Investment Calculator
Future value of regular contributions with compound growth.
Use one currency throughout. Results are in the same currency as your inputs.
Projected balance
How this investment calculator works
This tool projects the future value of an investment when you start with a balance and add a fixed contribution every month, earning a constant annual return that compounds monthly. It is a planning sketch for retirement accounts, brokerage savings, or any goal where regular deposits matter as much as the starting amount.
What the model assumes
Returns are constant every month. Real markets go up and down; sequence of returns, fees, and taxes can change outcomes a lot. Treat the result as “if I earned this average return steadily,” not as a promise.
Formula idea
The starting balance compounds for the full period. Each monthly contribution compounds for the remaining months after it is added. The combined future value is the sum of those pieces. The page also shows how much of the ending balance came from contributions versus growth.
Worked example (dollars)
Start with $5,000, contribute $300 per month for 20 years, and assume 7% annual return compounded monthly. The ending balance is typically tens of thousands higher than contributions alone, because early deposits have many years to grow. Raising the monthly contribution usually moves the needle more than a small rate tweak over the same horizon.
Illustrative UK / Europe example (£)
Illustrative planning sketch only — not investment advice. Tax wrappers (such as a UK ISA), fees, and real market returns are not modeled. Past performance is not a guarantee.
Try a starting balance of £2,000, a monthly contribution of £250, 20 years, and an assumed 5% annual return. Note the projected balance and how much came from contributions versus growth. Then raise the contribution to £300 and compare. Finally, drop the assumed return to 4% with the original £250 contribution — many planners find the contribution lever is both larger and more controllable than debating a one-point return difference.
| Change | Keep fixed | What you learn |
|---|---|---|
| +£50 / month | Years and return | How much contribution rate moves the ending balance |
| Return −1 percentage point | Contribution and years | Sensitivity to a more conservative assumption |
| +5 years | Contribution and return | Value of a longer horizon at the same monthly amount |
How to use it
- Enter your current invested balance (use 0 if you are starting from scratch).
- Enter the monthly amount you can invest.
- Enter an assumed annual return as a percent (historical averages are not guarantees).
- Enter the number of years you plan to keep contributing.
- Review projected balance and total contributions, then stress-test with a lower return.
Decision checklist
- Contribution first — Test +£50 (or +$50) before arguing about 6% versus 7% returns.
- Conservative case — Plan against the lower return assumption, not only the optimistic one.
- Fees and tax — Not in this model; net returns after charges can be lower.
- Lump sum only? — Use the interest calculator when you are not making regular deposits.
Frequently asked questions
What return rate should I assume?
There is no guaranteed rate. Try a conservative and an optimistic assumption and plan against the lower outcome. Past market averages are not promises.
Are contributions assumed at the beginning or end of each month?
This model treats contributions as a regular monthly deposit in a standard future-value of annuity style calculation. Exact payroll timing can differ slightly.
Can I model pounds sterling?
Yes. Enter £ amounts for starting balance and monthly contribution; interpret the projection in the same currency under your assumed return.
Contribution rate usually beats rate chasing
Over multi-decade horizons, the monthly contribution you can sustain often moves the projected balance more than arguing about 6% versus 7% returns. Try this stress test: keep years and return fixed, then raise the monthly contribution by £50 (or $50). Compare that change with dropping the assumed return by a full percentage point. Many planners find the contribution lever is both larger and more under their control.
If you are not making regular deposits yet, the interest calculator shows growth on a lump sum alone. For the difference between compound and simple models on a fixed principal, see compound vs simple interest.
Limitations
No inflation adjustment, expense ratios, or tax drag. Results are estimates only and are not financial advice.
Results are estimates only and are not financial advice.