Search "rent vs buy calculator" and you'll find dozens of tools that ask for your home price, your rent, and spit out an answer. Most of them are quietly comparing the wrong two numbers: your mortgage payment against your rent payment. That comparison is close to useless on its own, because it ignores almost everything that actually separates the two options financially.
Start with the price-to-rent ratio, not the payment
Divide the home's purchase price by the annual rent for a comparable property. A $500,000 home renting for $2,500/month ($30,000/year) has a price-to-rent ratio of about 16.7. As a rough starting heuristic — not a rule — ratios under 15 tend to favor buying, 15-20 is genuinely competitive either way, and above 20 tends to favor renting. Most major world cities today sit well above 20; a handful of more affordable metros sit under 15. This ratio is a useful first filter, but it says nothing about your specific mortgage rate, tax situation, or how long you'll actually stay.
The opportunity cost of the down payment is the piece almost everyone skips
A 20% down payment on a $500,000 home is $100,000 that stops being available to invest elsewhere. If that $100,000 would otherwise sit in a diversified index fund earning, say, 6-7% annually over the long run, that's a real, ongoing cost of buying — separate from the mortgage payment itself. This is the single biggest reason "my mortgage is cheaper than my rent" isn't the same as "buying is cheaper than renting." A true comparison has to include what the capital tied up in a down payment (and in home equity generally) could otherwise be earning.
Amortization means your early payments barely touch the principal
On a 30-year mortgage, the first several years of payments are overwhelmingly interest, not principal. Early on, you're building equity slowly — which matters a lot if there's any real chance you'll sell within 3-5 years, since you'll have paid a large amount in interest (plus transaction costs) without building much ownership stake to show for it.
Transaction costs are the reason short holds punish buyers
Buying and then selling within a few years is expensive regardless of what happens to the price. Closing costs on the way in typically run 2-5% of price; agent commissions and closing costs on the way out often run another 5-8% combined, depending on the market. That's a real, guaranteed cost that has nothing to do with appreciation — you can lose money on a sale even if the home's value went up, once you account for what it cost to buy and sell it. This is why almost every serious rent-vs-buy framework treats expected holding period as the single most important input, more important than the mortgage rate.
The "5% rule": a rough way to compare an apples-to-apples annual cost
One widely-used mental model, popularized by economists comparing housing costs, breaks the unrecoverable annual cost of owning into roughly three pieces, each around 1-2% of the home's value per year: property tax (varies hugely by location, roughly 1% in many US states, higher in some, near-zero in others), maintenance (a commonly used rule of thumb is about 1%/year, though older homes run higher), and the opportunity cost of capital tied up in the home (equity that isn't earning what it could elsewhere). Add those up — often landing around 5% of the home's value per year — and compare that single number to your annual rent for a similar property. If annual rent is meaningfully below 5% of a comparable home's price, that's a real signal renting is financially ahead for now; if it's close to or above that, buying starts looking more competitive. This isn't precise for any individual situation, but it's a far better sanity check than comparing payments directly.
Appreciation assumptions are the part most calculators quietly get overconfident about
Long-run home price appreciation, adjusted for inflation, has historically been much more modest than people assume — often close to flat to low-single-digits in real terms across many long-run studies, with huge variation by specific city and time period. Calculators that assume steady 4-6% nominal appreciation for 20-30 years straight are baking in an assumption that may or may not hold for your specific market. It's worth running the numbers with a conservative appreciation assumption, not just whatever the calculator defaults to, and treating the output as a range rather than a single confident number.
The real question is rarely "which is cheaper," it's "for how long"
Almost every rent-vs-buy comparison has a breakeven holding period — the point at which cumulative costs cross over and buying starts coming out ahead of renting-and-investing-the-difference. Below that horizon, renting usually wins once you account for transaction costs and opportunity cost; above it, buying usually wins once you've amortized those upfront costs and built real equity. The honest version of "should I buy or rent" is almost always "how confident am I that I'll stay past my personal breakeven point" — which is a question about your life plans, not just the market.
What local rules change everything
Property tax structure (some places cap annual growth, some don't), transfer taxes (some cities stack multiple layers), rent control coverage, and mortgage interest tax treatment vary enormously by country and even by city, and each one can shift the math meaningfully. That's why a single global rule of thumb only gets you so far — see our city-by-city buy vs rent guides for the specific tax and financing mechanics that apply to major cities around the world.