🇬🇧 UK-Specific Calculator

🇬🇧 UK Compound Interest Calculator

Calculate how your savings and investments grow with compound interest.

Final Balance

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Total Contributions

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Total Interest Earned

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Effective Annual Rate

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Principal Growth

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Contributions Growth

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GUIDE

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Calculator inputs

This calculator projects a UK savings balance in pounds from five entries. Initial Deposit is the amount already in the account on day one. Monthly Contribution is the amount you plan to add each month. Annual Interest Rate is the nominal yearly rate used by the formula, before any tax or charges. Time Period is entered in years, and the calculator rounds it to the nearest whole year. Compound Frequency tells the model when interest is added: monthly, quarterly or annually. That choice also controls when regular deposits enter the calculation. With monthly compounding, one monthly contribution is added at the end of each month. With quarterly compounding, three monthly contributions are grouped and added at each quarter end. With annual compounding, twelve are grouped at each year end. Use zero for Monthly Contribution when modelling a fixed lump sum. Use zero for Initial Deposit when you are starting only with future deposits. The tool recalculates whenever an input changes and does not require a separate account type selection.

Compound formula

The opening balance uses P × (1 + r ÷ n)^(n × t). P is the Initial Deposit, r is the Annual Interest Rate divided by 100, n is the number of compounding periods in a year, and t is the rounded number of years. The calculator sets n to 12 for monthly, 4 for quarterly and 1 for annual compounding. Regular deposits use an ordinary annuity formula: C × (12 ÷ n) × [((1 + r ÷ n)^(n × t) - 1) ÷ (r ÷ n)]. C is the Monthly Contribution. The factor 12 ÷ n combines monthly deposits into the selected period, so the model credits each combined payment after that period rather than on its actual calendar date. The final balance is the opening-balance result plus the deposit result. If the entered rate is zero, the calculator skips division by the rate and simply adds P to C × 12 × t. It does not model a negative rate.

Worked calculation

Suppose you enter an Initial Deposit of £8,000, a Monthly Contribution of £150, an Annual Interest Rate of 4.2%, a Time Period of 7 years and monthly compounding. The calculator uses 84 monthly periods and a monthly rate of 0.042 ÷ 12. The opening £8,000 grows to £10,728.76 before display rounding. The stream of end-of-month contributions grows to £14,618.37. Added together, those parts produce £25,347.13, which the page displays as a Final Balance of £25,347. You supplied £20,600 in total: the £8,000 opening deposit plus 84 payments of £150. The difference is £4,747.13, displayed as Total Interest Earned of £4,747. The Effective Annual Rate is 4.2818%, shown as 4.28%. With the same entries but annual compounding, the model groups each year's twelve payments at year end and displays £24,974. That comparison reflects this calculator's payment timing as well as its compounding frequency.

Reading the results

Final Balance is the sum of two separately calculated pots: Principal Growth and Contributions Growth. Principal Growth means the original deposit plus all interest attributed to it; it is not interest alone. Contributions Growth means the regular deposits plus the interest attributed to those deposits. Total Contributions includes both your Initial Deposit and every planned monthly payment, despite the shorter label. Total Interest Earned is Final Balance minus Total Contributions. Effective Annual Rate converts the nominal rate and selected frequency into one yearly compounded rate using (1 + r ÷ n)^n - 1. The page rounds each money result independently to the nearest pound and shows the effective rate to two decimal places, so displayed parts can occasionally differ from a displayed total by £1. None of the figures deducts income tax, product fees, withdrawal penalties or inflation. The results also do not separate interest paid in a particular UK tax year, which matters when checking taxable interest outside an ISA.

AER and gross rate

UK savings adverts commonly show AER and a gross rate, but they are not interchangeable inputs here. AER means annual equivalent rate. It expresses what the yearly return would be if interest were left in the account and compounded over a full year, which makes accounts with different payment schedules easier to compare. Gross describes interest before income tax is taken into account. This calculator treats Annual Interest Rate as a nominal rate and then applies the chosen compounding frequency. If a provider gives you a nominal gross rate that compounds monthly, enter that rate and choose monthly. If the only figure you have is AER, the simplest way to reproduce one year's advertised annual growth is to enter the AER and choose annually. Choosing monthly as well would compound an already annualised figure again and overstate the result. To model monthly crediting from an AER precisely, convert it first: the nominal percentage is 12 × ((1 + AER ÷ 100)^(1 ÷ 12) - 1) × 100.

UK savings products

For an easy-access account, enter your present balance, expected monthly deposits and the account's stated rate. If an easy-access account has a variable rate, one long projection should not be read as a promise. Run the calculator again with lower and higher rate assumptions if the provider can change the rate. A fixed-rate savings bond usually holds a lump sum for a stated term and may restrict additions or early access. To mirror that structure, use the amount paid in as Initial Deposit, set Monthly Contribution to zero, and use the fixed term only if it is a whole number of years. A regular saver may cap what you can pay each month and may calculate interest from each deposit date. Enter only the monthly amount permitted by the account, then check whether the provider credits interest monthly or merely pays it annually. The calculator's annual option groups a year of monthly deposits at year end, so it can understate growth when a real account accrues interest daily on each earlier deposit.

ISA and savings tax

A Cash ISA is a tax wrapper for eligible cash savings, not a separate compounding method. Interest kept inside the wrapper is free of UK Income Tax, and ISA subscriptions share an annual allowance set for each tax year. The allowance controls how much new money you can subscribe, not how large an existing ISA balance may grow. This calculator does not track tax years or allowance use, so check planned deposits across all of your ISAs before treating the full Monthly Contribution as eligible. Outside an ISA, the Personal Savings Allowance can make some savings interest taxable at a zero rate. Whether it applies, and how much is available, depends on your Income Tax band and other income. HM Revenue and Customs considers taxable savings interest across relevant accounts, while ISA interest does not use the Personal Savings Allowance. Because the calculator always shows gross growth and deducts no tax, compare its Total Interest Earned with your own tax position rather than treating Final Balance as an after-tax figure.

Protection and disclosure

UK banks, building societies and credit unions may be covered by the Financial Services Compensation Scheme when they are authorised and the deposit is eligible. Protection is organised per eligible person and per authorised firm, not necessarily per brand. Two savings brands can share one banking licence, so balances held under that licence are considered together. The scheme's limit can change, and separate rules can apply to temporary high balances from qualifying life events. Check the current FSCS position and the provider's authorisation rather than inferring protection from the calculator. The FCA's Banking: Conduct of Business sourcebook requires direct-offer financial promotions for savings accounts to include a summary box. Use it to check the rate, whether it can change, how and when interest is paid, access conditions and any bonus expiry. The FCA's Consumer Duty also requires firms to support consumer understanding and assess fair value. Those protections govern the provider's conduct; they do not turn this projection into a regulated quote or guarantee the entered return.

Offer comparisons

Compare like with like. First confirm whether each quoted percentage is AER or a nominal gross rate. Then match Compound Frequency to the way the provider credits interest, not simply to how often you intend to deposit. Keep Initial Deposit, Monthly Contribution and Time Period identical when testing two offers. The difference in Final Balance then comes from the rate and frequency assumptions rather than a changed savings plan. Check the product summary for introductory bonuses, balance tiers, monthly deposit limits, withdrawal limits and maturity instructions. This calculator has no fields for any of them. For a fixed-rate bond that does not accept later deposits, a non-zero Monthly Contribution creates a result the product cannot deliver. For an easy-access account, assuming today's variable rate for several years may be too optimistic or too cautious. Compare several rate cases. Also check whether a higher AER requires conditions you may not meet, because the calculator applies the entered rate to every pound for the whole rounded term without testing eligibility.

Model limits

The projection assumes one constant rate, one constant monthly contribution and no withdrawals for the entire rounded number of years. It has no start date, so it cannot allocate interest between UK tax years or account for the exact number of days in a month or leap year. It offers monthly, quarterly and annual compounding only; daily accrual cannot be selected. Contributions are treated as end-of-period payments and are bundled into quarters or years when those frequencies are chosen. The model does not handle tiered rates, limited-time bonuses, rate changes, fees, early-access charges, missed deposits, contribution caps or a maturity rollover rate. It also leaves inflation out, so Final Balance is a future cash amount rather than a statement of what that money will buy. Inputs must be complete finite numbers written with valid locale grouping and decimal separators; a typed pound sign, malformed grouping, partial numeric prefix or surrounding whitespace is invalid. Negative amounts and rates are rejected, and Time Period must round to a whole year above zero. An invalid entry clears the current results to dashes and shows an error. Money results are independently rounded to whole pounds. Use the result as a consistent scenario, then test it against the account's own terms and illustration.