Money decisions made clear.
Calculate savings growth, compare debt payoff strategies, and understand what compound interest really means for your financial future — with formulas, examples, and decision checklists.
How much will my savings be worth in 10 years?
Which debt should I pay off first?
How long will it take to clear my credit card?
How much do I need to save monthly for retirement?
Is daily compounding meaningfully better than monthly?
What happens if I increase my repayment by £100/month?
Recommended tools
Calculators for this decision
Compound Interest Calculator
Project growth from principal, contribution, rate, and compounding frequency.
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How interest-on-interest grows savings or debt over time at different compounding frequencies.
Savings Goal Calculator
Calculate monthly savings needed to reach a target by your deadline.
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How much to save monthly to reach a specific target by a specific date.
Credit Card Payoff Calculator
Find payoff time, interest cost, and better monthly payment targets for card debt.
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How long to clear a card balance and the total interest paid under different monthly payments.
Debt Snowball Calculator
Plan debt payoff from smallest to largest balance with motivational milestones.
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Which debt to pay first and how long until you are debt free using the snowball method.
Debt Avalanche Calculator
Plan debt payoff from highest interest rate first to reduce total interest.
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Total interest saved using the avalanche method versus paying minimums or snowball.
Retirement Savings Calculator
Estimate retirement savings progress and required monthly contributions.
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How much a retirement pot grows with regular contributions and different return rates.
Practical guide
How to use a financial calculator before making a money decision
A financial calculator does not replace a financial plan — it informs one. The value of a compound interest or savings calculator is not the number it produces, but the scenarios it lets you test. A saving rate that looks insufficient at 3% return looks very different at 6%. A credit card balance that takes 11 years to clear at the minimum payment can be cleared in 3 years with a modest extra monthly payment.
Before using any money calculator, gather real inputs: your actual balance, your actual interest rate from your statement, and your actual monthly contribution amount. Rough estimates produce rough results. If your credit card statement shows 23.9% APR, enter that rate. If your savings account pays 4.75% AER compounded monthly, enter that.
Always run at least two scenarios: a conservative case (lower return, higher rate, smaller contribution) and a realistic case. The gap between them shows how sensitive the decision is to one variable. If the result looks good in both scenarios, the plan is more robust. If it only works in the optimistic case, the risk is higher than it looks.
For large decisions — remortgaging, paying off a large debt early, setting a retirement contribution — use the calculator to get directionally correct, then verify the final numbers with your bank, pension provider, or a qualified financial adviser. Tax treatment, early repayment charges, and benefit interactions are not captured in a standard calculator.
Worked example
See it in action
Scenario: Paying off a £5,000 credit card at 22% APR — minimum vs. fixed payment
- 1Minimum payment strategy: Paying 2% of the balance (≈£100/month initially) takes approximately 35 years and costs over £7,500 in interest at 22% APR.
- 2Fixed £200/month payment: A fixed £200 monthly payment clears the same balance in about 28 months and costs approximately £830 in interest.
- 3Fixed £300/month payment: Increasing to £300/month clears the balance in about 18 months with approximately £510 in interest.
- 4The difference: Going from minimum to £300/month saves over £7,000 in interest and 33 years of repayment time — for an extra £200/month.
Result
On a £5,000 balance at 22% APR: minimum payments cost 35 years and £7,500+ in interest. A fixed £300/month costs 18 months and £510. The decision to pay more is clearly worth it — the calculator makes the trade-off visible.
Watch out
Common mistakes to avoid
Looking only at the monthly payment and ignoring total interest over the loan or card term.
Confusing APR and AER — APR is typically used for borrowing, AER for savings. They compound differently.
Forgetting that compound interest works against you on debt just as it works for you on savings.
Using round-number estimates (5% return, 3% inflation) when real account rates are available — use the actual rate.
Treating a retirement projection as a guarantee. Returns vary year to year; the calculator shows a directional estimate, not a commitment.
Comparing debt and savings rates without adjusting for tax. A 4% savings account may net less than 4% after income tax depending on your bracket.
Before you decide
Decision checklist
Have you entered the actual interest rate from your statement or account, not a rough estimate?
Have you run a conservative scenario (lower return, higher rate) alongside the expected one?
Have you checked total interest paid or received — not just the monthly figure?
If planning to pay off debt early, have you checked for early repayment charges with your lender?
For retirement projections, have you accounted for inflation in your target amount?
For tax-advantaged accounts (ISA, 401k), have you reflected the tax benefit in your assumed return?
Frequently asked questions
What is the difference between compound interest and simple interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus previously accumulated interest — meaning interest earns interest. For long-term savings, compounding significantly increases the final amount. For long-term debt, it significantly increases the total repaid.
Is daily compounding meaningfully better than monthly compounding for savings?
At typical savings rates (3–6%), the difference between daily and monthly compounding is small — often less than 0.1% per year. Annual compounding shows a bigger gap. The frequency of your deposits matters more than compounding frequency at these rates.
What is the debt avalanche method and when should I use it?
The avalanche method pays off the highest-interest debt first regardless of balance size. It minimises total interest paid. The snowball method targets the smallest balance first for motivational wins. Mathematically, avalanche saves more money; psychologically, snowball may be easier to stick to. Both are better than paying minimums on everything.
How accurate are online retirement calculators?
Retirement calculators use constant-rate projections. Real investment returns vary year to year. A calculator showing £500,000 at age 65 assumes steady growth — actual results could be higher or lower depending on sequence of returns, inflation, fees, and contribution consistency. Use them for directional planning, not precise forecasting.
Should I pay off debt or invest the extra money?
The break-even depends on the debt interest rate vs. expected investment return, adjusted for tax. High-interest debt (credit cards at 15%+) is almost always worth paying off before investing in non-tax-advantaged accounts. For low-rate debt (mortgage at 4%), the comparison is closer. Max out tax-advantaged investment accounts (ISA, pension, 401k) first, then compare rates on any remaining debt vs. investment.
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