How to Calculate Mortgage Payments: Formulas, Amortisation & APR Breakdown
Purchasing a home is typically the largest financial transaction of a person's life. While lenders provide repayment estimates, understanding how monthly installments are computed empowers you to save tens of thousands in interest fees over the lifespan of your loan.
Written by the Toolseza team · Updated 2026-10-31
The Standard Amortisation Mortgage Formula
Fixed-rate mortgages utilize standard annuity mathematics. The monthly payment M is calculated as:
M = P × [ r(1 + r)^n ] ÷ [ (1 + r)^n − 1 ]
Where: P is the Principal loan amount, r is the monthly interest rate (Annual rate divided by 12 months), and n is the total number of monthly payments (Loan term in years × 12).
Example: Borrowing $300,000 at 6.0% annual interest over 30 years (360 months). Monthly interest r = 0.06 ÷ 12 = 0.005. The formula calculates a monthly principal and interest payment of exactly $1,798.65.
The Front-Loaded Nature of Amortisation
In the first five years of a 30-year loan, more than 80% of every monthly check goes toward interest charges rather than reducing debt principal. In our $300,000 example, Month 1 entails $1,500.00 in interest and only $298.65 in equity.
By year 20, the proportion flips: interest drops as the remaining loan balance shrinks, and the majority of each payment accelerates principal equity building.
The Massive Financial Impact of Loan Term (15 vs 30 Years)
Choosing a 15-year term instead of a 30-year term increases your monthly payment, but cuts your total lifetime interest bill dramatically:
• 30-Year Mortgage ($300k at 6%): Monthly $1,798.65 | Total Interest: $347,514 | Total Repaid: $647,514
• 15-Year Mortgage ($300k at 6%): Monthly $2,531.60 | Total Interest: $155,688 | Total Repaid: $455,688
Opting for the 15-year loan saves an astounding $191,826 in pure interest charges!
How Early Overpayments Slash Loan Terms
Because interest is calculated on your remaining balance each month, making an extra payment of even $100 per month on a $300,000 30-year mortgage knocks over 4 years off the mortgage and saves over $30,000 in interest.