Methodology: how the calculators work
This page shows the working behind the calculators: what each one includes, what it leaves out, and where simplified assumptions are used.
Reviewed: 19 May 2026. Tax-sensitive assumptions are checked against GOV.UK and other official source pages for the 2026/27 tax year.
What the calculators are for
The calculators are designed for planning and comparison; useful for questions like “what happens if I contribute more?” or “which option looks stronger if I change the return assumption?”. They are not built to replace regulated financial advice, tax advice or provider-specific illustrations.
Core compounding calculators
The Investment, Savings and Retirement calculators use standard monthly compounding. Contributions are added regularly and growth is applied based on the annual return or interest rate you enter.
They do not guarantee returns. They also do not automatically deduct platform fees, fund fees, inflation, taxes or behavioural changes unless the page specifically asks for those inputs.
Emergency fund calculator
The Emergency Fund calculator multiplies your monthly essential expenses by your chosen target number of months. It is intentionally simple because emergency fund planning is about resilience, not precision.
MoneyHelper’s general rule of thumb is three to six months of essential outgoings in instant access savings, but the calculator lets users choose a different target based on income stability, dependants and personal circumstances.
Drawdown calculator
The Drawdown calculator projects a pot forward month by month while withdrawals are taken. It assumes a steady annual return, which is a major simplification. Real retirement drawdown is affected by market volatility, inflation, tax, fees, changing spending and sequence risk.
Use it as a stress-test tool. If a retirement plan only works when you enter a high return and a low withdrawal, that is a useful warning rather than a prediction.
Salary sacrifice calculator
The Salary Sacrifice calculator estimates the take-home pay change from sacrificing salary into a pension. It models Income Tax, employee Class 1 National Insurance, the Personal Allowance taper above £100,000, employer National Insurance and optional employer NI give-back.
It currently uses England, Wales and Northern Ireland Income Tax assumptions. Scottish income tax bands can differ. It does not model student loans, child benefit tapering, every employment benefit, mortgage lender treatment, scheme-specific rules or future tax changes after the review date.
Current assumptions include Personal Allowance of £12,570, the higher-rate threshold at £50,270, additional rate above £125,140, employee NI at 8% then 2%, employer NI at 15%, and a standard pension annual allowance of £60,000 for most people.
ISA vs pension calculator
The ISA vs Pension calculator starts with a monthly amount the user could afford from take-home pay. The ISA route invests that amount directly. The pension route estimates the gross pension contribution that could be funded for the same take-home cost, using either pension tax relief or salary sacrifice.
For pension withdrawals, it applies a simplified retirement-tax estimate: 25% of the pension pot is treated as tax-free and the remaining 75% is taxed at the rate selected by the user. This is deliberately simple, because real retirement withdrawals depend on other income, allowances, timing and future tax rules.
The calculator flags the current £20,000 ISA allowance, the standard £60,000 pension annual allowance for most people, and the fact that high earners may need to consider tapered annual allowance rules.
Mortgage vs invest calculator
This calculator compares two routes: overpaying a repayment mortgage, or investing the same spare cash from day one. It compares the end value under the assumptions entered by the user.
The investment side can be modelled as ISA, pension/SIPP or GIA. ISA growth is treated as tax-free. Pension/SIPP contributions are grossed up based on the selected marginal tax assumption, and retirement withdrawals are simplified as 25% tax-free and 75% taxable. GIA results include a simplified dividend-tax drag and Capital Gains Tax allowance treatment.
It does not model early repayment charges, product fees, remortgaging, changing rates, platform fees, adviser fees, inflation, sequence risk or changes in tax rates over the full period.
How to read a result
A calculator result is not a verdict. It is a scenario. The useful question is often not “what number did it produce?” but “what changes the answer?”. Adjust growth rates, timeframes, contribution levels and tax wrappers until you can see which assumption is doing the work.
Current UK source links
- GOV.UK Income Tax rates and Personal Allowances
- GOV.UK National Insurance rates
- GOV.UK Individual Savings Accounts
- GOV.UK dividend tax rates
- GOV.UK Capital Gains Tax rates and allowances
- GOV.UK pension scheme rates
- GOV.UK salary sacrifice changes from April 2029
Known limits
All calculators simplify reality. This site deliberately favours understandable assumptions over false precision. If a decision involves large sums, tax complexity, debt, pension access, benefits, inheritance planning or regulated investment advice, use the calculator as a starting point and consider speaking to a qualified professional.