Interest Calculator
Free UK calculators for loans, mortgages, savings, investments, credit cards and late payment. Work out monthly repayments, compound growth and statutory interest in seconds – no sign-up.
Loan interest calculator
Work out the monthly repayment, total interest and representative APR on a personal or bank loan.
Mortgage interest calculator
Enter the property price, deposit, base rate and lender margin to get your monthly repayment and a rate stress test.
Savings & deposit interest calculator
Work out the final balance of a savings account or fixed-rate bond, with monthly or lump-sum deposits.
Investment & fund calculator
A long-term returns calculator: starting amount, monthly investment, expected fund return and charges.
Statutory interest calculator
Work out the statutory interest on an invoice or tax bill from the due date to the payment date (by actual days).
Credit card & BNPL calculator
Estimate the payoff time and cost of a credit card, Klarna or Clearpay balance.
What is an interest calculator and when do you need one?
An interest calculator works out the cost of borrowing or the growth of savings from three simple inputs: an amount, an interest rate and a period of time. Whether you're weighing up a personal loan, checking a mortgage repayment, projecting how an ISA will grow, or working out the interest owed on a late invoice, the right calculator gives you a clear pound figure in seconds.
Use the tabs above to switch between a loan, mortgage, savings, investment, late-payment and credit card calculator. Each one uses the standard financial formula for that job, so the numbers line up with how UK lenders, banks and HMRC work them out.
Calculators for every need
- Loan calculator – monthly repayments and total interest on any personal or car loan.
- Mortgage calculator – repayments, total interest and a rate stress test.
- Savings & investment calculator – compound growth of a pot over time.
- Late payment calculator – statutory interest on overdue invoices and tax.
- Credit card calculator – how long to clear a balance and the true cost of the minimum payment.
Bank-specific calculators
If you already have a quote from a particular lender, our bank calculators let you check it. Enter the figures to compare an offer from Barclays, HSBC, Lloyds, NatWest, Santander, Halifax or Nationwide on a like-for-like basis. These tools are independent and not affiliated with the banks.
How reliable are the figures?
The calculators use established formulas: the annuity (amortisation) formula for loans and mortgages, and compound interest for savings and investments. For statutory late-payment interest we apply the Bank of England base rate plus 8% as set by the Late Payment of Commercial Debts (Interest) Act 1998, and the base rate plus 4% for HMRC interest on late tax. We update these rates when the Bank of England or HMRC changes them.
Results are indicative estimates to help you compare and plan. They are not financial advice, and a lender's or HMRC's own figure may differ. Always rely on an official quotation or statement for exact amounts.
How to use the calculator
1. Pick the right tab
Six tabs sit at the top of the page: loan, mortgage, savings, investment, late payment and credit card. Choose the one that matches what you're working out.
2. Enter your figures
Fill in only the fields you know. If you're unsure of a rate or margin, use the “Fill example” button to load a realistic set of numbers, then adjust them to your own case.
3. Read the headline result
The most important number appears first – your monthly repayment, final balance or interest owed. Below it you'll find a full breakdown, a comparison and a month-by-month repayment schedule.
4. Print, share or export to Excel
Download any repayment schedule as a CSV file to open in Excel or Google Sheets, or copy the result to share it.
Interest types compared
Different jobs use different formulas. This table shows which one applies where, so you can go straight to the calculator that fits your situation.
| Type | How it works | Typical use | Calculator |
|---|---|---|---|
| Simple interest | Interest on the original amount only, counted by exact days. | Late invoices, statutory interest, short bridging loans. | Daily interest |
| Compound interest | Interest earns interest – the balance grows on itself each period. | Savings, ISAs, long-term investing. | Compound interest |
| Annuity (amortising) | A fixed monthly payment; early instalments are mostly interest, later ones mostly capital. | Mortgages, personal loans, car finance. | Annuity loan |
| APR | The nominal rate plus all fees, expressed as one yearly figure for fair comparison. | Comparing loan and credit offers. | APR |
| Statutory interest | Bank of England base rate plus a fixed margin set by law. | Overdue B2B invoices and late tax. | Late payment |
Common situations
First-time buyer
You want to know how much you'd borrow on a £280,000 home with a £60,000 deposit, and what the monthly repayment looks like over 25 years. Enter the price, deposit, base rate and your lender's margin to see the payment and a rate stress test.
Business cash flow
A customer has missed an invoice due date. Work out the statutory interest owed 30 days on, plus the fixed compensation you're entitled to charge per invoice.
Long-term saver
You'd like to see what £200 a month at 5% grows to over 20 years, and how much of the total is compound interest rather than your own contributions.
Clearing a card
You want to know how long a £3,000 credit card balance takes to clear at £100 a month on a 22% APR – and how much interest that costs along the way.
Student
Interest accrues on student finance while you study. Estimate the balance at graduation and what repayments look like once you're earning above the threshold.
Worked examples
Example 1: £10,000 loan, 5 years, 6.5%
| Payment | Capital | Interest | Instalment | Balance |
|---|---|---|---|---|
| 1 | £141.49 | £54.17 | £195.66 | £9,858.51 |
| 12 | £150.16 | £45.50 | £195.66 | £8,250.55 |
| 36 | £170.94 | £24.72 | £195.66 | £4,392.32 |
| 60 | £194.61 | £1.05 | £195.66 | £0.00 |
Total repaid: £11,739.69, of which £1,739.69 is interest (excluding any arrangement fees).
Example 2: £220,000 mortgage, 25 years, 4.7% total rate
- Monthly repayment: £1,247.94
- Total interest over the term: £154,382
- Total repaid: £374,382
- Stress test (+2%): the payment rises to about £1,513 – roughly £265 more a month.
How interest is calculated
Every calculator on this site uses the same standard financial formulas that banks and lenders apply, so you can see exactly where the figures come from.
Annuity loan (fixed monthly payment)
payment = P × k / (1 − (1 + k)⁻ⁿ)
P is the loan amount, k is the monthly rate (the annual rate divided by twelve) and n is the number of payments. The monthly payment stays the same throughout the term. Early on most of it is interest, and later most of it is capital repayment.
Compound interest (savings and investment)
final balance = P × (1 + k)ⁿ + growth of regular deposits
Interest is earned on interest already accrued. The longer the term and the more often interest is compounded, the larger the final amount. This is the single most important factor in long-term saving.
Simple interest and late-payment interest
interest = principal × annual rate × days / 365
Simple interest is used for short periods and for late-payment interest on invoices. For UK commercial debts the statutory rate is the Bank of England base rate plus 8 percentage points, charged on the actual number of days a payment is overdue.
Representative APR
The representative APR combines the nominal rate and every fee into a single comparison figure. It is the only number that lets you compare loan offers fairly, because a headline interest rate ignores arrangement and monthly fees.
How to compare loans and savings properly
A headline interest rate rarely tells the whole story. When you weigh up finance options, look at the following.
- Compare on APR, not the nominal rate. Two loans with the same nominal rate can cost very different amounts once arrangement and monthly fees are included.
- Check the total amount repayable. A longer term lowers the monthly payment but increases the total interest you pay. The "total payable" line makes the difference clear.
- Run a rate stress test. Test how a one or two percentage point rise would affect your payment before committing to a variable-rate loan.
- Mind the compounding frequency on savings. Interest added monthly grows faster than interest added once a year, even at the same annual rate.
- Distinguish the repayment method. An annuity keeps the monthly payment level, while a reducing-balance (equal-capital) schedule shrinks the payment over time and pays less interest overall.
Common mistakes in interest calculation
These errors come up again and again when interest is worked out by hand or from memory.
- Confusing the nominal rate with the APR. Comparing on the nominal rate alone almost always leads to the wrong conclusion.
- Forgetting the fees. An arrangement fee and monthly charges can raise the true cost of the cheapest-looking loan significantly.
- Using the wrong day count for late interest. Late-payment interest runs from the day after the due date, on the actual number of days, not in whole months.
- Ignoring the compounding period. The same annual rate produces a different result depending on whether interest is compounded monthly or yearly.
- Overlooking tax and inflation. Savings returns are subject to tax, and inflation reduces the real value of the return over time.
Frequently asked questions
Enter the loan amount, the annual interest rate and the term into the loan calculator. It uses the annuity formula to work out your fixed monthly repayment and the total interest over the term – the same method UK lenders use.
Compound interest pays interest on your interest, not just on what you put in. The savings calculator adds your monthly contribution each month and compounds the return, so you can see how a pot grows over 5, 10 or 20 years – the longer the horizon, the bigger the effect.
For a business-to-business invoice you can charge statutory interest of the Bank of England base rate plus 8% (11.75% in the first half of 2026), plus fixed compensation of £40, £70 or £100 per invoice, under the Late Payment of Commercial Debts (Interest) Act 1998.
Yes. Every calculator on InterestCalc.co.uk is completely free, with no sign-up or registration, and you can use them as many times as you like.
They'll be very close. A lender's or HMRC's final figure can differ by a small amount because of rounding, exact day counts or fees specific to your agreement. Use our results as an indicative guide and check the official figure before you commit.
On a tracker or variable mortgage, your rate is usually the base rate plus your lender's margin, so a base rate change feeds straight through to your monthly payment. On a fixed rate, nothing changes until the fix ends. The mortgage tab lets you enter a base rate and margin separately and run a stress test for a 1% or 2% rise.
APR (annual percentage rate) rolls the interest rate together with compulsory fees into a single yearly figure. Because it includes the cost of fees, it lets you compare two loans fairly – a lower headline rate with high fees can work out dearer than a higher rate with none.
Cash savings give a fixed, low-risk return and instant access, which suits short-term goals and an emergency fund. Investing aims for a higher return over the long run but the value can fall as well as rise. The savings and investment tabs let you compare the two side by side. This is general information, not financial advice.
Yes. Your most recent calculations are kept in your browser, and you can download any repayment schedule as a CSV file to open in Excel or Google Sheets.
Glossary of key terms
- Principal – the amount you borrow or save, before any interest is added.
- Nominal rate – the headline annual interest rate, before fees.
- APR – the yearly rate including compulsory fees, used to compare offers.
- Base rate – the Bank of England's official rate, which many variable rates track.
- Margin – the percentage a lender adds on top of the base rate.
- Amortisation – paying a loan off in equal instalments of capital plus interest.
- Compound interest – interest calculated on your growing balance, not just the original sum.
- Statutory interest – the interest the law lets you charge on overdue commercial debts.
- Term – the length of a loan or savings plan, usually in years.
- Overpayment – paying more than the required instalment to clear a loan sooner and cut total interest.
What people say
“I found out my mortgage payment would go up by about £265 a month if rates rose 2%. That told me exactly how much headroom to leave in the budget.”
— James, Manchester
“I used the late payment calculator for an overdue invoice and got the exact statutory interest and compensation to put on the reminder.”
— Priya, Bristol
“Seeing £200 a month grow to nearly £82,000 over 20 years made the case for starting now rather than waiting.”
— Tom, Leeds