Investment Returns Calculator

How compounding with regular contributions actually grows money

This calculator models the single most important idea in long-term investing: money left to grow earns returns, and those returns themselves earn returns. Internally it steps forward month by month, not year by year, because most people add money monthly. Each month it applies one-twelfth of the annual rate to the running balance and then adds your monthly contribution. In formula terms, each month does balance = balance × (1 + rate/12) + monthly. Repeating that for every month of your horizon produces the growth curve you see plotted.

Rather than ask you to guess a single return, the tool runs three fixed scenarios side by side so you can see a realistic range instead of one optimistic number:

The summary cards at the top always report the moderate (7 percent) scenario, while the table below breaks out final value, total invested, total returns, CAGR, and the Rule of 72 for all three.

A worked example

Start with $10,000, add $500 every month, and let it run for 20 years at the moderate 7 percent rate. Over those 240 months you personally contribute $10,000 + $500 × 12 × 20 = $130,000. The calculator compounds month by month and ends near $310,000. The gap between the roughly $310,000 ending value and your $130,000 of contributions — about $180,000 — is pure compound growth that you never deposited. That is the whole point of starting early: most of the final balance is money the market earned, not money you saved.

Reading CAGR and the Rule of 72

The CAGR column is the compound annual growth rate computed as (Final ÷ Initial)^(1 ÷ years) − 1. Note an important subtlety: it compares the final value to your initial lump sum only, so when you make ongoing monthly contributions the displayed CAGR runs higher than the scenario's headline rate, because each later deposit had less time to compound. CAGR answers "what single steady rate turns my starting amount into the end result?" — it is a clean way to compare outcomes, not a claim about the market's annual return.

The Rule of 72 column is a mental-math shortcut: divide 72 by the annual rate to estimate the years to double your money. At 7 percent that is 72 ÷ 7 ≈ 10.3 years; at 10 percent, about 7.2 years; at 4 percent, about 18 years. It is approximate but remarkably close for rates in the single digits.

Inflation-adjusted (real) returns

Ticking Inflation Adjusted subtracts a flat 2.5 percent from each scenario rate before compounding — so the moderate case is modelled at 4.5 percent rather than 7. This shows growth in today's purchasing power. A future balance of $310,000 sounds impressive, but if prices roughly double over 20 years it buys far less than $310,000 does now. Toggling this on gives a sober, real-terms figure that is more useful for planning what your money will actually buy.

Practical tips

• Increase the monthly contribution before chasing higher returns. Adding $200 a month is fully within your control; earning 10 percent instead of 7 is not.
• Lengthen the horizon. Compounding rewards time non-linearly — the last decade of a 30-year run adds far more dollars than the first.
• Plan against the conservative column. Treating the 4 percent outcome as your baseline and anything above it as a bonus protects you from over-optimistic budgeting.
• Use the real-returns toggle for retirement goals, where what matters is purchasing power decades out.

What this model leaves out

The projection assumes a constant rate every single year, which never happens — real markets zig-zag, and the order of good and bad years (sequence risk) affects outcomes the model cannot capture. It also excludes taxes and fees; a 1 percent annual fee quietly turns a 7 percent gross return into a 6 percent net one, which compounds into a large gap over decades. Treat the output as an educational illustration of how compounding behaves, not a guarantee or a personalised financial plan. Everything is computed in your browser; no figures you enter are sent anywhere.