Two investors can look at the exact same rental property and disagree about whether it's a good deal — because they're measuring different things.
Rental property investors use several return metrics, and two come up constantly: cap rate and cash-on-cash return. They can point in completely different directions on the same property, and mixing them up is one of the more common mistakes new investors make when comparing deals.
Capitalization rate — cap rate — is net operating income divided by the property's purchase price (or current value). It deliberately excludes your mortgage payment, because it's meant to measure how the property performs as an asset, independent of how any particular buyer chooses to finance it. This makes cap rate useful for comparing properties against each other, or against a market average, since it isn't distorted by one buyer's down payment size or interest rate.
Cash-on-cash return divides your annual pre-tax cash flow — rent minus expenses and minus your mortgage payment — by the actual cash you put in, usually your down payment plus closing costs. This number reflects leverage: a smaller down payment means less of your own cash tied up, which can push cash-on-cash return higher even though the property's fundamentals (its cap rate) haven't changed at all.
A property with a modest 5% cap rate can still produce a strong 12%+ cash-on-cash return if financed with a low down payment at a favorable rate — leverage amplifies the return on your specific cash investment, for better or worse. The same leverage that boosts cash-on-cash return in a good year also amplifies losses in a bad one, which is why relying on cash-on-cash return alone can hide how thin a property's underlying margins really are.
Use cap rate to compare the underlying quality of different properties or markets on a level playing field. Use cash-on-cash return to judge whether a specific deal, financed the way you actually plan to finance it, makes sense for your own capital. Serious investors typically look at both, alongside the property's total return including any paydown and appreciation, rather than leaning on a single number.