Last updated · 20 April 2026

Methodology

This page explains how the calculators and tools on MJBurrows are built — what data they use, what assumptions they make, how they’re validated, what they can’t do, and how their methodology evolves over time. If you’ve ever used a financial calculator and wondered “where does this number actually come from?”, this is the answer for everything on this site. For the broader editorial framework, see the Editorial Policy.

Universal principles

Three rules apply to every calculator on the site:

Primary sources only. Every threshold, rate, and rule used comes from a primary government source (HMRC, gov.uk, Bank of England, ONS) — never from a secondary aggregator or competitor’s calculator.
Conservative defaults. Where an assumption must be made (a long-term growth rate, an inflation projection), the assumption is explicit, conservative, and documented.
Estimates, not predictions. Every output is an estimate based on the inputs you provide and the rules in force on the day of calculation. Outputs are not predictions, guarantees, or personalised financial recommendations.

Calculator privacy

The calculators on this site are entirely client-side. The numbers you enter — your salary, your estate value, your pension contributions, your gains — are processed in your browser and are not sent to any server, not stored, and not logged.

The site has no record of what you input. Site analytics do not capture calculator input values. Whatever you put in stays in your browser.

This applies to every calculator on the site without exception.

Data sources

The data behind the calcs comes from a small set of authoritative UK sources:

HMRC — tax thresholds, rates, allowances, and the rules behind them; pension annual allowance and tapered allowance; ISA allowance limits; CGT exempt amount and rates; IHT thresholds and reliefs; Stamp Duty Land Tax bands
gov.uk — statutory amounts, official guidance, benefit thresholds
Bank of England — Bank Rate (the reference rate behind mortgage impact calculations)
Office for National Statistics (ONS) — Consumer Price Index (CPI) for inflation calculations
Department for Education / Student Loans Company — Plan 1, Plan 2, Plan 4, and Plan 5 thresholds and rates

Where a third-party data feed provides live market data — Twelve Data, Yahoo Finance, and CoinGecko power the ticker and asset-price feeds — attribution is given in the site footer.

How each category works

The site’s 20 calculators sit in five categories. Below is what each category contains, the methodology that applies, and the known edge cases for that category.

Mortgages & Property

Mortgage Repayment Calculator — monthly repayment from loan amount, interest rate, and term
Remortgage Comparison Calculator — current vs new mortgage cost over the new term
BOE Rate Impact Calculator — payment change projected from a Bank of England Bank Rate movement
UK Stamp Duty Calculator — HMRC SDLT bands for England and Northern Ireland; first-time-buyer relief and second-home surcharge applied where relevant
Buy-to-Let ROI Calculator — gross rental yield, net yield, and total ROI from rental income, costs, and capital growth

Methodology Mortgage payments use the standard amortisation formula: P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]. Stamp Duty applies HMRC’s published SDLT bands directly. Buy-to-Let yield uses standard rental-yield formulas with all costs the user supplies (mortgage interest, maintenance, voids, management fees).

Edge cases Mortgage Repayment assumes standard repayment or interest-only as the user selects; offset, part-and-part, and lifetime mortgages are not modelled. Stamp Duty for Scotland (Land and Buildings Transaction Tax / LBTT) and Wales (Land Transaction Tax / LTT) are not covered — the calculator is England and Northern Ireland only.

Investing & Growth

Compound Interest Calculator — future value from initial capital, monthly contributions, growth rate, time horizon
Dividend Income Calculator — annual dividend income, yield, and post-tax income; applies UK dividend allowance and dividend tax rates
ISA vs SIPP Calculator — projected after-tax outcomes from comparable contributions to each account type

Methodology Compound growth uses the standard future-value formula with monthly contribution streams. Dividend tax applies HMRC’s £500 dividend allowance and the relevant dividend tax bands. ISA vs SIPP applies tax relief on SIPP contributions, projects both pots to a common time horizon, then applies tax-free withdrawal for ISA and tax-on-withdrawal (assuming basic-rate retirement income unless specified) for SIPP.

Edge cases Compound Interest uses monthly compounding by default; annual or quarterly compounding can be approximated by adjusting the contribution frequency. ISA vs SIPP assumes basic-rate income tax in retirement for SIPP withdrawals — users expecting higher-rate retirement income should adjust the assumption manually. The calc does not model lifetime allowance or annual allowance breaches.

Retirement Planning

Retirement Pension Calculator — pension pot projection from contributions, employer match, and growth assumptions
Pension Drawdown Calculator — drawdown sustainability under different withdrawal rates
Pension Annual Allowance Calculator — HMRC tapered annual allowance for high earners (£60,000 standard, taper for adjusted income above £260,000)
Salary Sacrifice vs Standard Pension Calculator — net cost comparison of contributing via salary sacrifice vs after-tax

Methodology Pension projections use compound growth on contributions. Annual Allowance applies HMRC’s published taper rules for the current tax year. Salary sacrifice comparisons factor in employee NI savings (12% basic / 2% above the upper earnings limit) and employer NI on the sacrificed amount.

Edge cases Drawdown calc assumes a fixed withdrawal rate; variable strategies (4% rule with adjustments, dynamic drawdown) are not modelled. Annual Allowance handles the standard taper but does not model Money Purchase Annual Allowance (MPAA, £10,000) or carry-forward across previous years. Salary Sacrifice does not model employer schemes that re-invest NI savings into the pension.

Tax, Income & Inflation

UK Take-Home Pay Calculator — Income Tax bands, NI thresholds, student loan deductions, pension contributions; supports England/Wales/Northern Ireland and Scottish bands
Inflation Purchasing Power Calculator — ONS CPI data to show real value of money over time
Marriage Allowance Calculator — tax saving from transferring 10% of personal allowance between spouses
Savings Interest Tax Calculator — Personal Savings Allowance (£1,000 basic / £500 higher / £0 additional rate) applied to interest income
Student Loan Repayment Calculator — Plan 1, Plan 2, Plan 4, and Plan 5 thresholds and repayment rates
Employer NI Calculator — employer NI contributions on employee salaries using the secondary threshold and rates

Methodology All Income Tax calcs use the current UK tax year (2026/27) HMRC published bands. Scottish bands are applied where the user selects Scotland as the location. Student loan plans are applied based on user-selected plan type. Inflation calcs use ONS CPI data, refreshed periodically.

Edge cases Take-Home Pay does not handle split-year residency, K-codes for tax debts, or unusual tax codes (BR, 0T, etc.). Inflation calc uses CPI as default; CPIH and RPI would give different results. Marriage Allowance assumes both partners are basic-rate taxpayers — the transfer is invalid if either crosses into higher-rate income.

Wealth & Estate Planning

Capital Gains Tax Calculator — annual CGT exempt amount (£3,000 in 2026/27), basic-rate vs higher-rate CGT rates, residential property surcharge where relevant
Inheritance Tax Calculator — Nil-Rate Band (£325,000), Residence Nil-Rate Band (£175,000) with taper, spouse/civil partner transfer, charity exemption, charity rate discount (40% → 36%), 7-year gift taper

Methodology CGT applies HMRC’s published rates and the current annual exempt amount. IHT applies the full HMRC framework: NRB, RNRB with taper for estates above £2m, spouse exemption, the 36% charity discount where 10%+ of the estate is gifted to charity, and the 7-year gift taper (0% < 3yr, 20% 3–4yr, 40% 4–5yr, 60% 5–6yr, 80% 6–7yr).

Edge cases CGT does not model Business Asset Disposal Relief (formerly Entrepreneurs’ Relief) at the reduced 10% rate, or Investors’ Relief. IHT does not fully model Business Property Relief (BPR), Agricultural Property Relief (APR), trust assets, or non-UK domicile complexity. For estates with these features, professional advice is essential.

Tax-year handling

Tax-related calculators reflect the current UK tax year, which runs from 6 April to 5 April the following year. The site is currently on the 2026/27 tax year.

All HMRC rates and thresholds are updated for the new tax year as soon as HMRC publishes them. Where rates change mid-cycle (Budget announcements, Autumn Statement adjustments), calculators are updated within five working days of the official announcement.

Modelling for previous tax years is not currently supported by default but can be added on request.

Validation against HMRC examples

Every tax-related calculator is validated against worked examples published by HMRC or in official guidance. Where HMRC publishes a worked example for a specific tax scenario (for instance, Income Tax on a £50,000 salary), the calculator is run with those inputs and the output cross-checked against HMRC’s published answer.

For calculators without an official worked example (compound interest, mortgage amortisation), validation uses peer-reviewed financial mathematics against multiple independent reference sources.

If validation reveals a discrepancy, the calculator is fixed before re-publication and a note added to the change log on the affected page.

Known limitations

Beyond the per-category edge cases listed above, the calculators share these structural limitations:

Personal circumstances are not modelled. A calculator can’t know about your bonus structure, your spouse’s income, your specific mortgage terms, your employer’s pension matching scheme, or any individual edge case. Outputs are illustrative based on the inputs you provide.
Future projections depend entirely on assumptions. Where a calc projects a future value (pension projections, compound interest, mortgage costs over a long term), the result is sensitive to the assumed growth rate or interest rate. Actual returns and rates will differ.
Tax rules change. The calculation reflects the rules in force on the day. If you’re modelling a future year, the rules may be different by then.
Rounding. Outputs are typically rounded to whole pounds for readability. The underlying calculation is held to higher precision.
UK only. All calcs assume UK residency and UK tax rules. They do not handle non-resident, dual-resident, or overseas-asset scenarios.

For situations where these limitations matter to your decision, this is exactly the territory where professional advice is the right step. The site does not provide advice. See the Editorial Policy → Never Advice for more.

Update cadence

Calculators are reviewed for accuracy on a rolling schedule:

Quarterly review of all calcs — thresholds, rates, formulas, and validation re-checked
Event-driven updates when HMRC publishes new rates (Budget, Autumn Statement, new tax year on 6 April)
Five-working-day target for deploying rate changes after the official announcement
“Last reviewed” date on each calculator reflects the most recent honest review — not just a marketing stamp

Methodology change log

When the methodology behind a calculator changes — not just the data inputs (rates, thresholds), but the actual logic, formula, or assumption — the change is documented here.

A change of inputs (e.g. HMRC publishing new tax-year rates) is a routine update, handled per the Update cadence above. A change of methodology is something different: a recalibration of approach. Examples include:

Adopting a new HMRC formula for an existing calculation
Switching the underlying inflation index used (CPI to CPIH, etc.)
Adjusting how a tapered allowance is modelled
Adding or removing a calculation step

Each change is logged with: date, calculator(s) affected, what changed, why, and the source for the new approach.

As of 20 April 2026, no methodological changes have been logged. Future changes will appear here.

Reporting calculation errors

If you spot a calculator producing an incorrect output — or you can show that the calculator’s logic doesn’t match HMRC’s published method — please email contact@mjburrows.com with:

The calculator name and URL
The input values you used
The output you got
The output you expected, with reasoning or source

The full corrections workflow is documented on the Corrections page.

Contact

For methodology enquiries — questions about how a specific calculator works, suggestions for new tools, or requests for additional functionality — the address is contact@mjburrows.com.