mortgage calculator excel formula

The core mortgage calculator Excel formula

The standard mortgage payment formula in Excel uses the PMT function. If your loan amount is in B2, annual rate in B3, and years in B4, the formula is:

=PMT(B3/12, B4*12, -B2)

This returns your monthly principal-and-interest payment. The minus sign before B2 makes the result show as a positive payment amount.

What each part means

• B3/12 = monthly interest rate
• B4*12 = total number of monthly payments
• -B2 = present value (loan principal as cash outflow)

Recommended Excel layout

Use this clean structure so your formulas stay readable:

• B2: Loan Amount (example: 350000)
• B3: Annual Interest Rate (example: 6.5%)
• B4: Loan Term in Years (example: 30)
• B5: Monthly Payment formula

Then place this in B5:

=PMT(B3/12,B4*12,-B2)

Break payment into principal and interest

If you want an amortization schedule, use IPMT and PPMT for each month.

• Interest (month 1): =IPMT($B$3/12,1,$B$4*12,-$B$2)
• Principal (month 1): =PPMT($B$3/12,1,$B$4*12,-$B$2)

Copy these down for each month by replacing the period number (1, 2, 3, ...).

Excel formula for payoff with extra payments

If you pay extra each month, you can estimate how many months are needed with NPER:

=NPER(B3/12, -(PMT(B3/12,B4*12,-B2)+B6), B2)

Where B6 is your extra monthly payment. This quickly shows how much faster the mortgage can be paid off.

Common mistakes to avoid

• Forgetting to divide annual rate by 12.
• Using years instead of total months for number of periods.
• Missing the negative sign on loan amount, causing sign confusion.
• Mixing percentage format (6.5%) with decimal format (0.065) incorrectly.

Practical tips for accurate mortgage models

1) Keep assumptions in one input section

Put loan amount, rate, term, taxes, insurance, PMI, and extra payment in a dedicated input block. This makes scenario testing much easier.

2) Separate P&I from total housing cost

The PMT formula gives principal and interest only. Your actual monthly payment may also include escrow, HOA dues, and mortgage insurance.

3) Build scenario columns

Create “Base,” “+1% Rate,” and “Extra $200” columns. This turns a simple calculator into a decision tool for refinancing or faster payoff planning.

Quick FAQ

Is PMT enough for a full mortgage calculator?

For monthly principal-and-interest, yes. For full budgeting, add taxes, insurance, and fees separately.

Can I use this for bi-weekly payments?

Yes. Change the period math to 26 payments per year and adjust the rate per period accordingly.

What if interest rate is 0%?

Then payment is simply loan amount divided by months. The calculator above handles that case automatically.