Your cooler has a hard physical limit.
Find out how close it is getting.

An evaporative cooler cannot blow air colder than the outdoor wet-bulb temperature — no matter its age, brand, or price. Measure what yours actually delivers, compare it to that limit, and you will know whether the machine is at fault, your house is, or the weather is.

Now check yours

Run the cooler for 15 minutes, then measure the air at your nearest supply register. That reading against the outdoor conditions is the whole diagnosis:

effectiveness = (outdoor − measured) ÷ (outdoor − wet-bulb)

What the number means

The top boundary depends on your pads, because their construction sets what the machine was ever capable of: 0.85 for 8–12 in rigid media, 0.70 for a 4 in rigid block, 0.65 for aspen. The bands below are written at 0.80, between the three; the calculator uses the boundary for the pad type you pick, and starts on 8–12 in rigid media.

• 0.80 and above — working as designed

  Your cooler is capturing most of the cooling physically available to it. If the house is still warm, the problem is airflow or relief air, not the unit.

• 0.65 to 0.80 — underperforming

  Maintenance will likely help. Start with pad wetting: dry streaks let air slip through without evaporating anything.

• 0.40 to 0.65 — significantly degraded

  Something is wrong. Scaled or collapsed pads, a weak pump, or a slipping belt. Work through the troubleshooting tree.

• Below 0.40 — not evaporating

  The pads are essentially dry. Check that the pump runs, that water reaches the distribution tubes, and that the sump is full.

• Above 1.00 — check the measurement

  Cooler output air below the wet-bulb is thermodynamically impossible for a direct cooler, so something in the inputs is off — a probe sitting in water carryover, stale weather data, or a two-stage unit, which legitimately can undercut the wet-bulb.

Where your air actually sits

Each dot is a state of the air — a temperature and a relative humidity. Outdoor air starts warm and comparatively dry, at the right. As it evaporates its way through the pads, it slides along the curve: cooling and gaining humidity at the same time, because that trade is the entire mechanism. The hollow dot is the typical, well-maintained figure from the panel above; the filled dot is where your cooler's own process actually stops — usually close by, since both are a matter of a pad in decent shape. If the machine could keep going, it would continue on to the wet-bulb floor at the top left — fully saturated air, and the hard limit nothing can beat.

How cold should the air be?

The honest answer is that it depends entirely on the weather, which is why nobody can quote you a single number. But for a well-maintained cooler at sea level, here is what to expect at the register:

Two things in that table are worth sitting with. Read across any row and the cooler output air warms by 10 °F as humidity climbs from 10 to 40 % — the machine has not changed, only the air it was given. Read down the 40 % column and a cooler on a 105 °F day delivers 86 °F air, which is why people in humid climates conclude these things do not work. In their climate, they are close to right.

Read down the 10 % column instead and the same machine turns 105 °F into 72 °F. That is the case for evaporative cooling, and it is entirely a case about climate.

How to take the measurement

Bad measurements cause more wrong conclusions on this subject than bad coolers do. Five minutes of care here is worth more than any diagnostic tool.

1. Run the cooler at least 15 minutes with the pump on. Pads that have just been wetted are still cooling themselves down. An early reading reads warm and will make a healthy unit look broken.
2. Measure at the nearest supply register, or at the cooler's discharge. Not a distant bedroom — duct losses are real, and they are a different problem from evaporation.
3. Keep the probe out of direct water carryover. Droplets on the sensor read wet-bulb, not air temperature, and produce impossible numbers.
4. Measure room temperature separately. It is not the same quantity and will read considerably warmer. Comparing cooler output air against room air tells you about the house; comparing it against wet-bulb tells you about the cooler.
5. Use an actual thermometer. A $10 digital probe or an instant-read kitchen thermometer is entirely adequate. Your phone's weather reading is not a measurement of anything inside your house.

Read further

• How it works

  Adiabatic saturation, the wet-bulb floor, and why arid climates are the natural habitat for these machines.

• Common problems

  Nine failure modes, ranked by how often each one turns out to be the actual answer.

• Troubleshooting

  Start from the symptom you can see and work toward the cause.