Investment Calculator
Free investment calculator with compound interest and monthly contributions. See how a starting balance and a monthly deposit grow over 5, 10, 20, or 30+ years.
📊 Multi-Portfolio Comparison
Compare up to 5 scenarios side by side. Inflation-adjusted toggle. Live chart.
🔥 Retirement / FIRE Calculator
When can you retire? Required nest egg, monthly savings, projected age.
Try the mobile app
Investment Calc for Android — save scenarios, compare strategies, year-by-year charts.
How to Use This Investment Calculator
- Enter your starting balance and how much you plan to add each month.
- Pick a realistic annual return — 7% real is a common stock-market planning number.
- Choose your horizon. The year-by-year table shows the snowball effect.
How Compound Growth Is Calculated
This calculator compounds monthly. Each month your balance grows by 1/12 of the annual rate, and then your monthly contribution is added. Over decades, that monthly compounding is meaningfully more powerful than annual compounding.
balance(m+1) = balance(m) × (1 + r) + contribution
The "Total contributed" line shows your out-of-pocket — initial plus monthly × months. Everything above that line is interest earned.
Why time matters more than amount
Compound growth is the quiet engine behind this calculator. Each year your returns earn returns of their own, so the balance doesn't grow in a straight line — it curves upward, and the steepest part of the curve is near the end. That's why when you start usually matters more than how much you put in. A modest monthly contribution begun in your twenties can finish ahead of a much larger one begun a decade later, simply because it had more years to compound.
Two cautions when reading the result. First, the return rate is an assumption, not a promise — markets don't deliver a smooth percentage every year. Second, inflation quietly erodes the future number, so a "real" (inflation-adjusted) return gives a more honest picture of what the money will actually buy.
I lost count, over twenty years in IT support, of capable and well-paid colleagues who left money sitting in a current account "until they figured out what to do with it," sometimes for years. The thing that finally moved people wasn't a lecture; it was seeing the gap on a calculator like this between starting now and starting in five years. When you watch the curve, the cost of waiting stops being abstract. I'm not a financial adviser, and I'd never tell anyone what to buy — but the one pattern I'm confident about is that the people who started early and just kept going did better than the ones who waited for the perfect plan.
— Hill, 20 years in IT supportThis is a projection based on the numbers you enter, not investment advice or a guarantee of returns. Investments can lose value; for decisions about your money, consult a qualified financial adviser.
Frequently Asked Questions
Are returns guaranteed?
No. Investment returns vary year by year — stocks have averaged ~7% real return historically but with big swings. Use this calculator to model scenarios, not to forecast certain outcomes.
Should I use a real or nominal return?
For retirement planning, use a real (inflation-adjusted) return like 5–7% — that way your future value is in today's dollars. For short horizons, nominal returns work fine.
How does compounding work?
Each year your existing balance earns a return, and that return is added to the balance. Next year's return is earned on a slightly larger pot. Over decades this snowballs — most of your final balance comes from growth on previous growth, not your original contributions.
Does starting early really make that much difference?
Yes — often more than the amount you contribute. Because returns compound, the years closest to the end produce the biggest gains, so an early start gives your money more time in the steep part of the curve. Try entering the same contribution with a start five years earlier and compare the ending balance.