Dehumidifier and Air Mover Sizing: How Many Do I Need?

The short answer

Count air movers from the wet surfaces and size dehumidifiers from the volume of the space, using the starting formulas the IICRC publishes for its S500 water damage standard:

• Air movers: one per wet room, plus one per 50 to 70 square feet of wet floor, one per 100 to 150 square feet of wet wall and ceiling above 2 feet, and one per wall inset or offset deeper than 18 inches.
• Dehumidifiers: cubic feet ÷ a factor for the class of water loss = pints per day needed. Divide that by one unit's AHAM rating to get the number of units.

Those numbers start the job. Daily moisture readings decide when it is done.

Step 1: Measure the drying area

The drying area, or drying chamber, is the space you close off and dry together. Measure its length, width and ceiling height.

• Square feet = length × width. Use it for the wet floor.
• Cubic feet = length × width × ceiling height. Use it for dehumidifiers and HEPA air scrubbers.
• Note which surfaces are wet and how far up the walls the water wicked. A thermal camera shows surface-temperature differences that can point to hidden moisture; confirm every suspect spot with a moisture meter.

Step 2: Decide the class of water loss

The IICRC sorts water losses into four classes by how much wet, porous material there is to dry, measured in the current S500 as a share of the room's combined floor, wall and ceiling surface:

• Class 1: under about 5% of surfaces wet, with little or no wet carpet.
• Class 2: about 5% to 40% wet. A room of soaked carpet and pad with water wicked less than 24 inches up the walls usually lands here.
• Class 3: over about 40% wet, often water from above soaking the ceiling, walls, insulation and floor.
• Class 4: water held deep in dense materials such as hardwood, plaster, concrete or stone, which need specialty drying.

Class is not category, which describes how clean the water is. This guide assumes clean (Category 1) water from a supply line or fixture, caught early; clean water that sits for days can degrade. Sewage or a toilet backflow, outside floodwater or storm surge, and washing machine or dishwasher water need an appropriate experienced professional, not a rental setup.

Step 3: Count the air movers

The IICRC air mover worksheet gives a low and a high count. Round each step up to the next whole number, then add the steps together.

1. One air mover in each affected room.
2. Wet floor: square feet ÷ 70 for the low count, ÷ 50 for the high count. This also covers the bottom 2 feet of the walls.
3. Wet wall above 2 feet plus wet ceiling: square feet ÷ 150 for the low count, ÷ 100 for the high count.
4. One more for each wall inset or offset deeper than 18 inches, such as a closet or alcove.

In a small space under about 25 square feet, such as a half bath, one air mover may be enough.

When only the lower walls got wet

If water mostly soaked the bottom of the walls and spread less than 2 feet into the room, the worksheet uses one air mover for every 14 feet of affected wall instead. Use one method or the other in a room, not both. Older editions gave 10 to 16 feet, the source of that common rule of thumb.

Small or large air movers

The worksheet counts machines, not airflow. Use the small 1/2 HP units (about 2,200 CFM) in single rooms and along wall bases, and the large 1 HP units (4,000+ CFM) in open floor plans and long wet runs.

Set each one almost touching the wall, angled about 5 to 45 degrees as the current S500 describes, so the air flows across the wet surface rather than straight at it. Point every outlet the same way around the room. See the air mover rental page for more.

Step 4: Size the dehumidifier

The IICRC initial dehumidification chart turns cubic feet into the pints of water per day your dehumidifiers must be rated to remove:

• LGR (low grain refrigerant): divide cubic feet by 100 for Class 1, 50 for Class 2, 40 for Class 3 and 40 for Class 4.
• Conventional refrigerant: divide by 100, 40 and 30 for Class 1 to 3. The chart gives no Class 4 factor.

Divide the result by one unit's AHAM rating and round up. That is the number of dehumidifiers.

Use the AHAM rating, not the saturation rating

The HBS commercial LGR dehumidifier is rated at 180 pints a day at saturation and 85 at AHAM. The IICRC formula uses the AHAM figure, measured at 80°F and 60% relative humidity.

Saturation figures come from much wetter test air (Dri-Eaz lists its saturation ratings at 90°F and 90% relative humidity). Size with 85; sizing with 180 would give you less than half the dehumidification the formula calls for.

Why the fleet uses LGR units

At Class 2 and 3 the chart asks for fewer rated pints from an LGR unit than from a conventional one. As Dri-Eaz's owner's manual explains, LGR units keep removing moisture as the air gets drier.

Worked examples with the HBS fleet

Three clean-water leaks, sized with the IICRC formulas and the HBS LGR dehumidifier's 85-pint AHAM rating.

Bedroom with wet carpet and pad

A 12 × 14 foot bedroom with an 8-foot ceiling and one closet. A supply line soaked the carpet and pad, and water wicked less than 2 feet up the walls.

• Air movers: 1 for the room, plus 168 square feet of floor ÷ 70 = 3 (low) or ÷ 50 = 4 (high), plus 1 for the closet. Total: 5 to 6 small air movers.
• Dehumidifier: 12 × 14 × 8 = 1,344 cubic feet ÷ 50 = about 27 AHAM pints a day. One LGR unit covers it.

Open kitchen and family room

A refrigerator ice-maker line spread water across about 400 square feet of a 20 × 24 foot open area with a 9-foot ceiling and a pantry alcove deeper than 18 inches.

• Air movers: 1 for the area, plus 400 ÷ 70 = 6 (low) or 400 ÷ 50 = 8 (high), plus 1 for the pantry. Total: 8 to 10, with large air movers along the long runs.
• Dehumidifier: 20 × 24 × 9 = 4,320 cubic feet ÷ 50 = about 86 AHAM pints a day. One 85-pint unit falls just short, so the formula says two.

Room under a ceiling leak

A burst supply pipe above a 14 × 16 foot room with an 8-foot ceiling soaked the ceiling, two walls from floor to ceiling, and the carpet.

• Air movers: 1 for the room; 224 square feet of floor ÷ 70 = 4 or ÷ 50 = 5; 224 square feet of ceiling plus 180 square feet of wall above 2 feet = 404 ÷ 150 = 3 or ÷ 100 = 5. Total: 8 to 11.
• Dehumidifier: 14 × 16 × 8 = 1,792 cubic feet ÷ 40 for Class 3 = about 45 AHAM pints a day. One LGR unit covers it.
• Wet cavities may need the Injecti-Dry wall and cavity drying system. A sagging ceiling is a structural hazard: keep people out from under it and call an appropriate experienced professional.

Sizing a HEPA air scrubber (AFD)

HEPA air scrubbers, also called air filtration devices or AFDs, are sized by air changes per hour (ACH). The IICRC chart notes that its airflow formula also works for AFDs: cubic feet × ACH ÷ 60 = the CFM you need.

Treat the math as a starting point. Legend Brands' technical guidance says 2 to 4 ACH is normally enough for construction dust control, but as a general rule an air scrubber should reach at least 6 ACH, and more is better when you can get it. If a professional wrote a plan for the job, use their number.

• The open kitchen and family room above, 4,320 cubic feet at 6 ACH: 4,320 × 6 ÷ 60 = 432 CFM. One HEPA 500 at 500 CFM covers it.
• A 30 × 40 foot floor with a 9-foot ceiling, 10,800 cubic feet at 6 ACH: 1,080 CFM, so pair the HEPA 1000 with a HEPA 500. For construction dust control at 4 ACH, the same floor needs 720 CFM, which the HEPA 1000 covers alone.

Leave margin: delivered airflow drops as filters load and when ducting is added. An air scrubber cleans the air; it does not dry materials or remove mold.

Starting points, not a drying plan

Every room dries differently: dense materials, cabinets over wet drywall and how soon drying started all change the result. That is why the IICRC calls its chart an initial recommendation.

• First read the same material in a room the water did not reach. That is the IICRC's dry standard; wet materials are done when they read close to it.
• Read the same marked spots every day with the same meter, and log the room's temperature and humidity with a hygrometer.
• If the readings stop falling, the S500 says to adjust equipment placement or add or change equipment. Call HBS to talk it through.
• Keep the area closed and the machines running around the clock.

Stop and call an appropriate experienced professional instead of renting when you find:

• Sewage or a toilet backflow, outside floodwater or storm surge, or washing machine or dishwasher water.
• Mold growing in the area. Do not run air movers there; the EPA recommends professional help for mold covering more than about 10 square feet.
• A sagging ceiling or other structural damage.
• An electrical hazard, such as wet outlets, wiring or a wet electrical panel.

Unsure whether to run the job yourself? Read rent drying equipment or hire a dry-out.

Common questions

How many air movers do I need for one room?

Start with the IICRC worksheet: one for the room, one for every 50 to 70 square feet of wet floor, one for every 100 to 150 square feet of wet wall above 2 feet and wet ceiling, and one for each inset or offset deeper than 18 inches. A 12 by 14 foot bedroom with wet carpet and a closet comes to 5 or 6 small air movers.

What size dehumidifier do I need for water damage?

Divide the room's cubic feet by the IICRC class factor to get the AHAM pints per day needed, then divide by one unit's AHAM rating. For an LGR unit in a typical Class 2 loss the factor is 50, so one HBS commercial LGR dehumidifier, rated 85 pints a day at AHAM, covers up to about 4,250 cubic feet.

Should I size a dehumidifier by its saturation rating?

No. The IICRC formula uses the AHAM rating, measured at 80°F and 60% relative humidity. The HBS unit's 180 pints a day is a saturation figure from much wetter test air; size with its 85 pints a day at AHAM.

How do I know when the room is dry?

Read the same material in a room the water did not reach, then read the wet spots every day with a moisture meter. The job is done when the wet materials read close to that dry reference, not when the surface feels dry.

What size HEPA air scrubber do I need?

Multiply the room's cubic feet by the air changes per hour you need and divide by 60. At the 6 air changes per hour Legend Brands gives as a general minimum, a 4,320 cubic foot room needs about 432 CFM, which one HEPA 500 covers. Larger spaces may need the HEPA 1000.

Sources for the formulas

• IICRC air mover calculation worksheet
• IICRC initial dehumidification factors and formulas
• Legend Brands: air changes per hour for air scrubbers

Class definitions follow published summaries of the S500 by Haag and Restoration & Remediation magazine.