One tells you where the pressure puts you. The other tells you how the airplane will actually fly. Here's the difference — with a worked example where the two split apart.
The short answer: pressure altitude is your field elevation corrected for non-standard pressure — the altitude your altimeter reads with 29.92 inHg set. Density altitude is that pressure altitude corrected again for non-standard temperature (and humidity). Pressure altitude tells you where the air pressure puts you; density altitude tells you where the air density puts the airplane's performance. On a standard day they're identical. On a hot day they pull apart — and it's density altitude that decides whether you clear the trees.
Pressure altitude answers a narrow question: if the whole atmosphere were at standard pressure, what altitude would match the pressure you actually have? You get it by setting 29.92 inHg (1013.25 hPa) in the Kollsman window and reading the altimeter — or with the formula pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000. A low pressure setting pushes pressure altitude above field elevation; a high setting pulls it below. That's the whole story: it only accounts for pressure, nothing else.
Density altitude starts from pressure altitude and adds the piece that pressure altitude ignores — temperature (warm air is thinner) and, to a smaller degree, humidity (moist air is thinner still). The result is the altitude in the standard atmosphere where the air density matches what you actually have. It is a performance number, not a position: it's the altitude the airplane "feels" like it's flying at. Every degree above standard temperature raises it; every degree below lowers it.
Pressure altitude tells you where you are. Density altitude tells you how your airplane feels. If you only remember one sentence, remember that one. Pressure altitude is an intermediate step; density altitude is the answer you actually plan the takeoff around.
Take a field at 2,000 ft elevation with a standard altimeter setting of 29.92 inHg.
Pressure altitude = 2,000 + (29.92 − 29.92) × 1,000 = 2,000 ft. With a standard setting, pressure altitude equals field elevation.
Now the temperature. ISA standard at 2,000 ft is 15 − (2 × 2) = 11 °C. Suppose the actual OAT is a hot 35 °C — that's 24 °C above standard.
Density altitude ≈ 2,000 + 120 × (35 − 11) = 2,000 + 2,880 = 4,880 ft. The pressure altitude never moved off 2,000 ft — but the airplane now performs as if it's at nearly 5,000 ft. Same runway, same altimeter, and 2,880 ft of extra "altitude" the engine and wings have to fight. That gap is exactly why density altitude, not pressure altitude, is the number that ends up in an accident report.
See it live — open the Density Altitude Calculator
Want just the first step on its own? Use the Pressure Altitude Calculator to get pressure altitude from any elevation and altimeter setting, then bring it here to see the temperature correction. For the full method with more examples, see How to Calculate Density Altitude, and for what high numbers do to your climb, how density altitude affects takeoff and climb.
The gap between the two numbers isn't academic. It shows up on exactly the days pilots get caught out. A hot afternoon at a mountain strip stacks high elevation, low pressure, and high temperature together — a field at 6,000 ft can produce a density altitude over 9,000 ft, and a normally aspirated engine that already lost power to elevation loses more to heat. A humid sea-level departure looks harmless on the altimeter but adds a few hundred feet of density altitude from water vapour alone, which is enough to stretch a takeoff roll on a short runway near gross weight. And a cold high-pressure winter morning runs the other way — density altitude drops below field elevation and the airplane over-performs its book numbers, which is pleasant but worth understanding so you're not surprised by an early rotation.
In practice you rarely stop at pressure altitude — it's a stepping stone. The workflow is: start with field elevation, correct for pressure to get pressure altitude, then correct for temperature to get density altitude, then take that density altitude into your POH performance charts to read a ground roll and a climb rate. Skipping to density altitude without understanding pressure altitude is fine day to day, but it hides where the number came from — and on a checkride the examiner will expect you to walk both steps. Knowing which correction each number carries is what lets you sanity-check a suspicious result instead of trusting a tool blindly.