Financial calculators explained
A money calculator turns a few inputs into a clear answer, but the answer is only as good as those inputs. This guide shows which inputs matter most, with real examples, and how WorthChex tests every figure.
The right tool for the question.
| Your question | Use | Key inputs |
|---|---|---|
| What will I actually get paid? | Take-home pay | Salary, pension method, student loan, region |
| Is this pay rise worth it? | Salary calculator | Current and new salary |
| What will a mortgage cost? | Mortgage | Loan, rate, term, overpayment |
| How much Stamp Duty? | Stamp Duty | Price, nation, buyer type |
| Can I afford this? | Affordability | Income, bills, new payment |
| When will I be debt-free? | Debt payoff | Balance, APR, payment |
| What will a loan or car cost? | Loan / Car finance | Amount, APR, term, deposit, balloon |
| How fast will my savings grow? | Savings / Compound interest | Start, monthly amount, rate, years |
| How much VAT? | VAT | Amount, rate, add or remove |
Small input changes, big differences.
A £250,000 mortgage over 25 years
| Rate | Monthly payment | Total interest |
|---|---|---|
| 3.5% | £1,251.56 | £125,468 |
| 4.5% | £1,389.58 | £166,874 |
| 5.5% | £1,535.22 | £210,566 |
| 6.5% | £1,688.02 | £256,405 |
Each 1 percentage point adds roughly £140–£155 a month. When you use a mortgage calculator, try your quoted rate and a rate 1–2 points higher to see what happens when your deal ends.
Saving £200 a month for 20 years
| Growth rate | Paid in | Final value |
|---|---|---|
| 2% | £48,000 | £58,959 |
| 4% | £48,000 | £73,355 |
| 6% | £48,000 | £92,408 |
For long-term projections, the growth rate is the input that matters most, and the one nobody can know in advance. Try a cautious, middle and hopeful rate rather than relying on one number.
Three things to check.
- Nominal rate or APR? Mortgages quote a yearly interest rate; loans and cards quote APR, which includes fees. WorthChex converts APR to the correct monthly rate, so a 12.68% APR is treated as exactly 1% a month.
- Which tax year and region? Our tax calculators use 2026/27 rates and let you choose Scotland, where Income Tax bands differ.
- What is left out? Calculators cannot see your credit file, a lender's own rules, fees you have not entered or future rate changes. Treat results as a planning estimate.
How WorthChex keeps its figures right.
Every calculator runs on one shared engine, and the same engine produces the figures in our guides. Before any change goes live, automated tests compare the engine with answers worked out by hand from official rates. For example:
| Test | Expected answer |
|---|---|
| Take-home pay on £40,000 (England) | £32,319.60 |
| Income Tax on £40,000 | £5,486.00 |
| Stamp Duty on £300,000 (home mover, England) | £5,000 |
| LBTT on £300,000 (Scotland) | £4,600 |
| £250,000 mortgage, 4.5%, 25 years | £1,389.58 |
Read more about our sources and assumptions on the methodology page.
Calculator FAQs
Why is my payslip different from the calculator?
Payroll works each pay period separately and may include things the calculator does not know about, such as a non-standard tax code, benefits in kind or overtime.
Do you store what I type?
No. The calculations run in your browser and the numbers you enter are not sent to us. See the privacy policy for details on cookies and ads.
Are the results financial advice?
No. They are estimates to help you plan and compare. For decisions like a mortgage or pension transfer, speak to a regulated adviser.
MoneyHelper tools and calculators · GOV.UK Income Tax rates. Figures are worked examples for planning, not financial advice.