Two machines, one job
Drying a wet building is two separate physical processes and both have to be running. Air movers push a high-velocity sheet of air across wet surfaces, which strips away the saturated boundary layer sitting on the material and lets more water evaporate. That water is now in the room air. Dehumidifiers pull that air across a cold coil or a desiccant and take the water back out of it, dumping it down a hose or into a bucket.
Run air movers alone and you have not dried anything. You have moved water from the carpet into the air, and the air carries it until it meets something cooler — the inside of an exterior wall, the underside of the subfloor upstairs, a window reveal — where it condenses. Opening windows is only sometimes a substitute, and only when the outside air is genuinely drier than the inside air, which on a humid day it is not. Run dehumidifiers alone and you dry the air very effectively while the water stays locked inside the material, because nothing is stripping the boundary layer. The pairing is the point.
How the counts here are built
Air movers are counted from floor area against a coverage figure you set, with an extra allowance for wet wall length because a wall being dried needs air along it rather than across the room. The default of 150 square feet each and one per 16 feet of wall are planning figures for getting equipment on a truck, not a specification. Set them to whatever your own machines and your own experience say.
Dehumidifier count comes from air volume rather than floor area, because the machine is working on the air in the room and a nine foot ceiling holds more of it than a seven foot one. The load figures — 30, 45 and 60 pints per day per thousand cubic feet for increasing wetness — are the coefficient in this calculator carrying the most uncertainty, because real moisture load depends on what the materials are, how saturated they got, and the temperature you can hold. There is an override field for exactly that reason. Treat the output as a starting equipment list you then adjust from readings.
| Output | Driven by | Confidence |
|---|---|---|
| Air mover count | Floor area and wet wall length | Good, and it is your own coverage number |
| Dehumidifier count | Air volume and assumed load | Moderate — the load assumption dominates |
| Amps and circuits | Nameplate draw you entered | Good |
| Days | Nothing here. Meter readings. | Not modelled at all |
Power is the constraint people hit first
Six air movers and two dehumidifiers is around 32 amps at 120 volts. A standard 15 amp branch circuit should not carry more than about 12 amps continuously, so that is three circuits, and a bedroom and hallway may well share one. The failure mode is not dramatic: a breaker trips overnight, nobody notices until morning, and eight hours of drying is gone from a job where hours matter. Before you plug anything in, find out what is on which breaker, spread the machines across them, and check the panel again after everything is running.
The other thing to plan is condensate. A dehumidifier pulling 130 pints a day fills a bucket several times a night. Gravity drain to a floor drain or a sink, or a condensate pump, is what lets the equipment run unattended rather than shutting itself off at two in the morning when the reservoir fills.
What this calculator will not do, and why
It will not tell you whether a material can be dried in place or has to be removed. It will not tell you how many days the job takes. It will not classify the water. Those decisions turn on things a form cannot see: what the water was, how long it sat, what the materials are, what is inside the wall cavity, and what a moisture meter says today compared to yesterday. Drying ends when readings on the wet materials match readings on unaffected materials of the same kind in the same building, and no formula predicts that date.
Some situations are not a do-it-yourself equipment problem at all. Water that has been in contact with sewage or has come from anywhere unsanitary, anything that has been wet for more than a couple of days, and anything that has reached drywall cavities or insulation belongs with a restoration professional, and is very often an insurance claim. Call the insurer before you tear anything out or discard anything, because the documentation of the loss is part of the claim. The real risk in a badly dried building is not the visible damage but the moisture you left behind a wall, which becomes a mold problem weeks later when everything on the surface looks fine — the mold removal guide covers that side. For finding where the water came from in the first place, see the water leak guide, and for sizing a permanent unit once the emergency is over rather than a rental fleet, use the dehumidifier sizing calculator.
Questions people ask
How many air movers do I need for a wet room?
The planning approach used here is one per 150 square feet of wet floor, plus one per 16 linear feet of wall you are actively drying. For a 600 square foot room with no wall drying that is four. Both of those numbers are adjustable in the form because equipment varies and because the right answer depends on how the air can actually circulate — a room broken up by furniture and partitions needs more than an open one to get air onto every wet surface. The count matters less than making sure no wet area is sitting in still air.
Can I just use fans and open the windows?
Sometimes, and only when the outdoor air is genuinely drier than the indoor air. On a cool dry day with a small spill, ventilation works. On a humid day you are pulling in air that cannot accept much more moisture and you have accomplished very little while a lot of air moved. Household box fans also move a fraction of the air a purpose-built air mover does and they do not direct it along a surface. The bigger issue is that without a dehumidifier the evaporated water stays in the building and condenses somewhere colder.
How long does drying take?
This calculator deliberately does not answer that, because there is no honest formula for it. Drying is finished when moisture readings in the affected materials match readings in the same materials elsewhere in the building that never got wet, and that depends on what the material is, how deeply saturated it got, the temperature you can hold, and whether air is actually reaching the wet surfaces. Three days is a common planning figure for a modest job with equipment running continuously, which is why the rental field defaults there, but plan by readings and not by the calendar.
When do I need to call a restoration professional?
When the water was sewage or came from anywhere unsanitary, when it has been sitting for more than a couple of days, when it has got into wall cavities or insulation, or when the affected area is large enough that you cannot get equipment on it quickly. Also whenever there might be a claim, which is most of the time — call the insurer early, photograph everything, and do not discard materials before it is documented. Professionals bring meters, thermal imaging and the ability to see inside assemblies, which is where the expensive version of this problem lives.
Why does mold appear weeks after everything looked dry?
Because surfaces dry long before cavities do. The face of a wall feels dry to the hand while the back of the drywall, the bottom plate and the insulation behind it are still wet, and that is a dark, still, damp space with organic material in it. That is the mold you find later, and it is the single strongest argument for using a meter rather than judging by touch, and for opening up an assembly that got wet rather than drying its face. Remediation itself is a separate topic covered in the mold guide.