Ten simple money calculations with UK examples: net worth, savings rate, emergency fund, real return, rule of 72, loan cost, debt-to-income, mortgage affordability, retirement savings and the true hourly cost of purchases.
You don't need to be good at maths to manage money well, but a handful of simple calculations make decisions much clearer. Each one below comes with the formula, a UK example, and the kind of calculator that does it for you.
1. Net worth
Net worth = what you own − what you owe. Include savings, investments, pensions and your home's value, minus mortgage, loans and card balances. Example: £15,000 savings + £40,000 pensions + £250,000 home − £180,000 mortgage − £3,000 credit card = £122,000. Track it yearly; the trend matters more than the number.
2. Savings rate
Savings rate = amount saved ÷ take-home pay × 100. Saving £300 from £2,500 a month is 12%. Include pension contributions if you want the full picture.
3. Emergency fund target
Target = essential monthly spending × 3 to 6. With essentials of £1,600 a month, aim for £4,800 to £9,600. Use more months if your income is irregular or you're the only earner.
4. Real return
Real return ≈ interest rate − inflation. Savings earning 4% with 3% inflation give a real return of about 1%. This shows whether your money is actually growing in buying power.
5. The rule of 72
Years to double ≈ 72 ÷ annual return. At 6% a year, money doubles in about 12 years. It works for debt too: a balance at 24% interest doubles in about three years if unpaid.
6. The true cost of a loan
Total cost = monthly payment × number of payments − amount borrowed. A £10,000 loan at 6.9% APR over three years costs about £308 a month, or roughly £1,100 in interest. Compare the total repayable, not just the monthly payment.
7. Debt-to-income ratio
Monthly debt repayments ÷ gross monthly income × 100. Lenders use this to judge affordability. Paying £600 a month on loans and cards from a £3,000 gross income is 20%. Lower is better.
8. Mortgage affordability
Lenders typically lend around four to four and a half times income, sometimes more, after checking spending and stress-testing against higher rates. A household earning £60,000 might borrow around £240,000 to £270,000. A mortgage calculator gives a fuller estimate, including payments at different rates.
9. Retirement savings rule of thumb
One rough guide is that a pension pot can sustainably provide about 4% a year (adjusted for inflation), though UK research suggests a lower rate may be safer, and it depends on investments, fees and life expectancy. On that basis, £10,000 a year of extra income on top of the State Pension would need a pot of roughly £250,000. Use a pension calculator and get guidance from Pension Wise if you're over 50.
10. The hourly cost of a purchase
Price ÷ your take-home hourly pay. If you take home £14 an hour, a £350 gadget costs 25 hours of work. A useful pause before a big purchase.
Bonus: cost per use
Price ÷ expected number of uses. A £120 coat worn 200 times costs 60p per wear; a £40 one worn five times costs £8 per wear.
Calculators that do the work
- Compound interest and savings goal calculators for calculations 4 and 5.
- Loan and credit card repayment calculators for 6 and 7.
- Mortgage calculators for 8.
- Pension calculators for 9, and the GOV.UK State Pension forecast.
- Budget planners for 2 and 3, such as MoneyHelper's.
Whichever calculator you use, check the assumptions: interest rate, inflation, fees and tax all matter.
Frequently asked questions
How often should I do these?
Once a year is enough for most, plus whenever you make a big decision.
Are online calculators accurate?
Only as accurate as their inputs and assumptions. Use them for estimates, not guarantees.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.