Methodology

Every number on SnapInterestCalc comes from a standard finance formula that you can verify. This page documents the exact math, assumptions, inflation and tax handling, and data sources used across all calculators.

Compound Interest Formula

FV = P · (1 + r/n)^(n·t) + C · [((1 + r/n)^(n·t) − 1) / (r/n)] · (n/12). Contributions treated as end-of-month deposits.

Simple Interest

I = P · r · t. No contributions added.

Inflation and Tax

Real value = FV / (1 + i)^t. After-tax value assumes taxes apply only to interest, not to principal or contributions.

Default Assumptions

Inflation defaults to 2.5% (long-run developed-market average). No fees are modelled. Rates are annual percentages.

Not Modelled

Sequence-of-returns risk, market volatility, currency conversion, and personal factors like debt or insurance are deliberately out of scope.