Enter the size printed on your old filter to see what it really measures, or measure the filter slot with a tape and get the size to buy. The actual sizes are a filter maker's own.
Your filter
A 16 × 25 × 1 filter really measures 15½ × 24½ × ¾ inNordic Pure's actual size · the slot is a little bigger: measure it inner edge to inner edge · MERV 8–13 for most homes
Quick answer: the size printed on a furnace filter is its name, not its measurement. A 16 × 25 × 1 filter really measures 15½ × 24½ × ¾ inches, by Nordic Pure’s own product pages, and a 20 × 25 × 4 measures 19½ × 24½ × 3⅝. American Standard: the real size “is usually ½ to ¾ inches smaller”. To find yours, read the old filter’s frame, or measure the slot from inner edge to inner edge and buy the closest size.
How to use the filter size finder
- Read the old filter’s frame: the size is printed on its edge, like 16x25x1. Enter it to see what the filter really measures.
- No size on it, or no old filter? Choose “A tape measure” and measure the slot from inner edge to inner edge, or the old filter itself. You get the common size that fits.
- Read the result: the name to buy, its real size, the filter face area and the MERV range for most homes.
Cleaning the air in one room? The air purifier size calculator sizes a room unit by AHAM’s rule.
The rules, and where they come from
Nominal and actual. Nordic Pure: “Almost all air condition filters have a different Actual Size (the exact dimensions) than Nominal Size (the name) printed on the air conditioning filter. It is best to measure the exact size of your A/C filter and compare it to our Actual Size.” Its product pages print the actual size of every filter; the table below is theirs, for their MERV 8 pleated filters.
How much smaller. American Standard: the actual size “is usually ½ to ¾ inches smaller for length (height) and width”. A 1-inch filter is about ¾ inch thick, a 4-inch about 3⅝ and a 5-inch about 4⅜ (Nordic Pure).
Measuring. American Standard: “Turn off the system and use a retractable tape measure to determine the length, width, and depth of the opening, from inner edge to inner edge. Then, you will buy a replacement filter that is closest to those measurements.”
MERV. American Standard: “HVAC professionals recommend a MERV rating between 8 and 13 for most residential applications”; “If the MERV rating is too high for your system, it can restrict airflow.” A 4″ or 5″ filter “can have a higher MERV rating … without restricting airflow”. Change it every 30 to 90 days.
Furnace filter size chart: nominal and actual
| Name on the frame (inches) | Actual size (inches) |
|---|---|
| 10 × 20 × 1 | 9½ × 19½ × ¾ |
| 12 × 12 × 1 | 11¾ × 11¾ × ¾ |
| 12 × 20 × 1 | 11½ × 19½ × ¾ |
| 12 × 24 × 1 | 11⅜ × 23⅜ × ¾ |
| 14 × 20 × 1 | 13½ × 19½ × ¾ |
| 14 × 24 × 1 | 13¾ × 23¾ × ¾ |
| 14 × 25 × 1 | 13½ × 24½ × ¾ |
| 15 × 20 × 1 | 14½ × 19½ × ¾ |
| 16 × 20 × 1 | 15½ × 19½ × ¾ |
| 16 × 24 × 1 | 15⅜ × 23⅜ × ¾ |
| 16 × 25 × 1 | 15½ × 24½ × ¾ |
| 18 × 20 × 1 | 17½ × 19½ × ¾ |
| 18 × 24 × 1 | 17½ × 23½ × ¾ |
| 18 × 25 × 1 | 17½ × 24½ × ¾ |
| 20 × 20 × 1 | 19½ × 19½ × ¾ |
| 20 × 24 × 1 | 19½ × 23 7/16 × ¾ |
| 20 × 25 × 1 | 19½ × 24½ × ¾ |
| 20 × 30 × 1 | 19¾ × 29¾ × ¾ |
| 24 × 24 × 1 | 23⅜ × 23⅜ × ¾ |
| 25 × 25 × 1 | 24½ × 24½ × ¾ |
| 16 × 25 × 2 | 15½ × 24½ × 1¾ |
| 20 × 20 × 2 | 19½ × 19½ × 1¾ |
| 20 × 25 × 2 | 19½ × 24½ × 1¾ |
| 16 × 20 × 4 | 15½ × 19½ × 3⅝ |
| 16 × 25 × 4 | 15½ × 24½ × 3⅝ |
| 20 × 20 × 4 | 19½ × 19½ × 3⅝ |
| 20 × 25 × 4 | 19½ × 24½ × 3⅝ |
| 16 × 25 × 5 | 15¾ × 24¾ × 4⅜ |
| 20 × 20 × 5 | 19⅝ × 19⅞ × 4⅜ |
| 20 × 25 × 5 | 19⅞ × 24⅞ × 4⅜ |
Nordic Pure’s own “Actual Size” for each filter, read from its product pages (MERV 8 pleated). Other brands’ real sizes differ a little: measure before you order.
Worked examples
A 16 × 25 × 1 filter really measures 15½ × 24½ × ¾ in: the most common size. Its slot is about 16 × 25 inside.
A 20 × 25 × 4 filter really measures 19½ × 24½ × 3⅝ in: a media cabinet filter. Nordic Pure’s 5-inch version is 19⅞ × 24⅞ × 4⅜.
Buy a 16 × 25 × 1 filter for what you measured, 15¾ × 24¾ × 1 in: a slot measured with a tape. The 16 × 25 × 1 leaves ¼″ each way.
Look for a 17 × 22 × 1 filter: no common size fits 17 × 22 × 1 in closely: an odd slot takes an odd size or an exact-size filter made to order.
Common mistakes
- Buying by the real size. Measure 15½ × 24½ and the filter you want is called 16 × 25. Search by the name on the frame.
- Measuring the wrong thing. Measure the slot inside edge to inside edge, or the old filter, never the furnace’s door.
- A filter that does not fit. American Standard: buy the same size again “unless it’s obvious that the filter is too big (edges are crushed or the filter is cut to fit) or too small”.
- Too high a MERV. Above 13 on a 1-inch filter “can restrict airflow” (American Standard). A thicker cabinet allows more.
- Swapping depths. A 4″ cabinet takes a 4″ filter: buy the depth printed on the old one.
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Planning the room around the furnace? Draw it to scale in the free 2D floor planner.
Furnace filter size questions
What furnace filter size do I need?
The size printed on your old filter’s frame, like 16x25x1. With no old filter, measure the slot from inner edge to inner edge and buy the closest size: a slot of about 16 × 25 takes a 16 × 25 filter.
What is on an air filter size chart?
The name of each filter (its nominal size) beside what it really measures. A 16 × 25 × 1 is 15½ × 24½ × ¾ inches, a 20 × 20 × 1 is 19½ × 19½ × ¾ (Nordic Pure).
What are the most common air filter sizes?
American Standard lists 16x25x1, 16x25x4, 16x25x5, 20x25x1, 20x25x4, 20x25x5, 16x20x1 and 16x20x4 among the most common.
Why is my filter smaller than the size on it?
The printed size is a name, rounded up, so the filter slides into a slot of that size. American Standard: the real size is usually ½ to ¾ inch smaller.
How do I measure a furnace filter size?
Turn the system off and measure the slot’s length, width and depth from inner edge to inner edge, as American Standard says, or measure the old filter. Then choose the size closest to it.
What MERV rating should a home filter be?
Between 8 and 13 for most homes, says American Standard. Higher can restrict airflow on a system made for a 1-inch filter.