Methodology
Calculation Methodology
A transparent explanation of the formulas, timing conventions, and limits behind the CompCalcs calculator.
Last reviewed: July 2026
What the calculator models
The calculator projects a starting balance plus a constant recurring contribution under a fixed annual rate. It is a reproducible mathematical model: changing principal, contribution, rate, years, or frequency changes the output immediately in the browser. It is not a forecast of any particular investment or market.
Core formulas
For a starting principal P, annual rate r, compounding periods n, and t years, the principal-only formula is A = P(1 + r/n)^(nt). With recurring contributions, the engine adds each contribution at its documented contribution date and applies the relevant periodic growth. This makes the contribution schedule visible in the year-by-year table instead of hiding it inside a single result.
Contribution timing
Monthly mode uses monthly compounding and end-of-month contributions. Annual mode uses annual compounding and adds each monthly contribution at the beginning of its modeled month, then applies the annual interest after the twelve monthly contributions. This is a deliberate sitewide convention: in annual mode, contributions made during the year are counted before that year's annual interest.
A reproducible example
To reproduce a result, record the five inputs shown in the calculator: starting principal, monthly contribution, annual rate, years, and frequency. For example, enter $100,000, $5,000, 5%, 10 years, and monthly mode. The annual table then shows the balance, amount invested, and modeled interest for each year. A second reader can enter the same values and receive the same cents-rounded result because the calculation runs from the shared finance engine.
Inflation, fees, and taxes
The nominal balance is not the same as spending power. Inflation can reduce the future purchasing power of the displayed amount, while account fees, taxes, contribution changes, withdrawals, and irregular deposits can reduce or change actual outcomes. Unless a page explicitly adds one of these assumptions, the calculator does not model them. For an inflation-adjusted comparison, use the stated inflation input in the relevant guide and treat the result as a sensitivity analysis, not a promise.
What the model cannot tell you
A constant return smooths away volatility, sequence of returns, liquidity needs, product rules, and the possibility of loss. The calculator does not choose an asset, estimate a safe withdrawal rate, calculate tax liability, or recommend a contribution amount. Its purpose is education: compare assumptions, inspect compounding mechanics, and understand how time and deposits affect a hypothetical balance.
Inputs and how they are interpreted
| Input | Interpretation |
|---|---|
| Principal | Initial amount at time zero |
| Monthly contribution | Recurring amount used for each modeled period |
| Annual rate | Fixed nominal annual assumption, not a forecast |
| Years | Number of modeled years, from 1 to 60 |
| Frequency | Monthly/end timing or annual/beginning timing |