Mortgage Calculator

How to Use the Mortgage Calculator

This calculator handles four mortgage scenarios: calculate your monthly payment, find the maximum loan you can afford, determine how quickly you can pay off a mortgage, and measure the impact of extra monthly payments. All calculations use standard amortization formulas with exact decimal arithmetic. Results include charts, amortization schedules, and payoff comparisons.

Calculation Modes

Payment — Find Your Monthly Payment

Enter the loan amount, annual interest rate, and term in years. The calculator computes the fixed principal-and-interest monthly payment.

Formula: M = P × [r(1 + r)^n] / [(1 + r)^n − 1]

Where P = principal, r = monthly rate (annual / 12), n = total months (years × 12).

Example: $300,000 mortgage at 6% for 30 years

• Loan Amount: 300000
• Interest Rate: 6
• Loan Term: 30
• Monthly Payment: 1,798.6516

Example: $250,000 mortgage at 5% for 15 years

• Loan Amount: 250000
• Interest Rate: 5
• Loan Term: 15
• Monthly Payment: 1,976.9841

After calculating, a stacked area chart shows how principal (green) and interest (red) shift over the life of the loan. Early payments are mostly interest; later payments are mostly principal. Below the chart, a paginated amortization schedule breaks down every payment into principal, interest, and remaining balance. The table shows 12 rows per page with navigation controls. A footer row displays total principal, total interest (in red), and total cost.

Amount — Find How Much You Can Borrow

Enter your target monthly payment, the interest rate, and the loan term. The calculator computes the maximum mortgage amount you can afford.

Formula: P = M × [(1 + r)^n − 1] / [r(1 + r)^n]

Example: You can afford $2,000/month at 5.5% for 30 years

• Monthly Payment: 2000
• Interest Rate: 5.5
• Loan Term: 30
• Max Loan Amount: 352,243.5262

Example: You can afford $1,500/month at 4% for 20 years

• Monthly Payment: 1500
• Interest Rate: 4
• Loan Term: 20
• Max Loan Amount: 247,532.7874

A bar chart compares the principal amount (green) against the total interest (red) over the term. Below the chart, an affordability summary table shows the max loan amount (in green), monthly payment, interest rate, loan term, total payments, total interest (in red), and total cost.

Term — Find the Payoff Time

Enter the loan amount, interest rate, and your monthly payment. The calculator computes how many months it will take to pay off the mortgage and compares it against a standard 30-year term.

Formula: n = −ln(1 − rP/M) / ln(1 + r)

Example: $200,000 mortgage at 5% with $1,200/month payments

• Loan Amount: 200000
• Interest Rate: 5
• Monthly Payment: 1200
• Payoff Time: 285 months (23 years 9 months)

A bar chart compares your payoff timeline against a standard 30-year term. A payoff timeline table shows your term in years and months, total payments, monthly payment, principal, total interest (in red), total cost, standard 30-year total cost, and interest saved versus the baseline (highlighted in green when positive). If your payment is too low to cover the interest, the calculator shows an error.

Example: $150,000 mortgage at 4.5% with $1,000/month payments

• Loan Amount: 150000
• Interest Rate: 4.5
• Monthly Payment: 1000
• Payoff Time: 221 months (18 years 5 months)

Extra — Calculate Extra Payment Savings

Enter the loan amount, interest rate, original term, and an extra monthly payment amount. The calculator shows how much faster you will pay off the mortgage and how much interest you will save.

The calculator first computes your normal monthly payment from the loan terms. It then recalculates the payoff time using the normal payment plus the extra amount. The result is the total interest saved.

Example: $300,000 mortgage at 5% for 30 years, paying an extra $150/month

• Loan Amount: 300000
• Interest Rate: 5
• Loan Term: 30
• Extra Monthly Payment: 150
• Interest Saved: 55,604.247

A bar chart compares the normal total cost (blue) against the accelerated total cost with extra payments (green). Below the chart, a savings summary table shows the monthly payment, extra payment, normal payoff time versus accelerated payoff time, time saved, total cost for both scenarios, total interest for both scenarios (in red), and interest saved (highlighted in green).

Example: $200,000 mortgage at 4.5% for 30 years, paying an extra $100/month

• Loan Amount: 200000
• Interest Rate: 4.5
• Loan Term: 30
• Extra Monthly Payment: 100
• Interest Saved: 31,745.735

Precision and Notation Settings

Use the settings bar above the tabs to control how results display:

• Decimal places: Adjust from 0 to 16 decimal places (default is 8)
• Notation: Switch between auto, fixed, exponential, and engineering notation (default is auto)

Your decimal places and notation settings save to your browser automatically. When you return to this calculator, your last chosen format loads without needing to adjust it again.

Common Use Cases

Buying Your First Home

You are pre-approved and want to know the payment on a $350,000 home at 6.5% for 30 years:

1. Switch to Payment
2. Enter 350000 as Loan Amount
3. Enter 6.5 as Interest Rate
4. Enter 30 as Loan Term
5. Monthly Payment: 2,212.2381

Setting a House-Hunting Budget

You can comfortably afford $1,800/month and rates are at 5.5% for 30 years:

1. Switch to Amount
2. Enter 1800 as Monthly Payment
3. Enter 5.5 as Interest Rate
4. Enter 30 as Loan Term
5. Max Loan Amount: 317,019.1736
6. Use this as your price ceiling (plus down payment)

Paying Off Your Mortgage Early

You have a $300,000 mortgage at 5.5% with 25 years remaining. What if you pay $250 extra each month?

1. Switch to Extra
2. Enter 300000 as Loan Amount
3. Enter 5.5 as Interest Rate
4. Enter 25 as Loan Term
5. Enter 250 as Extra Monthly Payment
6. Interest Saved: 62,856.087
7. You would pay off the loan 5 years 6 months sooner

Evaluating a 15-Year vs 30-Year Mortgage

You are borrowing $100,000 at 4%:

• 30-year payment: 477.4153/month, total interest 71,869.506
• 15-year payment: 739.68793/month, total interest 33,143.827
• Interest saved with 15-year: 38,725.68
• The 15-year option costs 262.27263 more per month but saves 38,725.68 in interest

Refinancing Analysis

You owe $280,000 at 6.5% with 25 years remaining. A lender offers 5.25% with $4,000 in closing costs:

1. First, calculate current payment in Payment mode: 280000, 6.5, 25. Monthly Payment: 1,890.5801

Now calculate the new loan with closing costs added to the principal:

1. Enter 284000 as Loan Amount
2. Enter 5.25 as Interest Rate
3. Enter 25 as Loan Term
4. Monthly Payment: 1,701.8635
5. Monthly savings: 188.71654
6. Break-even: approximately 21 months

Tips for Accurate Calculations

1. Enter the annual rate, not monthly: The calculator divides by 12 internally. Enter 6 for 6% APR, not 0.5
2. Rates must be positive: 0% uses simple division; negative rates are rejected
3. Payment must exceed monthly interest: In Term and Extra modes, a payment below the first month's interest triggers an error
4. Amortization schedules are paginated: Use page controls to navigate. The footer shows totals
5. Extra payments go entirely to principal: The calculator assumes the extra amount is applied directly to principal reduction
6. Interest saved compares to the original term: Extra mode assumes minimum payments for the full term
7. Chart colors match the table: Green represents principal or savings, red represents interest or costs
8. The amortization table rounds per-payment values: Individual rows may not sum exactly to totals; the footer is exact
9. Large mortgages may display in scientific notation: Switch notation to fixed for comma-separated formatting

Troubleshooting

Required

This appears when a field is left empty. Enter a value in all required fields before calculating.

Amount must be positive

Enter a positive loan amount. Zero or negative amounts are not valid.

Rate cannot be negative

Enter a positive annual percentage rate. Zero is accepted but negative rates are rejected.

Term must be positive

Enter a positive number of years. Partial years are accepted.

Payment must be positive

Enter a positive monthly payment amount.

Extra payment cannot be negative

Extra payments must be zero or positive. Enter the additional amount you want to pay.

Payment too low to cover interest

In Term or Extra mode, your payment does not cover the interest accrued in the first month. The loan balance would grow instead of shrink. Increase your payment, reduce the amount, or lower the rate.

Chart or table not appearing

The details section only renders after a successful calculation with no validation errors. Check that all inputs are filled and valid, then click Calculate.