WorthChex › Compound Interest Calculator

Browser-based calculation

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Starting balance

Regular contribution

Contribution frequency

Compounding frequency

Annual interest / growth assumption

Time period

years

MODEL ASSUMPTION

Compound growth is an illustration, not a promise.

This model keeps the selected annual rate constant, adds contributions at the end of each selected contribution period and shows the difference between money contributed and growth. Real savings rates can change and investments can rise or fall.

Source: MoneyHelper

Read the full WorthChex methodology →

UNDERSTANDING COMPOUND GROWTH

Time, contributions and the compounding assumption all matter.

Compound growth means that returns are added to a balance and can themselves affect later growth. The result is sensitive to the starting amount, the contribution pattern, the assumed rate and how often the model compounds.

Starting balance

A larger opening balance gives the assumed rate more capital to work on from the first period.

Regular contributions

Adding money changes the path of the balance. Compare a contribution scenario with the same starting balance at 0% growth to separate saving behaviour from assumed growth.

Rate is an assumption

A calculator can illustrate a fixed rate; it cannot guarantee an investment or savings return. Real rates, fees, taxes and market returns can vary.

Worked example

Suppose £5,000 starts in an account and grows at a fixed 5% annual rate with no further contributions. The interest in the first year would be £250 before compounding in later periods. The longer the balance remains invested and the more frequently interest is added, the more the path differs from simple interest.

MoneyHelper explains how interest rates and compound interest work and notes that the rate on savings or borrowing affects the amount paid or earned. MoneyHelper: interest rates explained →

REVIEWED SEPTEMBER 2026

Keep projected growth separate from money you actually have.

Use conservative assumptions when planning a goal with a fixed date. For investments, a projected return is not a promised outcome; for cash savings, the available rate can change. Compare the model with a 0% scenario so the contribution requirement remains visible.

Four inputs drive most of the illustration.

Starting balance gives the model a base. Regular contributions add new money. The annual rate controls the assumed growth and the time period determines how often that growth can be applied. Contribution and compounding frequency add another layer because money does not all enter the model at the same moment.

Starting balance

A larger starting amount gives the model more capital to experience growth from the first period.

Regular contributions

Contributions are separate from growth. This is why the result shows total contributions alongside the projected ending value.

Time and rate

Growth compounds over repeated periods. A small difference in the assumed rate or a longer time horizon can produce a large change in a purely mathematical projection.

Example: with £2,000 already saved and £250 added each month, most of the final amount in the early years can still come from your own contributions. Over a longer horizon, the accumulated growth can become a larger part of the projected total.

Separate saving from investing assumptions.

A fixed annual rate is useful for arithmetic but real products do not all behave the same way. Savings rates can change. Investment returns can be negative as well as positive and are not guaranteed. A compound-interest calculator therefore works best as a scenario builder rather than a forecast.

Try a range: a low or 0% case, a central assumption and a more optimistic case. Looking at the range can help you understand how sensitive the final figure is to the rate rather than anchoring on one attractive number.

Contributions still matter

When a target depends on regular saving, the contribution amount is often under your control while the future return is not. Keep the contribution realistic and treat the growth assumption as a variable.

Compound interest calculator FAQs

Is the projected value guaranteed?

No. It is based on a fixed rate assumption. Savings rates can change and investment returns are not guaranteed.

Why can the same rate give different results?

Starting balance, contribution amount, contribution timing, compounding frequency and the number of years can all change the outcome.

Does this include tax on savings or investments?

No. Tax treatment depends on the product and your circumstances. The calculator is a growth illustration rather than a tax calculation.

Should I use this for investment planning?

You can use it to understand compound-growth arithmetic, but it does not forecast market returns or assess whether an investment is suitable for you.