Financial Planning•6 min read
Monthly Payment Formula Explained
We peel back the curtain on the math lenders use to calculate your monthly debt obligations with the standard annuity formula.
Ever wondered how your bank arrives at that exact monthly payment number? It's not a secret—it's a standard mathematical formula.
The Annuity Formula
Lenders use the formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n - 1 ], where P is principal, i is monthly interest rate, and n is the number of months.
The Power of Math
Understanding this formula allows you to see how even small changes in interest rates or loan terms can have a massive impact on your monthly obligation.
Do banks use different formulas?
Most standard fixed-rate loans at major banks use this exact formula, though some specialized commercial or private loans may use different compounding methods.