Guide · Financial Calculator

Mortgage payment calculator — free online guide

A mortgage is one of the most straightforward Time Value of Money problems. You borrow a fixed amount today (PV), repay it in equal monthly instalments (PMT) over a set number of years (N), at a fixed interest rate (I/Y). Pebble's Financial calculator lets you solve for any one of these variables in seconds — no spreadsheet required.

This guide shows you exactly which numbers to enter, walks through real 15-year and 30-year examples, and explains how to use the calculator to compare loan scenarios. Whether you're a first-time buyer trying to understand affordability or refinancing an existing loan, the same five-variable setup applies.

Understanding mortgage math

A fixed-rate mortgage is an annuity: a series of equal periodic payments made to repay a lump sum borrowed today, with interest charged on the declining balance. Because interest is charged monthly on the remaining principal, early payments are mostly interest and later payments are mostly principal — this is called amortisation.

The exact split between principal and interest changes with every payment, but the monthly total stays constant. The TVM calculator handles all of this automatically — you just enter four variables and solve for the fifth.

The five TVM variables for a mortgage

Switch Pebble to Financial mode (tap the mode button and select 💰). You'll see five fields — here's what each means for a mortgage:

• N — Total number of monthly payments (years × 12)
• I/Y — Annual interest rate as a percentage (e.g. 6.75)
• PV — Loan amount — the amount you're borrowing after your down payment
• PMT — Monthly payment (what you want to find)
• FV — Remaining balance at end — enter 0 for a fully paid-off mortgage

Enter any four of these values, then tap CPT followed by the variable you want to solve. The calculator finds the missing piece.

Example: 30-year mortgage

You're buying a $400,000 home with a 20% down payment ($80,000), so your loan amount is $320,000. Your lender is offering 6.75% annual interest on a 30-year term. What is your monthly payment?

Over 30 years you'll pay roughly $747,000 in total — meaning about $427,000 goes to interest on a $320,000 loan. That's a useful number to have in mind when comparing loan options or deciding whether to put a larger down payment down.

Example: 15-year mortgage (same loan)

Same $320,000 loan at 6.75%, but over 15 years instead. How much more do you pay per month, and how much interest do you save overall?

The 15-year payment is about $756 more per month — but your total interest paid drops to around $190,000, saving you over $237,000 compared to the 30-year term. The financial calculator makes this comparison instant.

Solving for other variables

What loan can I afford?

If you know what monthly payment fits your budget, enter it as PMT (as a negative number — it's an outflow), fill in I/Y and N for your expected rate and term, set FV to 0, then tap CPT → PV. The result is the maximum loan amount you can support at that payment.

What rate do I need to refinance?

Know your current balance, remaining term, and target payment? Enter those as PV, N, and PMT, set FV to 0, and tap CPT → I/Y. You'll see the interest rate needed to hit that payment — useful when shopping lenders or evaluating whether refinancing makes sense.

How long to pay off with extra payments?

Planning to make extra principal payments each month? Enter your loan balance as PV, your rate as I/Y, your new higher payment as PMT (negative), 0 as FV, and tap CPT → N. The result is the number of months until payoff — you may be surprised how much time a small extra payment saves.

For more financial calculations, see the compound interest guide, the student loan calculator, or the financial independence guide.

Frequently asked questions