房贷计算器

Mortgage payment formula, worked by hand

Take a 375,000 home bought with 20% down, which leaves a loan of 300,000, at 7.03% for 30 years. That rate was Freddie Mac's average for a 30-year fixed mortgage in its survey published on 24 September 2026 (Primary Mortgage Market Survey).

1. Monthly rate: r = 7.03 ÷ 1,200 = 0.0058583.
2. Number of payments: n = 30 × 12 = 360.
3. Growth factor: (1 + r)^360 = 8.18945.
4. First month's interest: 300,000 × r = 1,757.50.
5. Payment: 1,757.50 × 8.18945 ÷ (8.18945 − 1) = 2,001.96.

Adding 1.1% property tax (375,000 × 0.011 ÷ 12 = 343.75) and 1,500 a year of homeowners insurance (125.00) gives a first monthly payment of 2,470.71. Excel's =PMT(7.03%/12, 360, -300000) returns the same 2,001.96 for principal and interest.

Keep every digit of r until the end. Rounding it to 0.00586 before raising it to the 360th power makes the payment 2,002.36, 40 cents a month and about 145 over the loan.

How amortization splits each payment

Each month's interest is the balance times r, and the rest of the payment repays principal. Because the balance falls slowly at first, early payments are mostly interest. This excerpt of the schedule is for the 300,000 loan above:

In the first five years you pay 102,602.45 of interest and repay only 17,514.88 of the loan. Principal first exceeds interest in payment 243, and the balance drops below half the loan after payment 262, almost 22 years in. Total interest is 420,703.98, 1.4 times the amount borrowed. If you expect to sell or refinance within a few years, the balance column, not the payment, tells you how much equity you will have.

Property tax, insurance, PMI and escrow

Principal, interest, taxes and insurance together are called PITI. A lender that sets up an escrow account collects the tax and insurance with each payment and pays the bills when they fall due. In the US, Regulation X lets the servicer collect one-twelfth of the expected yearly bills plus a cushion of no more than one-sixth of them (12 CFR 1024.17(c)). With 5,625 a year of tax and insurance, the cushion can reach 937.50. The CFPB notes that the escrow payment changes when tax and premiums change, while the calculator holds both at today's amounts.

A lender may require private mortgage insurance (PMI) on a conventional loan with less than 20% down (CFPB). With 5% down on the same home, the loan is 356,250, principal and interest are 2,377.32, and PMI at 0.5% a year adds 148.44 a month. The calculator stops PMI once the balance reaches 80% of the price (300,000), which happens after payment 130, for 19,296.88 in total. From that point you can ask in writing to cancel. If you never ask, the servicer must end PMI when the balance is scheduled to reach 78%, after payment 142, which costs 12 more months and 1,781.25. The CFPB defines original value as the lower of the sale price and the appraisal, so a low appraisal moves the 80% point later than the calculator shows.

In Canada, mortgage loan insurance is typically required with less than 20% down (FCAC). The premium is a percentage of the mortgage that is paid as a lump sum or added to the loan (CMHC). It is not a monthly charge that stops at 80%, so set PMI to 0. If the premium is added to the loan, lower the down payment so the loan amount includes it.

15-year vs 30-year mortgage

In the same Freddie Mac survey the 15-year average was 6.42%, 0.61 points below the 30-year rate. On the 300,000 loan:

The 15-year loan costs 598.19 more a month and 252,677.62 less in interest. Most of that saving comes from the shorter term: at 7.03% over 15 years, interest would be 186,273.42, so the lower rate saves a further 18,247.06. The 30-year loan's lower required payment leaves slack if income falls, and extra payments on it recover part of the gap, as shown below.

How the interest rate changes your payment

For a 300,000 loan:

On the 30-year loan each half point adds 93 to 104 a month and 33,400 to 37,300 of interest over the term. Payments scale with the loan, so for 450,000 multiply any row by 1.5. When comparing offers, use the note rate in this calculator: the APR also spreads points and fees over the loan, so it is usually higher than the rate (CFPB).

Extra payments and biweekly plans

Extra principal lowers the balance that every later month's interest is charged on. On the 300,000 loan at 7.03%:

The 166.83 row is one-twelfth of the monthly payment, which adds up to one extra payment a year. A biweekly plan, paying half the payment every two weeks, makes 26 half-payments a year and has close to the same effect. Entering principal and interest ÷ 12 as the extra payment models it. Check the loan note for a prepayment penalty before paying extra.

Mistakes that skew the result

• Canadian rates. Canada's Interest Act requires a blended-payment mortgage to state its rate "calculated yearly or half-yearly, not in advance" (section 6). This calculator compounds monthly, so convert first: 12 × ((1 + rate ÷ 2)^(1/6) − 1). A 5.00% semi-annual rate is 4.9487% monthly; on 300,000 over 25 years the payment is 1,744.81, not the 1,753.77 you get by entering 5%.
• Reading the total as permanent. The total monthly payment is the first month's. It falls by the PMI amount once PMI ends, and escrow is reset whenever tax or insurance bills change.
• Adjustable rates. The calculator assumes one fixed rate. For an adjustable-rate loan, run it once for the fixed period and again at a plausible reset rate.
• Currency. The currency setting changes the symbol and digit grouping, not the arithmetic. A home loan in India or the UK follows the same annuity formula; the loan EMI calculator adds tenure in months and one-off prepayments.

To work back from the payment you can afford to a loan size and price, use the loan affordability calculator. To weigh buying against renting, use the rent vs buy calculator. If you already hold a mortgage, the refinance calculator compares cash payments and the debt still owed when you expect to sell or refinance again.