Informational Guide

Sizing a Dehumidifier Correctly

All you need to know is the purpose, size, and space requirements of the dehumidifier. It isn’t as difficult as it may seem.

by Josh M

Dehumidifiers don’t get enough credit. Quite possibly one of the most overlooked aspects of home climate control options out there, dehumidifying systems are used in everything from construction to personal health to plant and wildlife care.

Finding a quality large or small dehumidifier for your home doesn’t have to be a challenge. All you need to know is the purpose, size, and space requirements of the dehumidifier. It isn’t as hard as it seems.

This article will help you answer “what sized dehumidifier do I need?” by showing you how to calculate for size and capacities, as well as the size needed for each room of your home or project space.

Air Humidifier on the Floor Near a Plant

When we talk about dehumidifier size, only rarely are we speaking of the physical dimensions. Just like heaters and air conditioners are sized by BTU output or tonnes, dehumidifiers are sized by the amount of humidity (moisture) they can remove in a single day.

This measurement is known as Pints per Day or PPD. This is a measurement of the actual amount of water pulled from the air and collected for draining, disposal, or even reuse. In some cases, the PPD is also called the Moisture Removal amount.

Home and personal use dehumidifiers are also sized by their power consumption in kilowatts per hour (kWh) or by the capacity of their water collection tanks, usually in pints, cups, or milliliters.

In general, there are a few major sizes to note. Each size has its pros and cons as well as its purpose. For example, a 30 PPD unit is great for a single room or apartment. They can generally cycle through the air in the home (known as air changes per hour, or ACH) enough to keep a relative humidity level below 60.

For larger spaces or small, specific needs, a 50 PPD unit may be needed. These are ideal for dialing in a humidity level to maintain, as the larger units tend to have more features. Things like auto-drain, humidity selection, timers, and remote operation are all found on larger units.

For industrial jobs, construction, or for use in extreme humidity areas, like your basement, a 70 to 90 PPD unit might be in order. These units can be stand-alone or connected to a permanent HVAC system in crawl spaces, basements, and flood-prone areas.

How to Determine What Size To Buy?

Now that we know what a dehumidifier is and how they are sized, what size do you need for your home or specific area? That will depend on a few factors. Let’s take a look at those factors now so we know what to calculate for.

Relative Humidity Rates

The most important number in any dehumidifier purchase is the relative humidity (RH). Relative humidity not only represents the amount of moisture in the air but the amount of food for bacteria and mold.

Being able to control the RH will not only make the air easier to breathe and better for your overall health, but it can starve mold and mildew spores, preventing them from growing and eventually even killing them off.

Most homes range between 50 and 70% relative humidity overall. Basements can reach up to 100% RH, and rooms without windows or proper ventilation can reach 100% as well. The ideal RH level for your home is between 30 and 60% RH.

For mold prevention, ozone production resistance and minimizing asthma attacks, the Mayo Clinic recommends a home humidity level below 50%.

Square Footage

The size of the space, in square feet, will also determine the size of your dehumidifier. You can’t expect a small capacity unit to move the air in a warehouse each hour. Likewise, having a system too big for the home can leave the air too dry, and your energy bill too high.

On average, a single-family home, apartment, or office space will utilize a dehumidifier that falls between 30 and 50 PPD. Larger spaces, or those prone to water accumulation, constant high humidity or flooding, such as a basement, may need a unit up to 70 PPD.

White Humidifier on Top of Bedside Table with Books
Average Temperature

Another base factor for the amount of humidity in the air is the ambient temperature. The temperature contributes to the humidity levels the air can hold, and it also accounts for the additional humidity between rapid temperature differences (like when it is 90 degrees outside and your air conditioner is set to 72 degrees).

Higher temperature fluctuations result in a higher relative humidity level. Knowing the average temperatures will help you understand your average RH, which, in turn, will help you determine the size dehumidifier you need.

Budget

Finally, your budget will come into play a bit, too. If you are looking for a table-top dehumidifier for next to the bed or in a single room of the home, you can find a 50 PPD dehumidifier that doesn’t break your bank in the slightest.

However, for whole-home integration, or specific uses (such as basement control, greenhouse uses, etc.), the dehumidifiers with the capabilities and features you need may need to be financially planned for.

How To Measure The Room For A Dehumidifier?

Measuring your room, home, or office for a dehumidifier can be a complicated process. There is one major question you need to ask yourself. Is the dehumidifier for general use or specific? For example, if you just want to maintain a general humidity range, the process is easier. However, if you need to keep your petunias healthy, you may need to do more math.

For general concerns, you need to know the conditions you have and the conditions you are trying to achieve. For this purpose, we will concern ourselves with achieving the ideal 45 – 55% RH range.

You need to know the current conditions as well as the square foot of the space. So, for example, if you have high humidity (moisture on the walls, but not seeping/running), and you are in a 700 square foot space, a 50 to 60 PPD unit is required.

We have a chart below outlining ideal sizes and types for various rooms and conditions for you.

For those that need specific control, though, you need to invest in a hygrometer. This tool will sample the air in real-time and give you the exact RH of the area. Using this number, along with the ambient temperature, indoor temperature, expected changes, and other factors (far beyond the capabilities of this article), your numbers will be more exact.

However, in these cases (greenhouses, for example), your dehumidifier should be a part of the HVAC system and not a portable or smaller unit that most homes will have in their bedroom. The more exacting controls, sensors and installation should be handled by a professional.

Calculating Dehumidifiers Capacity

Let’s break it down. You all know that humidity is shown as a percentage, but what is that a percent of? Humidity levels are technically the percentage that the air molecules are full of with moisture.

So essentially, if you were to zoom in on a single air molecule and saw it was half full of water, the humidity level would be 50% (this is very basic, of course). Air can hold moisture without dropping it, and that amount is shown as a percentage.

Likewise, the dehumidifier is rated by the amount of moisture it can remove from the air in pints, liters, or gallons per day. So when you see a dehumidifier rated at 50 PPD, that means that running 24 hours in the same spot, it will remove 50 pints of water from the air.

To calculate exactly what size capacity you need, there is an intensive formula to follow. We will break it down here.

Before you start the calculations, you need to know the following facts:

  • The size of the room in cubic feet/meters
  • Current temperature and humidity levels (use a hygrometer for this).
  • Maximum humidity levels of the space without any dehumidification.
  • The desired humidity level you require in the space.

Once you have these factors, you can begin.

  1. First, you need to convert relative to absolute humidity. You can do the complex math yourself or use an online resource, chart, or calculator where the work is done for you. Planetcalc.com has a great conversion tool to use.
  2. Subtract present humidity from desired humidity. This result will give you the number for 1 cubic foot/meter.
  3. Multiply this number by the volume of the room (room size above) to get the total volume of water to be removed from the room.
  4. The volume assumes an air exchange ratio of 1. You can do further math based on the capabilities of your chosen unit for higher exchange rates, though ACH of 1 is relatively normal.
  5. The final number is the needed capacity of your dehumidifier, running constantly to maintain the desired humidity levels.

For a practical guide that shows average usage numbers for various scenarios, see the chart below.

Dehumidifier Sizing Chart (By Room Size)

For most of us, a general guide is all that is needed. You only need to know the current conditions and size of the space.

The conditions are:

  • Slightly Damp: Air can feel “heavy.”
  • Moderately Damp: May smell a musty or moldy odor.
  • Very Damp: Wet spots are seen on the walls, but not running or streaking.
  • Wet: Heavy air, smell musty odors and water spots may run (a bathroom after a shower, for example).
  • Extremely Wet: Water pools on the floor, seeps or runs from the walls and drips from the ceiling. Usually found in flood areas, basements, and other wet areas.

One other note: The chart below uses the new (2019) standard of dehumidifier ratings. This change must be in effect starting January 1st, 2022. The old standard uses higher testing temperatures and a lower humidity level. When shopping, you will see both standards listed on the label. However, as of 2022, the old (2012) standard must be gone.

For wet areas that experience a lot of rainfall and especially extremely wet homes, a whole-home dehumidifier is recommended in all sizes and situations.

Humidity Conditions 300+ Sq Ft 500+ Sq Ft 800+ Sq Ft 1200+ Sq Ft 1500+ Sq Ft 2000+ Sq Ft
Slightly Damp 20 Pint 30 Pint 40 Pint 50 Pint 70 Pint 90 Pint
Moderately Damp 20 Pint 30 Pint 40 Pint 50 Pint 70 Pint 90 Pint
Very Damp 30 Pint 40 Pint 50 Pint 70 Pint 90 Pint 90+ Pint
Wet 40 Pint 50 Pint 70 Pint 90 Pint 90+ Pint 90+ Pint
Extremely Wet 40 – 50 Pint 50 Pint 70 – 90 Pint 90 Pint Min. 90 Pint Min. 90 Pint Min.

What Size Dehumidifier Do I Need? (By Room Type)

Most often, you will only need to have a dehumidifier in a specific room. Most commonly, these are the basement, bathroom, and kitchen. For those with respiratory disabilities, a dehumidifier in the bedroom may help improve sleep or minimize the moisture in the air you are breathing.

For each of the home’s main rooms, we have listed what sizes, based on various conditions, are generally needed for those rooms. So, if you don’t want to do the math or just want a good size to go out and buy right now, this is the section for you.

Dehumidifier Size for Basement

Basements are one of the rooms, aside from crawl spaces, that almost always need dehumidifiers. While it isn’t a rule, it should be expected. Our review of the best basement dehumidifiers covers everything you need to know to figure out if your basement needs one or not.

When you find that you do, You will most likely use a whole-home system or a heat-pump style transfer. This includes a fan, coils, and a complete draining kit. On average, a moderate to wet basement will require a 70-pint system, while extremely wet or very large (1500 sq ft or more) sized basements will need at least 90 pint.

Dehumidifier Size for Crawl Space

Like basement models, crawl space systems are usually permanent, vented into the home’s HVAC system, and have a drain kit involved. Some will require a pump to move pooled water out of the crawl space, too.

These industrial or commercial-style models range between 70 and 140 PPD recovery. Based on the humidity levels, water collection, and size of the crawl space, you may need any of these size ranges. Most homes, though, will get by with a 70 to 90 PPD system. See crawl space-specific dehumidifiers here.

Dehumidifier Size for Mobile Home

When living or traveling in a mobile home, RV or camper, you will find that moisture levels are generally higher. This is because the air conditioner in a home is designed to remove moisture as it cools or heats the air. Smaller ACs on these mobile units can’t keep up with the levels of humidity we sometimes face.

Adding a 1 to 10 PPD portable unit in the space is generally all you need, though. If you find that the mobile home has extreme moisture, it is due to an insulation problem that a dehumidifier will not be able to correct or compete with.

Woman on Humidifier Maintenance

Dehumidifier Size for Grow Room & Tent

For those with more specific needs, such as growing indoor plants, having a greenhouse, grow tent, or vivarium, humidity levels are critical. Having the right dehumidifier can make the difference in your results, the longevity of the plants, and even their coloring.

The actual type, size, and capacity of the dehumidifier you need will vary greatly depending on your setup. Small indoor areas can use a 1 – 5 PPD portable model, while full-blown grow tents or greenhouses may require 30 to 80 PPD units. Our full grow room review will help you pick the perfect unit for your exact needs.

Dehumidifier Size for Bathroom

Bathrooms should always use the exhaust fan when the shower or bath is in use. This not only helps recycle the air in the small windowless room but will also remove the moisture in the air. An exhaust fan, though, is generally not enough, and water accumulation on the walls, ceilings and floors can cause damage, lead to mold and mildew problems, or much worse.

The best-sized dehumidifier for bathrooms will fall between 1 to 30 PPD. A lot will depend on the size of the bathroom, the frequency of use (showers, baths, or running sink water), as well as the average indoor temperature and water temperature you use.

Dehumidifier Size for Garage

Inside your garage, the humidity will change more often than in any other room of the home. Often garages are non-insulated, stand-alone, and have a huge door that brings in humid air from outside more than anywhere else in your home.

For these reasons alone, you need a more capable dehumidifier unit. Depending on the size of the garage and the primary usage, your needs will differ. For example, for someone using the garage as a workshop where the door stays closed, you can use a 30 to 50 PPD model.

Likewise, those that use the space for a finished garage and living in high humidity areas may need a 60 to 70 PPD model. For more details, you can check out our best dehumidifier for garage use review.

Dehumidifier Size for Gun Safe

Finally, you need to protect the items that protect you. Guns use metal, and metal does not mix well with moisture and humidity. The inside of your gun safe is susceptible to humidity damage and a gun safe specific dehumidifier.

The average size for gun safes will be measured differently. Usually, rechargeable silica gel or electric rods are used. You can find canisters, glass-encased rods, or plug-in models that range between 50 cubic feet up to about 500 cubic feet in dehumidifying power.

People Also Ask (FAQ)

What size drain hose do I need for a dehumidifier?

Some brands or models attempt to make a small profit from offering their own non-standard sizes, so you can only discern this before buying the product. However, in almost all cases, the standard drain size for dehumidifiers is ¾ inch hose and connections.

Should I have one dehumidifier for each room in the house?

If you find that you require a dehumidifier in every room of the house, you should consider investing in whole-home dehumidifier systems. However, in most cases, you only need a dehumidifier in rooms with low air movement and higher humidity, such as bathrooms, kitchens, garages, and basements. The bedrooms are optional.

Can one dehumidifier do a whole house?

A whole-home dehumidifier connects to the HVAC system and will pull moisture from every room where you have ventilation. However, portable dehumidifiers will only perform as a whole-home system in a small apartment or area less than about 500 square feet. The fans are not strong enough to pull air for an exchange rate from other rooms of the home.

Conclusion

Finding the right size dehumidifier for your home, room, or specific needs can be a challenge. You also need to determine the type, output potential, and method of dehumidifying. For most instances, a 30 to 50 PPD model will work well in all rooms of most sizes.

For larger spaces, basements, and garages that have higher moisture levels, though, a larger or whole-home system is needed. Hopefully, this article gives you the information you need to make a well-informed decision on your next dehumidifier purchase.

Josh M

My name is Josh and I am obsessed with the HVAC industry. I created this website to help HVAC techs of all levels get the best out of their heating & cooling systems. I have spent thousands of hours studying air conditioners, heaters and home air products so you can learn & buy with confidence. Learn more about the team here.

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