Show HN: Brisk – Buy vs. Rent/Sell vs. Keep calculator tracking opportunity cost (opens in new tab)

Most buy vs rent calculators compare your mortgage payment to rent, factor in appreciation, and call it a day. But they ignore what happens to your downpayment if you don't buy—that $200K+ could go into index funds instead. S&P 500 has averaged ~14.5% over the last 15 years vs ~4% for housing. That difference compounds significantly.

And almost no calculators address the sell vs keep question: if you already own a property, should you sell and invest the proceeds, or hold onto it?

So I built a calculator that models both scenarios month-by-month.

Buy vs Rent

For buying: Full amortization schedule, mortgage interest deduction, property taxes, insurance, maintenance—all inflating annually except the fixed mortgage payment. Calculates net proceeds at sale including capital gains tax and exemptions.

For renting: Takes your would-be downpayment, invests it at your specified return rate. Each month, adds or subtracts the difference between buying and renting costs. Compounds monthly.

Example outputs for a $1.32M SF home, at 10% rate of return from investments:

5.5% mortgage: renting ahead by ~$250K at year 10, ~$5M at year 30

3% mortgage + $2K/mo income from renting a portion of home: buying ahead by $360K at year 10, $2.2M at year 30

Sell vs Keep

This is the harder problem. Say you bought a home 5 years ago for $1.5M, it's now worth $2M, and you have $1M in equity. Should you sell, or keep it?

For selling: Calculates what you'd net today after agent fees, taxes, and loan payoff. That lump sum gets invested. If it's your primary residence, you can toggle on renting costs that get deducted as your investment compounds.

For keeping: Tracks monthly cash flow—rental income minus loan payments, taxes, insurance, maintenance. Positive cash flow gets invested and compounds. At any year N, your "keep net worth" is the projected sale proceeds at that point plus your accumulated investment position.

You can also model refinancing—pull equity out to invest while servicing a larger loan. The calculator is flexible enough that it works for cars and other depreciating assets too.

All calculations run locally in browser, nothing sent to a server. There's a button to copy results into Claude/Gemini to verify the math and ask further questions.