Here are 8 steps which you can follow to estimate the size of the dehumidifier you need.
- With a hygrometer measure the amount of moisture in the area that you want to control
- If you don’t have a hygrometer, or don’t want to buy one, check if the area that you want to control has puddles of moisture. If so then the area probably has around 90-100% moisture.
If the area shows mould, smells very musky and/or shows stains then the area will probably show 80-90% humidity.
If you can smell mould in the area and the area feels very damp then you are looking at 70-80% humidity.
If the area smells mouldy only in damp or humid weather then the humidity level is probably around 60-70%.
- Check the following table for your ‘air changes per hour’ or ACH using the humidity percentages from above or from your hygrometer:
| % Humidity | ACH |
| 90-100 | 6 |
| 80-90 | 5 |
| 70-80 | 4 |
| 60-70 | 3 |
- Calculate the square feet of your area.
For example if your room measures 8 feet x 10 feet then the total square feet area is 80 feet.
- Now calculate the cubic feet in the area by measuring the height and multiplying that by the total square feet.
Therefore if the room is 8 feet high then the total cubic feet, for our example, will be 640 feet (8 X 80).
- Multiply the total cubic feet figure by the ACH and divide the answer by 60.
So if your humidity percentage is 75 then the ACH is 4.
In our example we would multiply 4 X 640 = 2560.
Next you should divide 2560 by 60 = 42.66.
You now know that you need 42.66 cubic feet of airflow per minute to properly adjust your area.
- Determine the pints of moisture that you need to extract from the air.
For moderately damp areas, which are those with a humidity reading of up to 70%, you need to extract around 10 pints of moisture for a 500 square feet area.
For every additional 500 square feet of area you need to extract another 4 pints of moisture.
So for a room that is 1250 square feet large you need to extract 16 pints of moisture
For very damp areas, those with a humidity reading of over 70-80%, you need to extract 12 pints of water for a 500 square feet area.
Add 5 pints for every additional 500 square feet of area.
For wet areas, or those with a humidity reading of 80-90%, you need to extract 14 pints of moisture per 500 square feet.
Add 6 pints for every extra 500 square feet of area.
For very wet areas you need to extract 16 pints of moisture for the first 500 square feet with an additional 7 pints for every 500 square feet of area.
- You should now know the size of the dehumidifier that you should need.
Read the product details of each dehumidifier to see if it is of suitable size.
Bear in mind that you can always buy a slightly larger dehumidifier and run it less often.
