Why two compound interest calculators can disagree.

The phrase “7% per year” can hide a monthly assumption.

If a calculator updates monthly, it needs a monthly growth rate. One common shortcut is to divide the annual rate by 12. A 7% annual rate becomes about 0.5833% per month. Another approach asks for the monthly rate that compounds to exactly 7% over a full year. That monthly rate is slightly lower.

Terra uses the second convention: the annual input is treated as an effective annual return, then converted into the monthly rate that would compound to that annual result.

Why the answers diverge.

If a calculator simply uses 7% ÷ 12 every month, the monthly compounding itself pushes the full-year effective return above 7%. Over 30 years of monthly contributions, that small convention difference can create a noticeable gap in the ending balance.

That does not mean one result is a prediction and the other is wrong. It means the calculators are defining the annual input differently. When checking one calculator against another, compare the return convention before comparing the final dollar amount.

Contribution timing matters too.

Monthly contributions made at the beginning of each month have slightly more time to grow than contributions made at the end of each month. Terra's quick compound calculator models month-end contributions. Other calculators may use beginning-of-month deposits or another timing assumption.

Use calculators for scenarios, not precision theater.

Real returns do not arrive at a steady monthly rate. Markets move unevenly, fees and taxes vary, contributions change, and inflation affects purchasing power. A compound calculator is useful because it makes the relationship between time, contribution rate and assumed return visible — not because it can predict a future balance to the dollar.

If you are comparing outputs, check the annual-return convention, contribution timing, fees, taxes and inflation before deciding that one tool is “off.”