Mistakes are common when choosing the right furnace filter size. Labels, filter thickness, MERV ratings, and dimensions are all things to consider when choosing the correct furnace filter size. This guide will provide you with a complete chart of filter sizes, explain nominal and actual sizes, and give you tips on avoiding buying the wrong size filter. If you want to quickly skip to the size chart, click here.
Contents
Nominal Size vs. Actual Size
The dimension printed on a furnace filter usually refer to the filter’s nominal size. This means the size of the furnace filter is slightly smaller than what is printed on it. A common mistake after measuring your furnace filter is looking for a replacement with the actual size instead of the nominal size. Just because the measurement of your filter is smaller than what’s printed on it, it’s not the wrong filter. Furnace filters are made slightly smaller than their nominal dimensions so they can fit into the filter slot.
For example, a furnace filter size may be labelled as 16 x 25 x 1 inches, but if you actually measure it, it might be 15.5 x 24.5 x 0.75 inches. Every manufacturer will have different dimensions. If you don’t fully understand actual size, buy filters based on the dimensions printed on your existing filter if possible. If you don’t already have a filter to look for dimensions on, check your manufacturer’s specifications.
The Furnace Filter Size Chart
| Nominal Size | Approximate Actual Size | Other Thickness Options |
| 14″ x 20″ x 1″ | 13.5″ x 19.5″ x 0.75″ | 1″, 2″, 4″ |
| 14″ x 25″ x 1″ | 13.5″ x 24.5″ x 0.75″ | 1″, 2″, 4″ |
| 16″ x 20″ x 1″ | 15.5″ x 19.5″ x 0.75″ | 1″, 2″, 4″ |
| 16″ x 25″ x 1″ (Common) | 15.5″ x 24.5″ x 0.75″ | 1″, 2″, 4″, 5″ |
| 18″ x 24″ x 1″ | 17.5″ x 23.5″ x 0.75″ | 1″, 2″, 4″ |
| 20″ x 20″ x 1″ | 19.5″ x 19.5″ x 0.75″ | 1″, 2″, 4″, 5″ |
| 20″ x 25″ x 1″ (Common) | 19.5″ x 24.5″ x 0.75″ | 1″, 2″, 4″, 5″ |
| 20 x 30 x 1 | 19.5″ x 29.5″ x 0.75″ | 1″, 2″, 4″ |
| 24 x 24 x 1 | 23.5″ x 23.5″ x 0.75″ | 1″, 2″, 4″ |
How to Measure Your Furnace Filter Size
If you don’t know what size to use for your filter or the label is unreadable you can figure out your furnace filter size in these three easy steps:
- Turn off your furnace system and remove your furnace filter. Remember the direction of the arrow on it for when you reinstall your filter.
- Measure the width, length, and thickness of your filter in inches. Now you have the actual size of your filter.
- Find the closest actual size on the furnace filter size chart and look for its equivalent nominal size. Reinstall the filter with the arrow pointing the same direction as before.
If you don’t have a filter to measure, you can measure the slot for your furnace filter and use the same process.
What If Your Furnace Filter Size Isn’t Standard?
Not every furnace filter size may appear in the chart. The easiest way to find your filter size is by checking the existing filter’s label. If there is no label, measure the filter and remember that you are measuring the actual size, but are purchasing a size that is usually rounded up. If there is no filter to measure, either check your manufacturer’s standard size or measure the size of the filter slot. You can easily find your furnace’s filter size by searching the model number on it.
If you need to force a filter into a slot because it’s slightly too big, it may contribute to shortening your HVAC system’s lifespan. If you are using a filter that leaves gaps around the edges, unfiltered air may enter your furnace and damage your system. Rarely, you might need to order a custom-sized filter.
MERV Ratings Explained
MERV stands for “Minimum Efficiency Reporting Value” and it measures the air filtration quality of a furnace filter. Most modern homes, filters rated from 8 to 13 are common, but you should follow your manufacturer’s recommendations. A higher rated filter will do a better job at capturing particles in the air and will improve air quality. If you want to actually know exactly what each filter captures, check out the EPA’s explanation of MERV ratings.
Common Mistakes to Avoid
Some common mistakes made when choosing a furnace filter size and installing it are:
- Ignoring filter thickness: just because the width and length of a filter are compatible with your system, the thickness isn’t always something you can change.
- Nominal vs. actual size: the physical dimensions of the filter when you measure it aren’t the same as the dimensions printed on the label.
- Buying a filter that doesn’t fit: your filter shouldn’t have gaps around the edges and shouldn’t be difficult to insert.
- Throwing away the old filter before checking the label: writing down the dimensions on your old filter’s label is the easiest way to know what size to buy.
- Installing the filter backwards: the arrow indicating airflow should point towards the furnace.
Frequently Asked Questions
How do I find my furnace filter size without removing it?
If you can see the label without removing the filter, look for the dimensions printed along the edge. The size will usually be written as width x length x thickness. If you can’t see the label, you can check your HVAC system’s manual or search your furnace model number to find the correct size.
What’s the most common furnace filter size?
Although there isn’t one universal furnace filter size, there are several common sizes. 16″ x 25″ x 1″ and 20″ x 25″ x 1″ are common sizes to encounter for modern HVAC systems.
Can I use a smaller filter than my slot?
Using a smaller air filter will let unfiltered air get through gaps. You shouldn’t use a smaller filter than what your system’s manufacturer recommends because it may damage your HVAC system.
How often should I replace my furnace filter?
For most homes using a pleated filter, it’s standard to change it every 60-90 days and even more often if you have pets or allergies. If you have a fiberglass filter, make sure to change it about every month. The lifespan of your furnace filter depends on many factors such as MERV rating, filter material, pets, and climate. If you want a complete explanation of when to replace your furnace filter, read our helpful guide.
Is furnace filter size the same as AC filter size?
In most homes, the HVAC system uses the same filter for heating and cooling because the furnace and air conditioner share the same air-handling system. This isn’t true for every system. Make sure to check the size printed on your existing filter because furnace and AC filters may be different sizes.
