TryBuildCalcEvaporative Cooler Calculator (CFM, Pad Size & Suitability Screening)
Size your evaporative cooler 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
Likely Effective in this climate. Arid conditions (under 30% relative humidity) are where evaporative cooling performs closest to its full potential — the drier the air, the more moisture it can absorb, and the more cooling it delivers.
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: Arid (under 30% RH)
Screening result: Likely Effective in this climate
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.
What Is an Evaporative Cooler Calculator?
This calculator sizes an evaporative (swamp) cooler: how much airflow (CFM) your space needs, how much cooling pad area that airflow requires, a rough estimate of water usage, and a preliminary screening of whether your climate's humidity is dry enough for evaporative cooling to work well at all.
The CFM formula follows a widely cross-sourced residential sizing rule, confirmed against a second, independently-sourced framing that turns out to be mathematically the same relationship. The humidity suitability screening is the one calculation on this page with no equivalent anywhere else in this site's HVAC category — evaporative cooling is fundamentally gated by ambient humidity in a way refrigerant-based AC and heat pumps simply are not.
Why this needs its own calculator, not just a flat sq-ft guess:
- Evaporative coolers size to air VOLUME, not floor area — a 20 ft vaulted ceiling needs more than double the CFM of an 8 ft flat ceiling for the identical floor area
- Ambient humidity genuinely gates how well the unit performs — above roughly 50% relative humidity it's usually a poor fit, though real performance depends on the coincident dry-bulb/wet-bulb conditions at your location, not RH in isolation, so it's worth checking before ruling it out
- Cooling pad area scales directly with required CFM and pad thickness — an undersized pad starves the airflow of evaporation surface, letting dry air pass through unwetted and undercooled
- Water usage varies enough with real-time weather that a single fixed number would overstate its own precision — worth seeing as a range before sizing a reservoir or water line
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 | Arid (under 30% 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 (Arid (under 30% RH)) | Likely Effective |
| 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 |
Therefore, this space needs approximately 2,000 CFM of airflow, a 5.8 sq ft cooling pad (6 in Rigid/Cellulose Media), a 4 sq ft relief/exhaust vent opening, and roughly 3.7-7.6 gal/hr of water. This climate's humidity level supports strong evaporative cooling performance.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Inputs & Sizing Method+-
- Room/zone area accurately measured, not guessed
- Ceiling height entered accurately, not left at the 8 ft default for a taller space
- Climate Humidity Level selected based on actual local weather data, not an optimistic guess
✓Humidity Suitability Screening+-
- Suitability screening result reviewed and understood as a preliminary, humidity-tier-based flag, not a definitive design determination
- If flagged: a refrigerated AC/heat pump alternative seriously considered, after checking the location's actual design wet-bulb temperature, not just this RH tier
✓Airflow & Pad Sizing+-
- Required CFM checked against the actual unit's rated CFM specification before purchasing, not assumed to match exactly
✓Installation & Operation+-
- Relief/exhaust path confirmed available and sized at or above the Relief Vent Area result before operating the unit
- Water supply line, float valve, and drain/bleed-off sized and connected per the unit manufacturer's installation instructions
- Structural support for a roof-mounted unit confirmed adequate by a qualified installer before installation
- Licensed HVAC/cooling contractor sign-off obtained before relying on this as a final design
Full QC Checklist+−
Verification checklist for an evaporative (swamp) cooler sizing estimate — covering inputs/sizing method, humidity suitability screening, airflow and pad sizing, and installation/operation. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Inputs & Sizing Method+-
- Room/zone area accurately measured, not guessed
- Ceiling height entered accurately, not left at the 8 ft default for a taller space
- Air Exchange Rate selection matches actual local climate conditions, not left at Standard by default
- Climate Humidity Level selected based on actual local weather data, not an optimistic guess
- Cooling Pad Type matches the actual media being purchased, not just a default
✓Humidity Suitability Screening+-
- Suitability screening result reviewed and understood as a preliminary, humidity-tier-based flag, not a definitive design determination
- If flagged: a refrigerated AC/heat pump alternative seriously considered, after checking the location's actual design wet-bulb temperature, not just this RH tier
- If flagged Reduced Effectiveness: expectations set appropriately for a smaller comfort improvement than the CFM figure alone suggests
- Local design wet-bulb temperature checked against a weather-data source for a more precise assessment beyond this simplified RH-tier screening, for a borderline or high-stakes installation
✓Airflow & Pad Sizing+-
- Required CFM checked against the actual unit's rated CFM specification before purchasing, not assumed to match exactly
- Once a specific unit is selected, its rated CFM re-entered into the Actual Unit CFM field so pad area, relief vent, and water usage reflect the real unit, not just the bare calculated minimum
- Pad Area checked against the specific unit's actual pad dimensions, not assumed to match this calculator's figure exactly
- Multiple smaller units (or ducted distribution) considered for a large or oddly-shaped space instead of one oversized single unit
- Water Usage range (not a single fixed number) used when sizing a reservoir, water line, or float valve
- Elevation/altitude correction consulted separately (manufacturer spec or a qualified installer) for an installation above roughly 2,000-3,000 ft
✓Installation & Operation+-
- Relief/exhaust path confirmed available and sized at or above the Relief Vent Area result before operating the unit
- Water supply line, float valve, and drain/bleed-off sized and connected per the unit manufacturer's installation instructions
- Pad wetness and water distribution checked evenly across the full pad face after startup
- Pad replacement/cleaning scheduled per the manufacturer's recommended interval, not left until performance visibly drops
- Unit drained and winterized before freezing weather, in any climate where it applies
- Structural support for a roof-mounted unit confirmed adequate by a qualified installer before installation
- Licensed HVAC/cooling contractor sign-off obtained before relying on this as a final design
Evaporative Cooler Reference Tables
Air exchange rate tiers, cooling pad face velocities, relief vent sizing, and humidity effectiveness tiers used by this calculator.
| Air Exchange Rate | Best For |
|---|---|
| 20 ACH | Mild climates, well-shaded/insulated spaces |
| 30 ACH | Standard — most climates |
| 40 ACH | Hot & dry desert climates |
Cooling Pad Face Velocity by Media Type
| Pad Type | Face Velocity | Notes |
|---|---|---|
| 4 in Rigid/Cellulose | 250 ft/min | Common on smaller/lower-CFM units |
| 6 in Rigid/Cellulose | 350 ft/min | Optimal thickness for typical residential 2,000-3,500 CFM units |
| Aspen/Excelsior Pad | Not modeled | Cheaper, common alternative — no consistent industry-wide face-velocity figure found; consult the pad manufacturer's own spec |
Relief/Exhaust Vent Sizing
| Required CFM | Relief Vent Opening |
|---|---|
| 1,000 CFM | 2 sq ft |
| 2,000 CFM | 4 sq ft |
| 4,000 CFM | 8 sq ft |
Suitability by Climate Humidity
| Relative Humidity | Effectiveness |
|---|---|
| Under 30% (Arid) | Highly effective |
| 30-50% (Semi-Arid) | Moderately effective |
| Over 50% (Humid) | Usually a poor fit — verify design wet-bulb conditions |
When should you use this calculator?
- Planning a new evaporative cooler installation and need a starting CFM figure to shop against.
- Getting a preliminary screening of whether your climate is dry enough for evaporative cooling to be worthwhile at all.
- Sizing a replacement cooling pad and need to know how much pad area your unit's airflow requires.
- Comparing how air exchange rate or pad type change required CFM and pad area (via Compare mode).
- Budgeting a rough water usage estimate before running or sizing a water supply line to the unit.
Quick Sizing Tips
- Check the humidity suitability screening BEFORE buying a unit — above roughly 50% RH, correct CFM sizing usually can't make up for the climate, though verify your location's actual design wet-bulb temperature rather than ruling it out from RH alone.
- Evaporative coolers need fresh outside air pulled through the space and pushed back out — a window or vent must stay open (or a dedicated relief vent installed) for the unit to work. This calculator's Relief/Exhaust Vent result sizes that opening from the unit's own required CFM (not attic area, which is what this site's Attic Ventilation Calculator sizes instead).
- Size to room VOLUME, not floor area — a vaulted or unusually tall ceiling needs meaningfully more CFM than the same floor area with a standard 8 ft ceiling.
- 6 in rigid/cellulose pad media needs less total pad area than 4 in for the same CFM, and typically lasts longer — worth the modest cost difference on a unit you'll run all season.
- Treat the water usage range as a planning figure, not a fixed spec — actual consumption climbs on hot, dry days when the unit is working hardest.
- At elevations above roughly 2,000-3,000 ft, lower air density is a real factor a full design should account for — this calculator does not model an elevation correction (see Limitations).
Common Mistakes
- Installing an evaporative cooler in a climate that routinely runs above 50% relative humidity without first checking the suitability screening and, ideally, the location's actual design wet-bulb temperature — performance there is usually meaningfully reduced.
- Sizing CFM to floor area alone, ignoring ceiling height — this understates required airflow for any room taller than a standard 8 ft ceiling.
- Running the unit without an open window, vent, or relief path for the air it pushes into the space — without somewhere for the air to exit, cooling performance drops sharply.
- Letting cooling pads run dry or clog with mineral scale before replacing them — a degraded pad can't fully wet the passing airflow, cutting cooling output well below what correct CFM/pad sizing would suggest.
- Treating the water usage estimate as a fixed number when budgeting a water line or reservoir, rather than the range it actually is.
- Assuming a bigger unit is always better — oversizing CFM beyond what the room needs wastes water and energy without meaningfully improving comfort.
Limitations
- The Suitability Screening is a preliminary, RH-tier-based flag, not a definitive technology rejection. Relative humidity alone is a simplification — real-world performance depends on the coincident dry-bulb and wet-bulb temperature at your location on a given day, not RH in isolation, and some higher-humidity conditions can still provide worthwhile (if not full-comfort) cooling. For a borderline result or a high-stakes decision, check your location's actual design wet-bulb temperature rather than relying on this RH tier alone.
- Elevation/altitude correction is not modeled. Lower air density at higher elevations is a real factor, but no consistent evaporative-cooler-specific correction factor was found across sources — a full design at elevations above roughly 2,000-3,000 ft should account for this separately.
- Aspen/excelsior pad media is not modeled for pad-area sizing. It's a real, common, cheaper residential alternative to rigid/cellulose media, but no consistent industry-wide face-velocity figure was found for it — consult the specific pad manufacturer's own specification.
- Water usage is a rough estimate range, not a fixed specification — actual consumption depends on real-time temperature and humidity and will vary meaningfully from day to day.
- Relief/Exhaust Vent Area sizes the required OPENING area from the sizing CFM (Actual Unit CFM if entered and larger, otherwise Required CFM) only — it doesn't verify that a specific window, vent, or path is actually unobstructed (screens, louvers, and duct runs can all reduce a physical opening's effective free area below its raw dimensions).
- The Actual Unit CFM override only ever increases the sizing basis for pad area, relief vent, and water usage — it doesn't model any OTHER spec differences between the calculated requirement and the specific unit you buy (motor type, cabinet size, duct connections, etc.).
- Does not model ducted whole-house distribution design, downdraft vs. sidedraft unit selection, or multi-unit zoning for larger homes.
- Does not verify local permit requirements or a specific unit's manufacturer installation instructions.
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.