How much will my money really grow?
Standard compound interest math, plus the inflation adjustment most calculators skip — see both the account balance and what it actually buys.
How to use this in 30 seconds
- Enter what you actually have today — the principal slider is the lump sum already invested. If you're starting from zero, leave it at $0 and lean on the monthly contribution.
- Set a realistic monthly contribution — what you actually save, not what you wish you saved. Most people overestimate. The 401(k) match employer contribution counts.
- Read the inflation-adjusted ("real") number, not the nominal — the headline figure assumes today's dollars. The scarier number is your actual buying power. Both are shown side by side.
The most common surprise: at 7%/2.5% real, $500/month for 30 years compounds to about $612K nominal — but only ~$292K in today's purchasing power. Run your numbers, then look at the real column.
How the math works
The formula is the standard compound interest equation with monthly contributions:
FV = P(1+r)n + PMT × ((1+r)n−1) ÷ r
— applied each year with the rate you set. The number that shows up on your brokerage statement.
We also run the same calculation a second time at (rate − inflation) to give you the real return. Most compound interest tools skip this step, which is why their projections quietly oversell the future. A 7% return with 2.5% inflation isn't a 7% return; it's a 4.4% real return. Over 30 years that's the difference between a $760K account balance and $324K of actual buying power.
Default assumptions: 7% annual return (the long-run real S&P 500 average), 2.5% inflation (current Fed target territory). Both are sliders. The 7% number comes from a century of equity data; lower it to 5% if you want to stress-test conservative assumptions.
All math runs in your browser. We don't store your inputs. Source on github.
Scenarios we've already crunched
If your situation looks like one of these, the article skips a few steps for you:
- $100/month from age 25 vs $200/month from age 35 — the early starter wins by $17K with half the monthly contribution. Time outranks amount.
- $50K at 30 plus $500/month → almost $1M by 60 — but inflation cuts that nominal $988K down to $406K of real purchasing power. Both numbers matter.
- Why your 7% return is really 4% — how to read account balances honestly when the market quotes nominal and reality charges in real dollars.
- The opportunity cost of an oversized emergency fund — $30K idle in a 2% savings account during 3% inflation isn't safe; it's slowly bleeding.