Drying Equipment Calculator — Air Movers & Dehumidifiers (IICRC)
Enter your wet area, ceiling height, and class of loss, and this runs the real IICRC S500 dehumidification formula — the same factor chart restoration crews use — to size how many dehumidifiers and air movers it takes to dry the space.
Not sure what class of loss you have? Take the 2-minute class identifier quiz first at /tools/water-damage-category-class-quiz, and if the water isn't clean (Category 1), start with the emergency triage wizard at /tools/water-emergency-what-to-do instead — contaminated water is removed, not dried.
How this works
This calculator uses the IICRC's official Initial Dehumidification Recommendation Factors and Formulas chart (Imperial rev 3.1.22) — the S500 industry standard restoration crews size equipment by. The factor chart divides your room's cubic footage by a factor that depends on the dehumidifier type and the class of loss:
Conventional refrigerant — Class 1 = 100, Class 2 = 40, Class 3 = 30, Class 4 = N/A. LGR (low-grain refrigerant) — Class 1 = 100, Class 2 = 50, Class 3 = 40, Class 4 = 40. Desiccant (in air changes per hour, ACH) — Class 1 = 1, Class 2 = 2, Class 3 = 3, Class 4 = 3. Note the N/A: conventional units simply can't dry a Class 4 loss, which is why this tool routes Class 4 + conventional to an LGR/desiccant recommendation.
For refrigerant and LGR units the formula is: cubic feet ÷ factor = required pints/day (AHAM) ÷ the unit's AHAM rating = number of units. Worked example: 400 sq ft × 8 ft ceiling = 3,200 cubic feet; a Class 2 loss with LGR uses factor 50, so 3,200 ÷ 50 = 64 pints/day required; at a typical ~70 PPD LGR rental rating, 64 ÷ 70 = 0.9, which rounds up to 1 LGR dehumidifier. Air movers run on a separate rule — about one per 350 sq ft, but never fewer than the working ratio of ~4 air movers per dehumidifier — so this same example lands on 4 air movers (max of the 2-per-sq-ft baseline and 4 × 1 dehumidifier). Desiccant units use a different formula: cubic feet × ACH ÷ 60 = required CFM ÷ the unit's CFM rating = number of units.
One honesty caveat the raw math hides: AHAM dehumidifier ratings are measured at 80°F and 60% relative humidity. A cool basement or an already-partly-dried room pulls 30–50% less water than the rating claims, so you often need more capacity than the formula suggests — which is exactly why pros run LGR units that keep working in cold, dry air, and why they monitor psychrometrics daily rather than trusting a single calculation. Structural drying typically takes 3–5 days when started inside the first 24–48 hours. Renting this much equipment for that long frequently approaches the cost of a pro visit that also includes moisture metering and insurance documentation you can't rent.
Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-07.
Sources
Frequently Asked Questions
How many air movers do I need per room?
Start from about one air mover per 300–400 square feet of wet floor, but check it against the working ratio of roughly four air movers per dehumidifier — on a small room the ratio wins. For a typical 400 sq ft Class 2 loss that's around 4 air movers paired with one LGR dehumidifier. Higher classes (water from above, saturated walls) push the count up.
Can I dry water damage with a regular dehumidifier?
For a small Class 1 or Class 2 clean-water loss, a conventional dehumidifier plus fans can work if you start within 24–48 hours. But conventional units stall in cool, already-dried air, and the IICRC chart lists no factor for conventional units on a Class 4 (deeply saturated hardwood, plaster, or concrete) loss — those need LGR or desiccant equipment. A household dehumidifier is also far below the AHAM rating this calculator assumes for a rental unit.
How long does structural drying take?
Typically 3–5 days once drying starts within the first 24–48 hours, assuming clean water and correctly sized equipment. Deeply saturated materials (Class 4) and cool basements take longer, because AHAM ratings assume 80°F/60% humidity and real extraction runs 30–50% slower in cold conditions. Pros monitor moisture daily and adjust rather than running a fixed timer.
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