How to Calculate Your Monthly Mortgage Payment

People often say “mortgage payment” when they mean the full housing bill. Lenders split it into pieces.

• Principal — the part that reduces your loan balance
• Interest — the cost of borrowing that period
• Taxes & insurance (T&I) — property tax and homeowners insurance, often collected in escrow
• HOA / PMI — association dues or private mortgage insurance when applicable

The classic formula in this guide calculates principal + interest (P&I) only. Add local tax and insurance estimates on top for a true “can I afford this?” number. A $2,100 P&I payment can become $2,600–$2,900 once escrow is included, depending on location and coverage.

For general installment loans (auto, personal), the same math family appears in our loan payment guide and the free loan calculator.

For a fixed-rate loan with equal monthly payments, the payment M is:

M = P × [ r(1+r)^n ] ÷ [ (1+r)^n − 1 ]

• P — principal (loan amount after down payment)
• r — monthly interest rate (annual rate ÷ 12)
• n — number of months (years × 12)

Worked example

Home price $400,000, 20% down → loan P = $320,000. Rate 6.5% APR → r = 0.065 ÷ 12 = 0.0054167. Term 30 years → n = 360.

M ≈ 320,000 × [0.0054167 × (1.0054167)^360] ÷ [(1.0054167)^360 − 1] ≈ $2,022 per month for P&I (rounded).

Total paid over 30 years ≈ 2,022 × 360 ≈ $728,000. Total interest ≈ 728,000 − 320,000 ≈ $408,000. Exact cents depend on rounding rules lenders use; your calculator should match within a dollar or two of bank quotes for plain fixed loans.

Skip the algebra with the free online mortgage calculator — enter price, down payment, rate, and years.

Early payments are interest-heavy. Later payments are principal-heavy. The total monthly P&I stays fixed on a standard fixed-rate loan; only the split moves.

Why the first years feel slow

Interest each month is charged on the remaining balance. At the start, the balance is highest, so interest takes a big slice of M and only a small slice pays principal. As the balance drops, interest shrinks and more of each payment retires debt.

Illustrative split (order of magnitude)

On a long 30-year loan at mid-single-digit rates, it can take roughly a decade before principal and interest are closer to even — exact crossover depends on rate and term. That’s normal amortization, not a “bad” loan.

Amortization schedule

A schedule lists every month: payment, interest portion, principal portion, and remaining balance. It’s the map for extra payments: any extra applied to principal shortens the schedule and cuts total interest because future interest is computed on a smaller balance.

Need “what percent of my payment is interest this year?” for taxes or planning — the percentage calculator helps with those ratios.

Shorter terms mean higher monthly P&I and much lower total interest. Longer terms lower the monthly bill and raise lifetime interest.

Term	Monthly P&I (illustrative)	Total interest trend
15-year	Higher	Much lower total interest
20-year	Middle	Middle
30-year	Lower	Highest total interest

Same loan, different terms

Using the earlier $320,000 at 6.5%: 30-year P&I was about $2,022. A 15-year term on the same principal and rate jumps monthly payment into the roughly $2,780 range (order of magnitude — run exact numbers in a tool) but can cut total interest by well over $200,000 versus 30 years. Exact savings depend on rate and whether you invest the payment difference elsewhere.

Choosing a term

• 15-year — fastest equity build; needs stronger cash flow
• 30-year — lower required payment; more flexibility if you invest or keep reserves
• Hybrid approach — take 30-year for optionality, then prepay principal when cash allows

Always leave room for maintenance, emergencies, and escrow shocks. A payment that looks fine at 28% of gross income can hurt after taxes, insurance, and repairs.

Payment scales with loan size and rate. Small rate moves matter a lot on large balances.

Down payment

On a $400,000 home: 5% down → $380,000 financed; 20% down → $320,000 financed. That’s a $60,000 smaller principal and often avoids PMI. At 6.5% for 30 years, the P&I difference between those two principals is hundreds of dollars per month — run both scenarios before you lock a budget.

Rate sensitivity

On a $320,000 30-year loan, moving from 6.0% to 7.0% can change P&I by roughly $200+ per month (order of magnitude). That’s why shopping lenders and locking thoughtfully matters as much as negotiating $5,000 off the purchase price.

APR vs note rate

The amortization formula uses the note interest rate for monthly r. APR folds in certain fees for comparison shopping. When modeling monthly cash flow, use the rate that actually accrues on the balance; when comparing loan offers, use APR and fee sheets together.

For non-mortgage borrowing math, compare with the loan calculator.

Enter the purchase price (or loan amount), down payment, interest rate, and term. Read monthly P&I, total interest, and any amortization details the tool shows.

1. Open the free mortgage calculator.
2. Enter home price and down payment (or paste the exact loan amount).
3. Enter the annual interest rate you expect or were quoted.
4. Choose term in years (15, 20, or 30 are common).
5. Note monthly P&I first — that’s the formula output.
6. Add estimates for tax, insurance, HOA, and PMI for full housing cost.
7. Rerun with a higher rate (+0.5% or +1%) to stress-test affordability.
8. Compare 15-year vs 30-year on the same principal to see total interest.

Tips for cleaner comparisons

Keep property tax and insurance constant when you only want to compare rates or terms. Change one variable at a time. Screenshot or log each scenario so “the cheap payment” isn’t a memory of a half-finished form.

Extra principal payments reduce balance early, which cuts every future interest charge. Even $100–$200/month on a long mortgage can shave years and five figures of interest — exact impact depends on rate and when you start.

• Confirm application — tell the servicer extra is for principal, not future months’ interest only
• Watch prepayment rules — most standard U.S. mortgages allow prepayment; still read your note
• Don’t starve the emergency fund — high-interest credit card debt usually beats mortgage prepayment priority

Common mistakes

• Comparing only monthly P&I and ignoring taxes, insurance, and HOA
• Using list price instead of likely financed amount after down payment
• Forgetting PMI when down payment is under 20%
• Assuming today’s teaser or adjustable rate lasts 30 years
• Ignoring closing costs and cash-to-close when “affordable payment” still needs a large check at signing

This is educational, not lending advice. Final terms come from your lender’s disclosures and underwriting.