Everyday money · General financial education

Compounding on debt

In saving, compounding can grow a balance. In debt, unpaid interest can be added to the balance and then itself bear interest. Same mechanic, other direction.

In more detail

Many cards and overdrafts charge interest periodically (often daily or monthly) on the open amount. If you pay less than interest plus principal, the balance can grow. The statement and contract formula show the rhythm; Finmon invents no rates.

Background

Compounding on investments and compounding on debt use related maths. Whether interest is added to the balance, and how, is in the contract, within limits set by local law (in Germany e.g. § 248 BGB). This module explains that mechanic and recommends no product. It sits next to compound growth on savings and to interest.

An example

A card shows €800. You pay only part of the accrued interest. The rest is added to the balance; the next cycle uses a higher amount.

A common misconception

Compounding only affects savings and investments.

Unpaid interest on debt can be capitalised and then itself bear interest.

Source: CFPB · Credit card interest calculation. Last reviewed: 2026-11-09. Editorially reviewed on 2026-11-09. Next review 2027-11-09. Version 0.2.0. Finmon explains. It does not advise. Not investment, tax or legal advice.