Simple Mortgage Calculator
Estimate the monthly principal-and-interest payment for a fixed-rate mortgage from the loan amount, annual interest rate, and loan term.
Mortgage inputs
Required. U.S. dollars; use a positive amount up to $1 trillion.
Required. Annual nominal rate from 0% through 100%.
Required. Years, up to 100; the value must equal a whole number of months.
Monthly payment estimate
Estimated monthly principal & interest
Formula: M = P × r × (1 + r)n ÷ ((1 + r)n – 1)
This estimate covers principal and interest only. Property taxes, homeowners insurance, mortgage insurance, HOA dues, closing costs, and other charges are outside this simple model.
How to use this simple mortgage calculator
What this calculator does
This calculator estimates the level monthly principal-and-interest payment for a fixed-rate, fully amortizing mortgage. It uses the loan principal, the stated yearly interest rate, and the repayment term to solve the standard payment formula. The result is a planning estimate, not a loan approval, quote, or complete housing-cost forecast. The Consumer Financial Protection Bureau explains that a lender's principal-and-interest calculation is driven by the same three core terms, while the payment you actually send may also include taxes and insurance. See the CFPB's explanation of how mortgage lenders calculate monthly payments.
When to use it
Use this calculator when you want a quick principal-and-interest estimate before requesting a formal quote, when comparing the payment effect of two fixed interest rates, when checking how a shorter or longer term changes the required payment, or when reviewing whether a lender's stated principal-and-interest figure is in the expected range. It is intentionally narrow: it does not add escrowed property taxes, homeowners insurance, mortgage insurance, association fees, or other ownership costs.
How to calculate
- The calculator opens with a complete demonstration: a $100,000.00 loan, 4.50% yearly interest rate, and 30-year term. The example immediately produces the payment and a validated Excel workbook.
- Replace Loan amount with the principal you expect to borrow. Enter U.S. dollars using plain digits or standard comma grouping; the dollar sign is optional.
- Replace Yearly interest rate with the nominal annual fixed rate. You may type the percent sign or leave it off. This is the interest rate, not APR, because APR can include additional loan charges.
- Set Loan term in years. The value must correspond to a whole number of monthly payments, so 30 or 15.5 years is valid, while a value that would create a fraction of a month is rejected.
- Read Estimated monthly principal & interest. Results update as you edit valid inputs. The three header pills labeled Principal, Annual rate, and Term mirror the current valid assumptions so you can verify the scenario at a glance.
- Select Download Excel to export the current validated inputs and calculated payment to a real .xlsx workbook. Select Reset to clear the demonstration values and the calculated result. After Reset, Excel export is disabled until all three required inputs form a complete valid scenario again.
Input guide
Loan amount is required and represents mortgage principal in U.S. dollars. Enter a positive amount no greater than $1 trillion, such as $100,000.00. A larger principal increases the payment proportionally when rate and term stay unchanged. Standard U.S. comma grouping is accepted, but decimal-comma input such as “100000,50” is rejected rather than silently reinterpreted.
Yearly interest rate is required and accepts 0% through 100%, for example 4.50%. A higher fixed rate raises the monthly payment for the same principal and term. Zero is permitted and uses the simple principal-divided-by-months edge case. Do not paste an APR into this field unless APR and the note rate are actually the same; CFPB's guide to the difference between a mortgage interest rate and APR explains why they can differ.
Loan term is required and is entered in years, from more than 0 through 100. The term must convert to a whole number of monthly payments. The startup example uses 30 years, which equals 360 payments. A longer term generally lowers the required monthly payment because principal is spread across more payments, although it can increase the amount of interest paid over the full life of a typical loan.
Output guide
Estimated monthly principal & interest is the calculated fixed monthly payment in U.S. dollars. It is driven by all three inputs. A higher loan amount or rate raises it; a longer term typically lowers it. With a valid positive principal and term, the result is positive even at a 0% rate. The Principal, Annual rate, and Term pills are scenario summaries, not separate calculations. This page deliberately excludes taxes and insurance from the displayed payment; CFPB's Loan Estimate explainer shows where principal, interest, taxes, insurance, and other projected payment components appear on a formal mortgage disclosure.
Worked example
For the opening example, P = $100,000, the annual rate is 4.50%, and the term is 30 years. The monthly rate is 0.045 ÷ 12 = 0.00375 and the number of payments is 30 × 12 = 360. Substituting those values into the amortizing-payment formula gives an unrounded payment of about $506.6853, displayed as $506.69 per month. The browser calculation, visible result, and exported workbook all use the same unrounded canonical value before formatting.
How the fixed-rate mortgage formula works
The payment formula chooses one level monthly amount that is sufficient to pay the principal plus interest over the stated term, assuming the interest rate remains fixed and every scheduled payment is made. Early in a typical amortizing mortgage, more of each payment goes to interest because the outstanding balance is larger. As principal is reduced, the interest portion generally falls and more of the same payment goes toward principal. The CFPB's overview of how mortgage amortization pays down principal describes this shift.
For a zero interest rate, the standard expression would divide by zero, so this calculator uses the correct limit case: principal divided by the number of monthly payments. That keeps the result finite and intuitive. For positive rates, the formula converts the yearly percentage to a monthly decimal rate and compounds it across the monthly payment count.
Planning boundary: a principal-and-interest estimate is only one part of housing cost. When comparing real offers, review the lender's Loan Estimate and account for taxes, insurance, mortgage insurance, fees, and any other charges that apply to your property and loan.
How changing assumptions affects the payment
Loan amount has a direct proportional effect in this model: if every other assumption is unchanged, doubling principal doubles the monthly principal-and-interest payment. Interest rate has a nonlinear effect because every payment reflects the cost of carrying the remaining balance over time. Even a seemingly small rate difference can produce a meaningful monthly change on a large principal. Loan term works in the opposite direction for the monthly payment: spreading repayment over more months usually lowers the monthly amount, while shortening the term raises it because principal must be repaid faster.
When comparing scenarios, change one input at a time first so the reason for the difference is clear. Then test the combinations you are actually considering. Keep the scope in mind: a lower calculated principal-and-interest payment does not by itself mean the overall loan is less expensive or more suitable. Fees, cash required at closing, taxes, insurance, and product features can materially change the complete cost picture.