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

Getting the wrong size of dehumidifier is like getting the wrong sized heater. Sure, it will work in the same way – but it isn’t going to warm a space properly and you’re either going to end up too hot or too cold. If you want to deal with humidity properly in your home then you need to get the right size dehumidifier.

I’ve used and installed dehumidifiers of all shapes and sizes and while it can seem a little complicated at first, it’s actually quite straightforward to determine which size of dehumidifier you need. In this guide, I’ll explain why the size matters, how to calculate the size you need, and share a handy dehumidifier size chart so you know exactly what size to look for.

Air Humidifier on the Floor Near a Plant

Does Size Matter?

The size of your dehumidifier will determine how much excess moisture it can remove from the air. It’s really important to get the right-sized dehumidifier to improve your air quality.

If your dehumidifier is too small then it won’t be able to exact moisture and remove the excess humidity. These high moisture levels leave you uncomfortable and can make your air a breeding ground for micro-organisms and dust mites. It can even lead to mold growth which can make your home smell musty and trigger allergies [1].

If your dehumidifier is too large then it will make your air too dry. The dry air can irritate your throat, skin, and eyes, and make you feel uncomfortable. You also need to consider the energy efficiency of using a large dehumidifier in a small space because you’re probably using more electricity than needed – which can inflate your energy bills for no reason.

How Are Dehumidifiers Sized?

The size of a dehumidifier is not the physical dimensions of the appliance, instead, dehumidifiers are sized by the amount of humidity (moisture) they can remove in a single day. This is usually expressed as Pints Per Day (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.

Most dehumidifiers used at home are also sized by their power consumption in kilowatts per hour (kWh) or by the capacity of their water tank (usually in pints, cups, or milliliters).

A Rough Guide To Dehumidifier Sizes

We’ll give a more specific size chart in a second, but as a general guide there are 3 sizes of dehumidifiers to consider:

  • Small Dehumidifiers – 30-40 PPD

These work well as a single-room dehumidifier, or in an apartment with limited floor space. A small dehumidifier of this size should be able to keep the relative humidity below 60%.

  • Medium Dehumidifiers – 50-70 PPD

These dehumidifiers can work in a larger home space and work well in more humid conditions where excess moisture needs to be maintained more frequently. They tend to have features like auto-drain, humidity selection, timers, and remote operation are all found on larger units. These higher-capacity models are often used in a laundry room, or other rooms susceptible to mold growth.

  • Large Dehumidifiers – 70-90 PPD

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.

5 Factors Which Influence The Size Of The Dehumidifier You Need

The rough guide above can give you an idea of where to start, but every home is different. Here are 4 key factors to consider:

Square Footage

The physical size of the space your dehumidifier needs to work over is the key factor to consider in determining which dehumidifier you need. A small dehumidifier can’t work in a warehouse, but a whole-house dehumidifier used in a small space can leave your air too dry.

It’s important that you measure your square footage accurately (and we’ll come onto that in a second), but here’s a very rough guide for different-sized rooms:

  • Small rooms (up to 499 square feet) – 30-50 PPD dehumidifiers
  • Medium rooms (500-999 square feet) – 50-70 PPD dehumidifiers
  • Large rooms (1000+ square feet) – 70 PPD + dehumidifiers

Remember, this is just a very rough guide to match the average square footage to the right size dehumidifier, but it totally depends on the conditions in your home – which is why you need to consider the next 4 points too.

Relative Humidity Rates

Relative humidity (RH) is the amount of moisture in the air. It’s usually described as a %. The relative humidity in your home should generally be between 30-60% (though it can vary from room to room). If your humidity is lower than 30% then it can make you uncomfortable, and if it’s over 60% then it can negatively impact your health [2].

Your dehumidifier needs to be large enough to handle the relative moisture levels in a room without drying it out. Basements can reach up to 100% relative humidity, and rooms without windows or proper ventilation can reach 100% as well.

The higher the relative humidity the larger the dehumidifier you need so it’s worth investing in a hygrometer to accurately measure the relative humidity in your home. If you live in a humid climate you should also add 10 pints to the dehumidifier size.

How You Use The Space

How you use the space and what you have in the space will determine how much moisture there is in the air. The higher the relative humidity, the harder your dehumidifier will need to work to extract moisture – so the larger it needs to be.

If there’s a washer or dryer nearby, or it’s being used in a bathroom you should add 5 pints onto the size. If it’s a living area with lots of people you should also add 5 pints. If there are a lot of doors and windows, add 5 pints.

You need to carefully consider what might add moisture to your room, and then consider whether you need a bigger dehumidifier to handle it.

White Humidifier on Top of Bedside Table with Books

Average Temperature

The temperature in your home determines the humidity levels the air can hold, and in colder conditions, the moisture will be more contained in the air. It also accounts for the additional humidity generated by 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, so you’ll need a bigger dehumidifier if you live in an area with dramatic temperature changes.

Budget

Budget is always a consideration and generally, you’ll find that the larger the dehumidifier, the more it will cost. Single-room dehumidifiers and usually affordable and you can find a 50 PPD dehumidifier that doesn’t break the bank.

However, for a whole-house integration, or for larger dehumidifiers with the capabilities and features you need may need then you’ll have to pay more. It’s worth checking what you can afford before you start browsing home appliance manufacturers’ websites for their high-end models.

Calculating The Dehumidifier Capacity You Need

To calculate exactly the exact size dehumidifier you need you first need to gather the following information:

  • The size of the room in cubic feet/meters (you should be able to measure this easily enough)
  • Current temperature and humidity levels (you will need a hygrometer for this).
  • Maximum humidity levels of the space without any dehumidification (again, your hygrometer can help)
  • The desired humidity level you want in the space. (this depends on the type of room and what you use it for)

Once you have these factors, this is the process to follow:

  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 size of the room 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 an ACH of 1 is relatively normal.
  5. The number you’ve ended up with is the required capacity or size dehumidifier you need.

Just note that it assumes the dehumidifier is running constantly to maintain the desired humidity levels. If you’re running your dehumidifier for less time then it will impact the size dehumidifier that you need.

Dehumidifier Sizing Chart (By Room Size)

Knowing how to calculate what size dehumidifier you need can be useful, but most of us just need a size chart to follow. Below is a chart that gives the size of the dehumidifier you need based on different conditions.

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 (common in laundry rooms).
  • Wet: Heavy air, a musty smell, 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.
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.

To note:

  • The chart above uses the new (2019) standard of dehumidifier ratings [3]. 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 by home appliance manufacturers. However, as of 2022, the old (2012) standard must be gone.

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

To get the right size dehumidifier you have to consider the exact conditions and humidity level it will be dealing with. This can vary massively, so here is a breakdown of the best-size dehumidifier for where they are most commonly used:

Basement Dehumidifiers

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 with high humidity will need at a least 90-pint dehumidifier size.

A basement room dehumidifier will often have to deal with cold conditions, so it’s worth looking at desiccant dehumidifiers which are less likely to freeze because there are no coils.

Crawl Space Dehumidifiers

Crawl space dehumidifiers should be at least 70-90 PPD. They often have a small area in terms of square feet, but there is a lot of moisture and humidity to deal with. You may need a dehumidifier with a large water tank so it doesn’t need to be emptied as often.

If you get the right dehumidifier for your crawl space you can protect your home and improve the overall air quality [4]. It may be investing in specialist crawl space dehumidifiers which can range from 70-140 PPD.

Mobile Home Dehumidifiers

A small-size dehumidifier of 10-30 PPD is usually enough for a mobile home. A portable dehumidifier is usually enough, but if you find the mobile home has extreme moisture then it could be down to an insulation problem which dehumidification or air conditioning will not be able to compete with. You’ll need to fix the issue first.

Grow Room & Tent Dehumidifiers

Small-size dehumidifiers of 1-10 PPD will work best in a small tent. However, full-blown grow tents of greenhouses will require 30-80 PPD units. Having the correct size dehumidifier will be important for your plants, so it’s important to consider how much moisture you’re dealing with.

Bathroom Dehumidifiers

The best-sized dehumidifier for bathrooms will fall between 10 to 50 PPD. A lot will depend on the area (in square feet) 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.

Bathrooms should always use the exhaust fan when the shower or bath is in use. This air conditioning will help to remove the moisture in the air and make the job easier for your dehumidifier.

Garage Dehumidifiers

If you use your garage as a workshop where the door stays closed then a 30-50 PPD dehumidifier will work. If you’re using the garage as a living space, or it has a greater square feet area then you’ll need a 60-70 PPD dehumidifier.

Garages will often have low temperatures and changing humidities so you need a more capable dehumidifier. You may need to run it more frequently or even plumb it in for continuous drainage.

Gun Safe Dehumidifiers

The average size for gun safes will be measured differently from most dehumidifiers. They will usually be desiccant dehumidifiers with rechargeable silica gel or electric rods. On average, you need models that range between 50 cubic feet to about 500 cubic feet in dehumidifying power. This will depend on the square feet measurements of your gun room.

Guns use metal, and metal does not mix well with moisture and humidity, so it’s important to get the right dehumidifier for your safety.

People Also Ask (FAQ)

Can one dehumidifier do a whole house?

A whole-home dehumidifier connects to the HVAC system and will pull moisture and regulate the humidity in every room where you have ventilation. However, a small portable dehumidifier or single-room dehumidifier will only perform as a whole-home system in a small apartment or area less than about 500 square feet because it isn’t large enough.

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

Most homes don’t need individual room dehumidifiers, but it’s advisable to have at least one dehumidifier on each floor if you have a multi-level home. If you find that you require a dehumidifier in every room of the house, you should consider investing in whole-home dehumidifier systems.

What size drain hose do I need for a dehumidifier?

The standard drain size for dehumidifiers is ¾ inch hose and connections. 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.

Conclusion

Getting the right size dehumidifier is the key to controlling high humidity. Portable dehumidifiers are often cheaper, but if they aren’t big enough then they just won’t do the job. It’s important you consider the square feet area the dehumidifier is covering, and the humidity levels it’s dealing with so you can get the right size model.

Hopefully, this guide has helped explain how dehumidifiers are sized, and you now know which size of dehumidifier is best for you.

References:

1- https://www.clinicalresearchcenter.com/home-humidity-levels-can-trigger-allergy-symptoms/#:~:text=Indoor%20humidity%20levels%20are%20an,humidity%20is%20kept%20below%2050%25.

2- https://www.mayoclinic.org/diseases-conditions/mold-allergy/symptoms-causes/syc-20351519

3- https://www.sylvane.com/blog/new-dehumidifier-testing-standards-for-2020/

4- https://indoorscience.com/blog/it-came-from-the-crawl-space/

Last Updated on January 25, 2023

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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