Flat rate
With principal
Interest on the original principal, split evenly.
- Behaviour
- Instalment stays level. Interest does not fall as the balance falls.
- Suits
- Short terms where a predictable, identical payment matters more than interest efficiency.
Total interest is the principal times the monthly rate times the term. It is divided evenly across the instalments, so every payment is the same size and the split never moves.
Model this in the lab