A workflow for anyone weighing up a home purchase: combine a mortgage payment calculation with the opportunity cost of the down payment to see the true financial picture. The plan is to compute monthly payment and total interest, then run the same down payment through a compound interest projection. Both tools run in your browser.
Open the mortgage calculator. Enter the property price, your down payment, the interest rate you expect, and the term in years.
Note the monthly payment. This is the affordability figure most buyers focus on, but it is only one part of the picture.
Note the total interest paid over the full term. This is the extra money you will hand over to the lender, on top of the principal.
Open the compound interest calculator and enter your down payment as the starting balance. Leave monthly contributions at zero (or set them to your planned principal contributions) to model the down payment alone.
Pick a realistic long-run annual growth rate. A diversified portfolio commonly returns 5 to 7 percent after inflation. Set the term to the same number of years as the mortgage.
Compare the two outputs. The mortgage's total interest is money out. The compound interest projection is money you could have made by investing the down payment instead. The difference is your true cost compared to renting plus investing.
Adjust the inputs to test scenarios: a bigger down payment, a shorter term, a higher interest rate, or a different investment growth assumption. The comparison shifts quickly.
Expected output and how to verify
You should end with three numbers: monthly mortgage payment, total interest paid over the term, and the projected investment value of the down payment over the same period. To verify, the monthly payment times the number of months should equal the principal plus total interest reported by the calculator (within rounding). The compound interest figure should be larger than the starting balance and grow steeply over longer terms.
Common pitfalls
Forgetting to use the same term in both calculators makes the comparison meaningless. Match the years exactly.
Picking an unrealistically high growth rate inflates the opportunity cost and makes the mortgage look worse than it probably is. Use a conservative figure and run a second pass with a higher one to bracket the range.
The comparison ignores property appreciation, maintenance, taxes, and the rent you would otherwise pay. Treat the numbers as a starting point for a deeper analysis, not the final answer.
Variations
For a refinance decision, run the mortgage calculator twice (once with current terms, once with the refinance offer) and compare total interest. For a side-by-side rent versus buy view, run the compound interest calculator with monthly contributions equal to the rent you would otherwise pay, so the investment grows in step with avoided housing costs.
Frequently asked questions
What is opportunity cost in this context?
It is the return your down payment could have earned if you invested it instead. Comparing this to mortgage interest helps you see the trade-off between buying and renting plus investing.
What growth rate should I use?
A long-run figure for a diversified portfolio sits in the 5 to 7 percent range after inflation. Pick a conservative number if you prefer a cautious comparison and run the numbers again with a higher rate to see the range.
Does the comparison account for rent?
Not by default. To get a fully fair picture, subtract the rent you would otherwise pay during the same term. The point of the workflow is to make the trade-off visible, not to give a single answer.
Why not just look at the monthly payment?
The monthly payment shows affordability today, but it hides total interest paid over the term and the opportunity cost of locking up the down payment. The workflow combines both views.