Principal vs. Interest Earned
Separate “money I put in” from “money growth created”
Every ending balance is really two numbers stacked together: the cash you supplied and the interest that arrived on its own. Splitting them apart is the single most useful habit when reading a compound interest result, because only one of those two numbers is under your control.
Do the split by hand first
Your principal is easy to compute without any tool. Take your starting amount, add your monthly contribution times twelve times the number of years, and you have every dollar you personally supplied.
For a $5,000 start with $200 a month over 20 years, that is $5,000 plus $48,000, so $53,000 of principal.
Subtract that from whatever the calculator shows as the ending balance. The remainder is interest earned, and seeing it as a standalone figure makes the concept concrete in a way the total never does.
- Principal is the start amount plus every contribution you make.
- Interest earned is the ending balance minus that principal.
- Early on, principal is nearly the whole balance.
- On long horizons at decent rates, interest usually overtakes principal.
- You control principal directly; interest depends on rate and time.
Find the year they cross over
There is a specific year when accumulated interest first exceeds total principal. Before it, you are carrying the account. After it, the account is carrying you.
Find yours by running the same inputs at several lengths — 10 years, 15, 20, 25 — and computing the split each time. The crossover tends to land in the late teens or twenties at moderate rates.
Marking that year on your plan is genuinely motivating. It converts a vague promise about compounding into a date you can wait for.
What this changes about your behavior
If you are years away from the crossover, the most productive thing you can do is add principal. Growth has nothing to work with until you supply it.
If you are past the crossover, protecting the balance becomes as important as adding to it. A withdrawal at that stage removes not just the cash but all the future growth that cash would have produced.
That reframes an early withdrawal from a car or a vacation. You are not spending $10,000 today; you are spending what $10,000 would have become by the end of your horizon.
Why the split matters for taxes
In a regular brokerage account, the two halves are treated differently when you eventually sell. Your principal comes back to you untaxed because you already paid tax on that income; only the growth portion is subject to tax.
That is why brokerages track your cost basis, which is just a formal name for the principal figure you calculated above. Keeping your own record of total contributions is a useful backup.
Inside a Roth account the distinction matters differently, since qualified withdrawals of both principal and growth come out tax-free. Knowing which half is which makes those account rules much easier to follow.
A caution about the interest number
The interest figure is the least reliable part of the projection because it depends entirely on a rate you assumed. Your principal is a fact, and the interest is an estimate.
Rerun the same scenario two points lower and recompute the split. Principal stays exactly the same; interest shrinks noticeably.
That contrast is the strongest argument for treating your contribution as the foundation of the plan and the growth as the bonus on top of it.
Try these inputs
- Baseline: $5,000 start, $200/mo, 6%, 20 years — split the result
- Longer: same inputs at 30 years — recompute the split
- Lower rate: $5,000 start, $200/mo, 4%, 20 years