Investment Growth Calculator

Project how your money grows with compound interest, monthly contributions and a chosen annual return.

Educational use only. This calculator produces estimates from published formulas and does not constitute financial or legal advice. For real decisions, consult a qualified financial planner or accountant.

This tool runs entirely in your browser — no data is sent to our servers. If JavaScript is disabled, the fully interactive version will not run; enable JavaScript to compute results.

About this investment growth calculator

Compound interest is interest earned on interest. Over decades, it does most of the heavy lifting in long-term wealth — which is why starting early matters far more than starting big. A small contribution started at 25 will usually beat a big contribution started at 45, all else equal, simply because compounding has more time to work.

This calculator takes a starting amount, a monthly contribution, an expected annual return and a horizon. It shows the projected end balance, the total you actually invested, and the compound gain (the difference). Seeing those three numbers side by side makes the power of long horizons very visceral.

Real-world returns are noisy — markets do not compound at a smooth 8% every year. Use realistic long-run averages: 6–8% real return is reasonable for a diversified stock portfolio over 20+ years. Higher numbers are possible; lower numbers are also possible. Inflation eats into nominal returns, so the "real" number is what matters for buying power.

How to use it

Enter how much you have already saved (if anything).
Enter what you can add each month — be realistic.
Enter an expected annual return — 6–8% is a defensible long-term average for diversified equities.
Enter the horizon in years.
Read the final balance and the compound gain.

Tips & notes

Start now, not after raising more money. Time is the most powerful input — small monthly amounts started today usually beat bigger amounts started later.
Use after-tax, after-fee return estimates. A "10% return" before tax and fees can become 6% in your pocket.
Re-run the calculation every year as your contribution rate grows. Most people can increase contributions over time as income rises.
When comparing this against a productive asset (rental property, solar system, small business), use the same discount rate in both models. The Power Generation Planner uses the same NPV framework so you can compare a renewable-energy project head-to-head with a stock portfolio.

Frequently asked questions

What return rate should I use?

For long-horizon diversified equities, 6–8% nominal (4–5% after inflation) is a defensible historical average. For bonds, 2–4%. For cash, 0–3%. Mix accordingly to your portfolio.

Is the projection guaranteed?

No. Markets are volatile and short-term returns can be very different from long-term averages. The calculator shows what would happen at a constant rate — useful for planning, not a guarantee.

How do I account for inflation?

Subtract roughly 2–3% from your nominal return to estimate the real (after-inflation) return. The "real" number tells you what your money will be worth in today purchasing power.

How does this compare to buying a solar system or other productive asset?

Productive assets can beat stock returns at low risk if the payback is short and the local tariff is high, but the money is tied up in one asset. Model the solar or renewable side with the Power Generation Planner (IRR + NPV at your discount rate) and compare both numbers using the same discount rate.