About the Mortgage Calculator
Buying a home is probably the biggest purchase you'll ever make, and the monthly mortgage payment is the number that decides whether it actually fits your budget. Our Mortgage Calculator breaks that number down into its real parts - principal, interest, property tax, home insurance, PMI, and HOA dues - so you're not just seeing a total, you're seeing exactly where every dollar goes.
Most online mortgage calculators only show principal and interest, which can make a home look far more affordable than it really is once taxes and insurance are added. We built this one to show the full picture from the start.
How It Works
The calculator first works out your loan amount by subtracting your down payment from the home price. It then applies the standard fixed-rate amortization formula to that loan amount, your interest rate, and your loan term to find the monthly principal & interest payment. On top of that, it adds 1/12th of your estimated annual property tax and home insurance, plus PMI if your down payment is under 20%, plus any HOA dues you enter.
Formula & Methodology
The formula comes from the present value of an annuity: a fixed monthly payment M, discounted at the monthly rate r over n payments, must equal the loan amount P today. Rearranging that equation for M gives the standard formula above. The monthly rate r is your annual rate divided by 12 and converted to a decimal (a 6.5% annual rate is r = 0.065 ÷ 12 = 0.005417), and n is your loan term in years multiplied by 12 (30 years = 360 payments).
Property tax and homeowners insurance aren't part of this formula at all - lenders add them separately as 1/12th of the annual bill, collected monthly into an escrow account and paid on your behalf when the yearly bills come due. PMI (private mortgage insurance) is calculated as a percentage of your loan balance, typically 0.3%–1.5% annually depending on your credit score and loan-to-value ratio, also divided into a monthly amount.
Step-by-Step: Calculating It By Hand
- 1Subtract your down payment from the home price to get the loan principal, P.
- 2Divide your annual interest rate by 12 to get the monthly rate, r (as a decimal - 6.5% becomes 0.065 ÷ 12).
- 3Multiply your loan term in years by 12 to get the total number of payments, n.
- 4Compute (1+r)^n.
- 5Plug P, r, and (1+r)^n into M = P × [r(1+r)^n] / [(1+r)^n − 1] to get principal & interest.
- 6Add 1/12 of your estimated annual property tax.
- 7Add 1/12 of your estimated annual homeowners insurance.
- 8If your down payment is under 20%, add estimated monthly PMI (loan balance × annual PMI rate ÷ 12).
- 9Add any monthly HOA dues to reach your total estimated payment.
Examples
20% down payment
$400,000 home, $80,000 down, 6.5% rate, 30-year term → about $2,022 in principal & interest, with PMI dropping to $0 since you're at 80% LTV.
10% down payment
Same home with only $40,000 down means a larger loan and PMI added until you build 20% equity, raising the total monthly payment noticeably.
Advantages
- Shows your real, all-in monthly payment instead of just principal & interest
- Instantly reveals how PMI disappears once you reach 20% equity
- Lets you compare 15-, 20-, and 30-year terms side by side
- No sign-up, so you can run numbers privately before talking to a lender
Common Mistakes
- Forgetting property tax and insurance and assuming principal & interest is the whole payment
- Not accounting for PMI when putting down less than 20%
- Using a national average interest rate instead of a real quote from a lender
- Ignoring HOA dues, which can add hundreds of dollars a month in some communities
Edge Cases to Watch For
- A 0% promotional interest rate makes the standard formula divide by zero - the payment simplifies to just P ÷ n in that case.
- PMI typically cancels automatically at 78% loan-to-value based on the original amortization schedule (not 80%, and not your current market value), per the Homeowners Protection Act.
- This formula only holds for fixed-rate loans - adjustable-rate mortgages (ARMs) recalculate the payment when the rate resets after the initial fixed period.
- Extra principal payments reduce your balance and total interest, but won't lower your required monthly payment unless you specifically request re-amortization from your lender.
- Lenders round to the cent using their own conventions, so a manual calculation can land a cent or two off from your actual statement.
Common Use Cases
- Deciding how much home you can comfortably afford
- Comparing a 15-year vs. 30-year mortgage
- Checking how a larger down payment changes your payment
- Estimating costs before getting pre-approved