Money guide
How Compound Interest Works
Understand principal, compounding frequency, monthly contributions, and the assumptions behind compound-interest projections.
Interest can earn interest
Compound interest applies growth to the current balance, including earlier interest. Principal is the starting amount. Contributions add new money. Interest earned is the ending balance above principal and total contributions.
How the calculator models growth
The Compound Interest Calculator accepts principal, an equal monthly contribution, a fixed nominal annual rate, whole-year duration, and annual, semiannual, quarterly, monthly, or daily compounding. It converts the nominal rate to an equivalent monthly rate, applies growth, then adds each contribution at month end.
Worked projection
Consider $8,000.00 initially, $250.00 contributed at each month end, a fixed 5.5% nominal annual rate compounded quarterly, and seven years.
| Measure | Illustrative result |
|---|---|
| Initial principal | $8,000.00 |
| Total contributions | $21,000.00 |
| Total invested | $29,000.00 |
| Projected interest | $8,247.56 |
| Projected final value | $37,247.56 |
These figures use the same engine as the tool. Currency display is rounded while the calculation keeps more precision.
Important limits
The projection assumes rate and contribution stay constant. It does not model changing returns, taxes, fees, inflation, withdrawals, market risk, or a particular account's timing rules. Use results as scenarios, not promises. Simple ROI is a separate point-to-point measure described in How ROI Is Calculated.