You just took out a $300,000 mortgage for 30 years. Your monthly payment is set, but you have questions: How much of that payment is actually paying off the house? Why does the balance seem to drop so slowly? What if you paid an extra $100 every month? You could try to build a spreadsheet from scratch using complex financial formulas. You would need to calculate interest for each month based on the remaining balance, subtract it from the payment, and repeat this 360 times for a 30-year loan. Or you could use an amortization calculator to instantly generate a complete schedule. It will show you that in the first month, only $300 of your payment goes toward the house, while $1,500 goes to interest. It will also show you exactly when that ratio flips. An amortization calculator computes the breakdown of every single payment over the life of a loan. It reveals the split between Principal (what you owe) and Interest (the lender's profit) for every month until the balance reaches zero. A...