Finance tools
Mortgages, loans, interest, tax and take-home pay. All in one place.
APR ↔ AER Converter
Convert between APR and AER with different compounding frequencies. Get equivalent monthly and daily rates. Optional simple fee modelling, share link and CSV export.
Open tool →Child Benefit & High Income Charge Calculator
Work out UK Child Benefit for your family and estimate the High Income Child Benefit Charge. Updated thresholds, correct weekly rates for 2024–25 and 2025–26, adjusted net income with pension and Gift Aid, backdating up to 3 months, share link and CSV export.
Open tool →Compound Interest Calculator
Project savings growth with compound interest and monthly contributions. See final balance, total contributions and interest earned. Share link and CSV export.
Open tool →Credit Card Payoff Calculator
Work out how long a credit card will take to clear with minimum payments, a fixed payment, or a solved payment for a target months. See interest, totals and a full schedule. Share link and CSV export.
Open tool →Investment Growth & Inflation Calculator
Free calculator to project investment growth with monthly contributions, fees and inflation. Compare nominal returns with real spending power. Works in GBP, USD, EUR and more.
Open tool →ISA Allowance Splitter
Get a suggested monthly split across Cash ISA, Stocks & Shares ISA and Lifetime ISA that respects the £20,000 annual limit and the £4,000 LISA cap. Auto months left to 5 April, warnings, share link and CSV export.
Open tool →ISA Contribution Tracker
Track ISA contributions for the current UK tax year. See remaining allowance, projected totals, daily target to year end and flexible ISA replacement capacity. Share link and CSV export.
Open tool →ISA Growth Calculator
Project an ISA balance with monthly contributions. Separate growth from fees, choose contribution timing, add a fixed fee and see a real-terms view after inflation. Share link and CSV export.
Open tool →ISA Transfer Benefit Calculator
Compare a Cash ISA to a Stocks & Shares ISA. Include fees, growth, contribution timing and inflation. See final balances, fees paid, break-even month and a clear better-by figure. Share link and CSV export.
Open tool →Jobseeker’s Allowance Calculator (New Style & legacy)
Work out New Style Jobseeker’s Allowance and see how earnings and pensions reduce it. Includes a legacy income-based JSA view for existing claims, a savings scale with tariff income, Universal Credit capital rules, share link and CSV export.
Open tool →Junior ISA Calculator
Project a Junior ISA to age 18. Cap to the £9,000 yearly limit, model monthly and lump contributions, growth and fees, and see a real-terms figure. Clear reminder: the money is the child’s, not the parents’.
Open tool →Lifetime ISA (LISA) Bonus Calculator
Work out Lifetime ISA growth and the 25% government bonus. Cap to the £4,000 yearly limit, model monthly and lump contributions, growth, fees and inflation. Includes an early withdrawal charge estimator, share link and CSV export.
Open tool →Loan Comparison Calculator
Compare two loans side by side. See monthly repayments, total interest, payoff time and savings. Supports extra monthly payments, share links and CSV export.
Open tool →Loan Repayment Calculator
Calculate monthly repayments, total interest and a full amortisation schedule. Supports extra monthly payments. Clean, quick and accurate.
Open tool →Mortgage Interest vs Inflation Impact
See how inflation changes the real cost of a mortgage. Compare nominal repayments with real payments in today’s money, track outstanding balance, and export a yearly schedule. Supports GBP, USD, EUR and more.
Open tool →Mortgage Overpayment Calculator
Work out mortgage repayments, see how extra monthly overpayments, one-off lump sums and annual top-ups reduce the term and interest. Clear schedule, share link and CSV export.
Open tool →Mortgage Refinance Calculator
Compare your current mortgage to a new deal. See new repayments, monthly saving, break-even time, total interest saved and NPV after fees. Share link and CSV export.
Open tool →Pension Pot Real Value Estimator
Estimate how your pension pot could grow, then see the real value after inflation. Model monthly or annual contributions, fees, drawdown income, optional 25% lump sum, and annuity comparisons. Multi-currency with clear visuals and CSV export.
Open tool →Pension vs ISA Calculator
Compare pension contributions against investing the same net monthly budget in a Stocks and Shares ISA. Model salary sacrifice, net pay or relief at source, employer match, fees, growth, inflation and contribution timing. See effective uplift, break-even ISA growth and full schedules.
Open tool →Savings Goal Calculator
Work out how much to save each month to reach a target, or how long it will take with your current monthly amount. Supports monthly compounding, start or end of month contributions, annual uplift, share link and CSV export.
Open tool →Tax-Free Childcare Calculator
Work out Tax-Free Childcare top-ups per child, check basic eligibility and compare against legacy Childcare Vouchers. Edit caps, rates and costs. Monthly and yearly view, share link and CSV export.
Open tool →UK Inflation Calculator (CPI)
Convert money between years using UK CPI. Supports monthly or annual indices from a JSON file, or a custom cumulative rate. Share link and CSV export.
Open tool →UK Take-Home Pay Calculator
Best-in-class UK take-home pay calculator. Supports Scotland and rUK tax bands, emergency tax (Month 1), salary sacrifice, net pay and relief-at-source pensions, student loans (Plan 1/2/4/5 + PG), NI, benefits in kind and presets. Breakdown per month, 4-weekly, weekly and daily. Share link and CSV.
Open tool →Make finance maths simple
This section brings together the calculators people use to make real money decisions. Mortgages, loan repayments, APR and AER, compound interest, overpayments, savings goals, ISA growth, pensions, salary to take-home and day-rate to salary are all covered with clean inputs and results that are easy to copy. If you just need the number, enter your figures and you are done. If you want to understand the method, each page includes a short explainer and a worked example in plain English.
Accuracy and speed come first. Calculations are shown clearly, with sensible defaults and rounding where it helps. Layout stays consistent across devices and there is no clutter. More tools will be added based on demand and search behaviour, including UK-specific tax years and thresholds, plus optional extras like fees and overpayment schedules where useful.
Bookmark this hub if you compare mortgage options, plan savings, check investment growth or sense-check payday figures. Stable Numbers is a practical resource designed to save time, reduce errors and give you confidence in the result. If there is a finance calculator you would like to see, send a request and it will be prioritised by demand.