Compound Interest Projection Generator

Compounding is famously slow and then not, and the reason is visible in the table: interest is credited monthly and immediately starts earning interest itself, so the balance grows on what it has already grown. This projects year by year, separates the money paid in from the money earned, and checks that those two plus the opening balance account for the closing figure exactly.

Opening balance (pence)

What is in the account before any contribution.

Monthly contribution (pence)

Added at the end of each month, after that month's interest.

Annual rate (tenths of a percent)

59 means 5.9%, applied monthly at a twelfth of that.

Years

Compounding is slow at first and then not — the last years do most of the work.

Your result will appear here.

What this generator does

Applies a twelfth of the annual rate each month, credits the interest, adds the contribution, and records each year's opening balance, contributions, interest and closing balance. The three components are reconciled against the closing figure.

How to use this tool

Enter the opening balance and the monthly contribution, in pence.
Set the annual rate in tenths of a percent.
Choose how many years to project.
Compare the paid-in and earned columns — the crossover is the interesting part.

Understanding the controls

Opening balance: What is there before any contribution. Starting earlier with less usually beats starting later with more, which the table makes visible.
Monthly contribution: Added at the end of each month, after that month's interest — the conservative convention.
Annual rate: In tenths of a percent. Applied monthly at a twelfth, which compounds slightly ahead of the headline figure.
Years: Compounding is unimpressive over five years and dramatic over thirty, which is the whole lesson.

Common use cases

Projecting a savings balance with regular contributions
Seeing when interest starts to outpace what you pay in
Comparing the effect of starting earlier against contributing more
Teaching compounding with figures that provably reconcile
Producing a savings projection for a plan

How this generator works

Each month the balance earns a twelfth of the annual rate, rounded to the penny, and then the contribution is added. Because interest is credited before the next month's calculation, it earns interest itself — which is the difference between compound and simple growth. Before display each year is checked to open exactly where the previous one closed and to add up from its own three components, and the closing balance is confirmed to equal the opening balance plus everything paid in plus everything earned.

Randomness and fairness

Nothing here is random. The same inputs always give the same projection. This is arithmetic for planning and teaching, not financial advice, and it knows nothing about your circumstances.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

A constant rate throughout, whereas real returns vary and can be negative — a projection is not a forecast.
No inflation adjustment, so the closing figure is in today's pounds only if the rate is read as a real rather than nominal return.
No tax, fees or platform charges, all of which reduce the outcome.
Contributions are the same every month, so increases in line with earnings are not modelled.This is arithmetic for planning and teaching, not financial advice, and it knows nothing about your circumstances.

Privacy and your data

The projection is computed in your browser. Your figures never leave the page and are not included in analytics.