounding.

Income Tax

Progressive bands with Personal Allowance taper above £100,000 (reduced £1 per £2 over). Scottish Income Tax uses six taxable bands. NI Class 1 is estimated annually at 8%/2%; actual payroll calculations operate by pay period.

Pension future value

FV = PV(1+r)ⁿ + PMT × ((1+r)ⁿ − 1) / r with monthly compounding.

Personal loan

The same fixed-payment amortisation formula as the mortgage calculator, with the last payment reduced to the remaining balance. Optional extra payments reduce principal after the month's interest is applied.

Credit card repayment

Monthly approximation: opening balance plus entered annual purchase rate ÷ 12 interest and new spending, minus the fixed payment. The comparison minimum is estimated as interest plus 1% of balance or £5, whichever is higher. Use the issuer statement for its daily interest, fees and contractual minimum.

VAT

Add VAT with gross = net × (1 + rate). Extract VAT withnet = gross ÷ (1 + rate); subtracting 20% from a VAT-inclusive amount is not the reverse of adding 20%.

Editable inputs

Interest rates, contribution amounts, growth assumptions, loan terms and transaction values can be replaced with the figures from the relevant quote, statement or planning scenario. Statutory rates remain tied to the tax year shown on the calculator.

Review dates

The UK tax year runs from 6 April to 5 April. Rule files are reviewed after a Budget, enacted tax change or update to a cited authority source. Stable formulas are reviewed when a defect or conflicting benchmark is reported.

Each calculator page displays its "last updated" date in the footer. If you spot a figure that disagrees with the official source, please use theContact page.

Using the estimates

Each calculator describes the scenario it models and the inputs that drive the result. Use the figure for planning, then confirm a transaction or filing against the relevant statement, contract or authority record.