Compound interest
One year per row. Leave a field blank to use the base rate; fill in only the years you want different.
duration amount. Duration takes y/years or m/months (no unit = months). Optional label after #.Frequently asked questions
How do I calculate compound interest when my monthly contribution changes over time?
Add a phase for each stage of your plan — for example three years at 1000 per month, then five years at 2000 — each with its own length (in months or years) and amount. The calculator compounds interest monthly across all phases and shows the final balance, total invested and total interest.
Can I see a timeline of my investment plan?
Yes. The timeline shows the whole plan as a line with one dot per phase; the distance between dots is proportional to each phase's length. It updates as you add, edit or reorder phases.
Can I use a different return and inflation for each year?
Yes. Turn on “Rates by year” and you get one row per calendar year of your horizon: fill in only the years you want different, and the rest stay on the base rate. It lets you test scenarios — a drop right at the start, a few years of high inflation — and see that the same set of returns produces different balances depending on the order they arrive in, because an early drop falls on less money. The values are your own assumptions: the calculator does not forecast markets.
Is it better to invest more at the start or increase contributions gradually?
Money invested earlier compounds for longer, so for the same total invested, front-loading usually ends higher — but most incomes grow over the years. Create one simulation per strategy and compare the final balances side by side; the calculator shows the difference, the decision stays yours.
Do my data leave the browser?
No. Everything is calculated and stored locally in your browser — no account, no server, no tracking. Your plans stay on your device.
Notice: This is a calculation tool, not financial advice. The results are estimates: they assume a constant rate that you choose yourself and leave out taxes, fees, currency exchange and real market risk. No return is guaranteed — investment decisions are your own responsibility.
Guide: invest more now, or increase your contributions over time? The maths, worked through →
Notice
This is a calculation tool, not financial advice. The results are estimates: they assume a constant rate that you choose yourself and leave out taxes, fees, currency exchange and real market risk. No return is guaranteed — investment decisions are your own responsibility.
A compound interest calculator for plans that change: instead of one fixed monthly contribution, you set phases — each with its own length and amount — and watch the balance grow month by month. And if you want, each calendar year can carry its own return and its own inflation, instead of one rate for the whole horizon.
How to use it
- Start from an example — Simple compound interest (one phase, the same contribution throughout) or Compound interest in phases (a plan that changes) — or press Clear to start from zero.
- Under Parameters, enter the starting capital, the annual return and, if you like, inflation.
- Under Phases, each row is a period: how many months it lasts and how much you put in each month. + Phase adds one; you can also type the whole plan as text.
- The result updates as you type: Key figures, the chart and the month-by-month Table, which you can export as CSV.
- + Simulation opens another plan in a tab, for comparison. The “i” next to each field explains it, and the calculator button, bottom right, is there for side sums.
What this calculator does
- Phases at whatever granularity you like, in months or years: three and a half years at 1000, one month at 600, two at 800, then five years at 2000.
- A table with every month of the period — contribution, that month's interest, total invested and balance — which you can export as CSV.
- Several simulations in tabs, effective or nominal rate, contribution at the start or the end of the month, and a real balance discounted for inflation.
- Rates by calendar year: give each year its own return and inflation — blank years stay on the base rate — and see how much the order they arrive in matters.
How the maths works
The monthly rate comes from the annual one by the route you pick — effective (1+r)^(1/12)−1, the right one for a compounded annual return, or nominal r/12. With the contribution at the start of the month, that month's money already earns in that month. The real balance discounts inflation from the nominal balance. Future returns are an assumption, not a promise: the number is for comparing plans, not for forecasting the market. No taxes, brokerage costs or fund TER. Nothing leaves your browser — the simulations are stored on this device only.