Housing

Mortgage repayment calculator

A mortgage payment is only one part of the housing decision. The useful view combines principal, interest, recurring property costs, and the share of household income committed each month.

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Editable source

# Mortgage pressure
home_price = 350000 in usd
down_payment = 70000 in usd
principal = home_price - down_payment
monthly_payment = 1469.65 in usd
property_costs = 250 in usd
household_income = 6500 in usd
housing_cost = monthly_payment + property_costs
housing_load = housing_cost / household_income
loan_to_value = principal / home_price

Inputs

What the model needs

Home price: The agreed or expected purchase price.
Down payment: Cash applied before the mortgage principal is calculated.
Interest rate: The annual mortgage rate used for the repayment estimate.
Term: The repayment period in years.
Property costs: Recurring tax, insurance, association, or maintenance allowance.
Household income: Monthly income used only for the housing-load check.

Method

Formulas stay visible

principal = home_price - down_paymentmonthly_rate = annual_interest_rate / 12payment = principal × monthly_rate × (1 + monthly_rate)^n / ((1 + monthly_rate)^n - 1)housing_load = (payment + property_costs) / household_income

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Worked example

A $350,000 home with 20% down

$280,000 principal
4.8% annual rate
30-year term
$250 monthly property costs

ResultThe modeled payment is about $1,470; total monthly housing cost is about $1,720.

Against $6,500 monthly household income, the modeled housing load is roughly 26.5% before utilities and irregular repairs.

Stress-test it

Move the interest rate by one percentage point in both directions.
Add a realistic maintenance allowance instead of using only the lender payment.
Compare the cash impact of a larger down payment with the liquidity it removes.

Use it when

Checking a likely payment before speaking with a lender
Comparing down-payment choices
Understanding how property costs change affordability

Know the boundary

The worked example uses a fixed-rate amortization model.
Taxes, insurance, fees, and rate changes vary by location and loan.
The affordability signal is informational and is not lending advice.

Questions

Does the mortgage payment include taxes and insurance?

The loan payment does not. Add recurring property costs separately so the household-load result reflects the expenses you expect to pay.

Why test a higher interest rate?

It reveals whether the decision is robust to a less favorable quote, future refinancing, or a variable-rate change.

Is a larger down payment always better?

It reduces principal and interest but also converts liquid cash into home equity. Model the emergency reserve left after closing as a separate decision.

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Open mortgage repayment calculator

Mortgage repayment calculator

What the model needs

Formulas stay visible

Stress-test it

Use it when

Know the boundary

Turn this example intoyour living model.

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