TryBuildCalcSwamp Cooler Humidity Calculator (Humid Climate Suitability Check)
Check your climate's evaporative cooling suitability instantly.
🕒 Last updated: September 14, 2026
Inputs
ℹ️Floor area of the space this unit will cool.
ℹ️Evaporative coolers size to air VOLUME, not floor area — a taller ceiling needs proportionally more airflow. Optional — defaults to 8 ft (a standard residential ceiling) if left blank.
ℹ️How many times per hour the unit fully replaces the room's air — hotter, drier climates want a faster exchange rate.
ℹ️Evaporative cooling's effectiveness is gated by ambient humidity, not just heat — it works best in dry climates.
ℹ️Thicker pads handle more airflow per sq ft of pad face, needing a smaller total pad area for the same CFM.
ℹ️If you've already picked a specific unit, enter its rated CFM. Manufacturers sell in fixed tiers, so a purchased unit is often larger than the bare requirement — pad area, relief vent, and water usage will scale to match, never below the calculated minimum. Leave blank to size off the calculated requirement alone.
Required Airflow
2,000 CFM
4,000 cu ft at 30 air changes/hr
Usually a Poor Fit — Verify Design Wet-Bulb Conditions. Above 50% relative humidity, evaporative cooling typically delivers a much smaller temperature drop and can add uncomfortable humidity — this RH tier alone doesn't guarantee no cooling, but real performance depends on the coincident dry-bulb/wet-bulb conditions, not RH in isolation. Check your location's actual design wet-bulb temperature before ruling it out; a refrigerated AC or heat pump is usually the better fit for this climate.
Room / Zone
Area: 500 sq ft
Ceiling height: 8 ft
Room volume: 4,000 cu ft
Airflow Sizing
Air exchange rate: 30 ACH
Required CFM (calculated minimum): 2,000 CFM
Suitability Screening
Climate: Humid (over 50% RH)
Screening result: Usually a Poor Fit — Verify Design Wet-Bulb Conditions
Cooling Pad
Type: 6 in Rigid/Cellulose Media
Face velocity: 350 fpm
Required pad area: 5.8 sq ft
Water Usage (estimate)
Estimated range: 3.7-7.6 gal/hr
Varies with real-time temperature and humidity.
Relief / Exhaust Vent
Required opening area: 4 sq ft
Open window, vent, or dedicated relief path — sized to this unit's own CFM, not attic area.
Assumptions Used
Required CFM is Room Volume (Area x Ceiling Height) multiplied by your selected air exchange rate and divided by 60, cross-checked against a second, independently-sourced rule of thumb (Room Volume divided by 1.5-3, depending on climate) — both formulas are mathematically the same relationship. A required minimum, rounded UP, never to nearest, computed from the exact Area/Ceiling Height values rather than a pre-rounded volume so a fractional input can't get silently rounded away. The Suitability Screening is an informational flag, not a hard block — evaporative cooling's effectiveness is gated by ambient relative humidity (cross-checked across multiple industrial and residential evaporative-cooling references): under 30% RH it performs close to its full potential, 30-50% RH delivers noticeably less cooling, and above 50% RH performance typically drops sharply, though this simplified RH-tier screening is not a precise pass/fail — actual performance depends on the coincident dry-bulb and wet-bulb temperature at your location, not RH alone, so verify your design wet-bulb conditions before ruling evaporative cooling out entirely. Cooling Pad Area, Relief/Exhaust Vent Area, and Water Usage are all sized from Required CFM UNLESS you enter an Actual Unit CFM that's larger — manufacturers sell in fixed CFM tiers, so a purchased unit (e.g. a 3,000 CFM model bought for a 2,001 CFM requirement) is often larger than the bare calculated minimum, and Phoenix Manufacturing's own owner's manual bases relief-opening sizing on the unit's actual rated airflow, not the load calculation alone; an entered value smaller than the calculated minimum never shrinks Cooling Pad Area, Relief/Exhaust Vent Area, or Water Usage below what the safer calculated minimum requires — but it IS surfaced as an amber undersized-unit warning above, since an undersized unit is a real problem even though this calculator's own downstream figures stay safe. Cooling Pad Area is that sizing basis divided by the selected pad's face velocity (250 ft/min for 4in rigid/cellulose media, 350 ft/min for 6in), a required minimum rounded UP — an undersized pad (too high a face velocity for the airflow) risks incomplete evaporation and water carry-over through the pad; dry spots or pooling on an installed pad are usually a water-distribution problem, not a sign the pad area itself is wrong. Relief/Exhaust Vent Area is approximately 2 sq ft of unrestricted opening per 1,000 CFM (cross-sourced U.S. evaporative-cooler manufacturer guidance) — a genuinely different formula from this site's Attic Ventilation Calculator, which sizes passive attic vent area from attic square footage, not cooler airflow. Estimated Water Usage is a rough LOW-HIGH RANGE (not a fixed spec), scaled from two independently cross-checked real-world data points — actual usage depends heavily on real-time temperature and humidity and will vary. This calculator does not model elevation/altitude correction (a real effect at higher elevations, but no consistent evaporative-cooler-specific correction factor was found across sources) or aspen/excelsior pad media (a real, common, cheaper alternative, but no consistent face-velocity figure was found for it) — see Limitations below.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Evaporative Cooler Calculator.
Evaporative cooler humidity suitability screening
Evaporative cooling depends on evaporating water into the air — above roughly 50% relative humidity, performance typically drops sharply. RH alone is a simplification, though: real performance depends on the coincident dry-bulb and wet-bulb temperature at your location, not RH in isolation, so this screening is a preliminary flag, not a definitive rejection.
This page defaults to the same 500 sq ft/8 ft/Standard scenario as the generic calculator, but with a Humid climate selected — a combination that deliberately demonstrates the Usually a Poor Fit suitability result — edit the inputs above to match your actual climate.
Evaporative Cooler Sizing Formula: How Is It Determined?
Required CFM comes from room volume and a selectable air exchange rate. Cooling pad area, relief vent area, and water usage all scale from a sizing basis — Required CFM, unless you enter a larger Actual Unit CFM. See Limitations for what the humidity screening and water-usage estimate don't model.
Room Volume & Required CFM
Room Volume (cu ft) = Area x Ceiling Height
Required CFM = ceil(Room Volume x Air Changes per Hour / 60)
Evaporative coolers size to air VOLUME, not floor area, since they cool the whole air mass in a room, not just what's over the floor. This ACH-based formula is cross-checked against a second, independently-sourced rule of thumb — Required CFM = Room Volume / Divisor, with a Divisor of 1.5-3 depending on climate — and the two are mathematically identical (Divisor 2 equals exactly 30 ACH, since Volume/2 = Volume x 30/60). A required minimum, rounded UP, never to nearest. This is the figure to shop against — it's never altered by the Actual Unit CFM input below.
Sizing Basis — Required CFM vs. Actual Unit CFM
Sizing CFM = Actual Unit CFM, if entered AND larger than Required CFM; otherwise Required CFM
Manufacturers sell in fixed CFM tiers, so a purchased unit is often larger than the bare calculated requirement — e.g. a 2,001 CFM requirement has no exact match on the market and rounds up to a 3,000 CFM unit. Pad area, relief vent area, and water usage should reflect what's ACTUALLY installed, not just the bare minimum, or they undersize the vent/pad for the unit you end up buying (Phoenix Manufacturing's own owner's manual bases relief-opening sizing on the unit's rated airflow, not a bare load calculation). Entering an Actual Unit CFM smaller than Required CFM never reduces the sizing basis below the calculated minimum.
Cooling Pad Area & Water Usage
Pad Area (sq ft) = ceil(Sizing CFM / Pad Face Velocity)
Water Usage (gal/hr) = Sizing CFM x [0.00185 to 0.0038]
Pad face velocity is cross-sourced at 250 ft/min for 4in rigid/cellulose media and 350 ft/min for 6in — these are upper limits: a face velocity above them moves air through the wet pad too fast for full evaporation and can carry unevaporated water downstream. Going below the limit doesn't create dry spots or pooling by itself (those are usually a water-distribution problem, not a velocity problem) — it just means the pad has more evaporation surface than strictly required for the airflow. Pad area is also a required minimum, rounded UP. Water usage is a genuinely condition-dependent figure (it swings with real-time temperature and humidity), so it's shown as a cross-checked low-high range rather than a single fabricated point estimate.
Relief / Exhaust Vent Area
Relief Vent Area (sq ft) = ceil(Sizing CFM x 2 / 1,000)
Evaporative coolers push outside air INTO the space, and that air needs a dedicated unrestricted path back out (open window, vent, or relief vent) — without one, pressure builds and airflow stalls. This scales with the COOLER'S OWN sizing CFM, not attic area, so it's a genuinely different formula from this site's Attic Ventilation Calculator (which sizes passive attic vent area from attic square footage). Cross-sourced U.S. evaporative-cooler manufacturer guidance converges on approximately 2 sq ft of unrestricted opening per 1,000 CFM. Also a required minimum, rounded UP.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Area / ceiling height | 500 sq ft / 8 ft |
| Air exchange rate | Standard (30 air changes/hr) |
| Climate humidity level | Humid (over 50% RH) |
| Cooling pad type | 6 in Rigid/Cellulose Media |
| Actual unit CFM | Not provided |
Sizing Steps
| Calculation | Result |
|---|---|
| Room volume (500 sq ft x 8 ft) | 4,000 cu ft |
| Required CFM (30 ACH) — the calculated minimum | 2,000 CFM |
| Suitability screening (Humid (over 50% RH)) | Usually a Poor Fit |
| Cooling pad area (6 in Rigid/Cellulose Media, 350 fpm) | 5.8 sq ft |
| Relief/exhaust vent area | 4 sq ft |
| Estimated water usage | 3.7-7.6 gal/hr |
This climate's humidity level is usually a poor fit for evaporative cooling — above 50% RH, performance typically drops sharply, though this simplified RH-tier screening isn't a precise pass/fail. Even correctly sized to 2,000CFM, expect a much smaller comfort improvement here than in a drier climate. Check your location's actual design wet-bulb temperature before ruling it out entirely — a refrigerated air conditioner or heat pump is usually the better fit for this climate.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.