Quick answer: To measure an air filter, pull out the old one and read the size printed on the side (like 16x25x1). If there's no print, measure the length, width, and thickness with a tape measure, then round each up to the nearest inch. That rounded number is the nominal size you order by.
Ordering the wrong filter size is the most common filter mistake, and it's an easy one to avoid. Get it wrong and air slips around the edges, dust gets past, and your system works harder. This guide shows you exactly how to measure an air filter so you order the right fit the first time. It takes about five minutes and one tape measure.
How to measure an air filter: the fast way
Most of the time you don't need to measure at all. The size is already printed right there.
- Step 1: Turn off your HVAC system at the thermostat.
- Step 2: Find the filter slot. It's usually in the return air grille (a large vent on a wall or ceiling) or in a slot near the furnace or air handler.
- Step 3: Slide the old filter out.
- Step 4: Look at the cardboard frame. The size is printed on the edge, like 16x25x1 or 20x25x1. The numbers are length x width x thickness in inches.
That printed number is your nominal size, the size you order by. Write it down and you're done. If you can read it, skip to the ordering section below or jump straight to our size finder to match it.
How to measure when the size is worn off
Sometimes the print is faded or the filter is old enough that you can't read it. No problem. Grab a tape measure and measure the filter itself (or the slot if the filter is missing):
- Length: Measure the longest side, edge to edge.
- Width: Measure the shorter side, edge to edge.
- Thickness (depth): Measure how thick the filter is, front to back. Common depths are 1", 2", 4", and 5".
Here's the part that trips people up. Your tape measure will read the actual size, which is about ½" smaller than the name. So a filter labeled 20x25x1 actually measures closer to 19.5" x 24.5" x 0.75". That gap is on purpose, so the filter slides in and out without binding.
Nominal vs actual size: why the numbers don't match
This is the single biggest source of sizing confusion, so let's make it dead simple.
- Nominal size = the rounded name on the box. This is what you order by. Example: 16x25x1.
- Actual size = what a tape measure reads, roughly ½" smaller on length and width. Example: 15.5" x 24.5".
So when you measure with a tape, round each dimension up to the nearest inch to find the nominal size you order. Measure 15.5" x 24.5"? Order 16x25. Measure 19.5" x 24.5"? Order 20x25. We cover this in detail in nominal vs actual filter size.
Quick conversion chart
| If your tape measures (actual) | Order this (nominal) |
|---|---|
| 15.5" x 24.5" | 16x25 |
| 19.5" x 24.5" | 20x25 |
| 15.5" x 19.5" | 16x20 |
| 13.5" x 19.5" | 14x20 |
| 19.5" x 19.5" | 20x20 |
| 17.5" x 23.5" | 18x24 |
Then add the thickness. A filter that measures about 0.75" thick is a "1 inch" filter. One that measures about 3.75" is a "4 inch." See 1 inch vs 4 inch filters if you want to know which depth lasts longer.
Measuring tips that save you a return
- Don't guess on thickness. A 1" and a 2" filter can have the same length and width but won't fit the same slot. Always measure depth.
- Length always comes first by convention, but if your slot is square (like 20x20), it doesn't matter. For rectangular filters, double-check which way it goes in.
- Measure the slot, not just the old filter, if you suspect someone forced in the wrong size before. The slot is the source of truth.
- Note the airflow arrow. While the filter is out, look for the arrow printed on the frame. It points the direction air flows, toward the furnace. Your new filter needs to go in the same way. More on that in air filter direction arrows.
Once you know your size, ordering is simple
Plug your nominal size into the Ironside size finder and it pulls up every matching filter. From there, pick your MERV level based on what you need to catch:
- Dust and pollen: Everyday Defense (MERV 6–8)
- Pets and allergies: Allergy & Pet (MERV 10–12)
- Smoke and fine particles: Maximum Protection (MERV 13–16)
- Odors and cooking smells: Odor & Smoke (carbon)
Or browse all Ironside filters in one place. Once you've locked in your size, a subscription keeps that exact filter showing up on schedule, with free shipping every time and your price locked in. No re-measuring, no running to the store.
FAQ
Where is the size printed on an air filter?
On the cardboard or plastic frame along the edge, usually in large numbers like 16x25x1. The format is length x width x thickness in inches. If it's worn off, measure with a tape and round each dimension up to the nearest inch.
Why does my tape measure show a smaller number than the filter size?
Because the printed size is the nominal size, and the actual measured size is about ½ inch smaller. This small gap lets the filter slide in and out easily. Always order by the rounded-up nominal number, not the exact tape reading.
What if my measurements fall between two sizes?
Measure again carefully, since being off by even a quarter inch can change which size you need. If you're truly between sizes, go with the size that matches your slot snugly without forcing it. Air slipping around a loose filter ruins filtration.
Do I need to measure thickness too?
Yes, always. Two filters can share the same length and width but have different depths (1", 2", 4", 5"). The wrong depth either won't fit the slot or won't seal. Measure all three dimensions every time.