Compound Growth Calculator

Compound growth is the reason starting early matters so much. Your money earns a return, then that return earns a return, and over decades the growth can dwarf what you put in. This calculator shows exactly how that plays out with your numbers.

What this calculator does

Enter a starting amount, a monthly contribution, an expected annual return, the number of years and an inflation rate. It shows:

• the future value of your investments,
• that same amount in today's dollars,
• how much you contributed, and
• how much came from growth.

The chart compares your balance with the total you put in. The widening gap between the two lines is compound growth at work.

The formula in plain English

Each month, your balance earns interest at (annual return ÷ 12), and your contribution is added. The standard formula is:

future value = start × (1 + r)ⁿ + monthly × [((1 + r)ⁿ − 1) ÷ r]

where r is the monthly return and n is the number of months.

Today's dollars = future value ÷ (1 + inflation)^years. This shows what the money will buy at today's prices.

A worked example

Rosa, a new physician assistant, starts with $10,000 and invests $1,000 a month at a 7% expected return for 25 years.

• She contributes $310,000 in total.
• Her balance grows to about $867,000.
• More than $557,000 of that — nearly two-thirds — is growth.
• At 2.5% inflation, that's about $468,000 in today's dollars.

If she waited five years and invested for 20 years instead, she'd end up with roughly $530,000 — about $340,000 less, even though she'd only have contributed $60,000 less. Those first five years do a lot of the work.

Why it matters at each career stage

If you're: Student

Even $50 a month makes the habit real. The amounts you can save now are small, but you have the most time on your side of anyone.

If you're: Resident / fellow

A few hundred dollars a month during training can grow into six figures by mid-career. A Roth IRA is a great place for it.

If you're: Employed

Your contributions now matter most. Raising your monthly investing after each raise is the simplest way to speed things up without feeling it.

If you're: Practice owner

Compound growth works on reinvested profits too. Compare what your practice earns on new equipment or space with what the same money could earn invested.

Picking realistic inputs

Return. A balanced or stock-heavy portfolio has historically earned more than cash or bonds over long periods, but with big swings along the way. For planning, many people use 6% to 7% before inflation for stocks and 3% to 4% for a high-yield savings account or bonds. If you're not sure, run a low, middle and high case — the range is more honest than a single number.

Monthly amount. Use what you can sustain, not what you hope to do in a great month. Automatic transfers right after payday are the easiest way to make it stick.

Years. Count to when you'll actually start using the money. For retirement, that might be 25 to 35 years. For a house down payment, it may be only five, and a lower-risk return makes more sense.

Inflation. The long-run U.S. average has been around 2.5% to 3%. The higher you set it, the less your future dollars will buy.

Three lessons from the math

Time beats amount. Starting early usually matters more than investing a larger amount later.

Consistency beats timing. Investing every month, no matter what markets are doing, captures growth without guesswork.

Small leaks add up. A 1% fee or an extra year of delay compounds in the wrong direction. The advisor fee calculator shows how much fees cost over time.

When you're ready to set a goal, try the FI number calculator, or project your workplace plan with the 403(b) calculator.