Investment Calculator Guide

An investment calculator projects your future balance based on a starting amount, an expected return rate, and recurring contributions. Ours compounds monthly, summarizes results per year, shows a growth chart, and provides a detailed yearly schedule.

You can switch between three modes: Classic to see a fixed horizon, Goal to solve “how long until I reach X?”, and S&P 500 to replay real yearly index moves. Switching tabs keeps your inputs so you can compare scenarios without retyping.

Each run produces a stacked growth chart plus a year-by-year table showing deposits and interest. You can change the currency formatting without altering returns.

Compound interest means growth on top of growth. Each month your balance is multiplied by a monthly rate derived from your annual return (e.g., 7% ≈ 0.565% per month). The next month, the larger balance earns interest again.

Example: $10,000 at 7% annual return for 30 years (no extra deposits) grows to ≈ $76,000. Add $200/month and the final balance is dramatically higher because contributions also compound.

Compounding works in both directions: strong months lift the base for future growth, while negative periods reduce the base. The yearly schedule shows how much comes from deposits versus market moves, so you can separate savings discipline from performance.

Monthly compounding is a practical middle ground. Daily would add complexity without changing long-horizon results much, and annual would hide volatility. We convert your annual rate to a monthly rate, apply it, then show yearly totals for clarity.

Contributions can be weekly, biweekly, semimonthly, monthly, quarterly, or annually. We normalize everything to a monthly equivalent for accurate compounding. You’ll see the effect as Total contributions and Interest in the chart and schedule.

Regular deposits turn volatility into an advantage via dollar-cost averaging—you buy more when prices are low and less when they’re high. Matching the frequency to your paycheck keeps the plan realistic.

Use the yearly schedule to sanity check totals. If you enter $100 weekly, you should see roughly $5,200 per year of deposits; that quick check catches typos before you rely on the projection.

Pick Classic when you want to see how a plan grows over a fixed horizon. Use Goal when you have a target amount and need a time estimate. Switch to S&P 500 to stress-test with real market history, then extend beyond the last data year with your own assumed return.

Overly optimistic returns: try a range (e.g., 4%, 6%, 8%) to stress test.

Ignoring fees/taxes: lower your return input to approximate after-costs.

Irregular contributions: pick the closest frequency and sense-check totals in the schedule.

Comparing apples & oranges: Classic ≠ backtest; use each for its purpose.

Our backtest uses yearly S&P 500 levels to compute year-over-year changes. We apply monthly contributions and compound each month using the monthly rate derived from that year’s change. Negative years reduce the balance like in real markets.

Choose any start/end year (not earlier than the first available data year). To continue beyond the last data year, set an assumed return for future years. This makes the tool useful both for historical analysis and forward projections.

Use the backtest to answer “what if I kept investing through past drawdowns?” and to benchmark your plan against long-run index behavior. Results are nominal and exclude fees/taxes, so lower your assumed return if you want a conservative view.

Return rate: dominates over long horizons; include fees/taxes by lowering it.

Contributions: larger or more frequent deposits accelerate compounding.

Time: the longer you invest, the more exponential the curve becomes.

Inflation: results are nominal; subtract expected inflation for real returns.

Data & assumptions: historical backtests use yearly S&P 500 changes; anything beyond the last data year relies entirely on your assumed return.