Custom HVAC Filter Sizes: Nominal vs Actual, Measuring and Getting a Fit
Custom HVAC filter sizes: why a 16x25x1 filter measures about 15.5 x 24.5 in, how to measure your slot, when to order custom, and why a snug fit matters.
Affiliate disclosure: product links on this page go to Amazon, or to the maker's own store for brands that sell only direct. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you; a link to a maker's store earns us nothing. No retailer or brand sees our picks before they are published or has any say in them. How we choose.
HVAC filter sizes are nominal, rounded labels; the actual filter is a little smaller. Filtrete's 16x25x1 measures 15.62 by 24.62 by 0.78 inches, Nordic Pure's 15.5 by 24.5 by 0.75, and Aerostar's 15.75 by 24.75 by 0.75, according to their listings. Measure your filter slot and pick the standard size whose actual dimensions fit snugly. Order a custom size only if no standard size does.
Fit matters because EPA says a filter should fit snugly so air does not leak around it, and you should not have to bend or crush it to fit. EPA's technical summary adds that bypass airflow around poorly fitted filters significantly decreases their effectiveness.
Nominal vs actual size
| Brand | Nominal | Actual |
|---|---|---|
| Filtrete MPR 1900 | 16x25x1 | 15.62 x 24.62 x 0.78 in |
| Filtrete MPR 1000 | 16x25x1 | 15.68 x 24.68 x 0.81 in |
| Nordic Pure MERV 13 | 16x25x1 | 15.5 x 24.5 x 0.75 in |
| Aerostar MERV 13 | 16x25x1 | 15.75 x 24.75 x 0.75 in |
| Honeywell FC100A1037 (4-inch media) | 20x25x4.5 | 19 15/16 x 24 7/8 x 4 3/8 in |
The differences between brands of the same nominal size can be a quarter inch, which is enough to leave a gap in a tight slot or jam in a loose one. Nordic Pure's listing says to measure your filter and compare it to the actual size to ensure proper fit.
How to measure
- Turn the system off.
- Remove the old filter and measure the slot or rack, not the filter: width, height and the depth of the slot.
- Measure to the nearest eighth of an inch.
- Compare with the actual sizes brands publish, and choose one that slides in without bending and without obvious gaps.
- Note the airflow direction; EPA says the arrow on the filter should point in the direction of airflow.
EPA notes that filters are commonly 1 to 5 inches thick. Do not put a 1-inch filter in a 2-inch slot; the gap behind it lets air bypass.
Where filters live
EPA's technical summary says furnace filters are installed either at the base of the air-handling unit or upstream in return grilles. It warns that at a return grille, leaky return ducts can draw air from unconditioned spaces around the filter, and recommends well-sealed ducts for return grille installations. If your filters are in grilles, measure each one; homes often have more than one size.
When you need a custom size
- No standard size's actual dimensions fit within about an eighth of an inch.
- The slot was built to an odd size, common in older or custom ductwork.
- You are using a filter cabinet whose maker no longer supplies filters.
Several online filter makers build custom sizes to order. Ask for the MERV rating, the actual finished dimensions and the frame type, and avoid cutting a larger filter down, which leaves an unframed edge for air to bypass.
Fixing a loose fit
- Try another brand of the same nominal size; actual sizes differ.
- Use a filter rack or frame designed for your slot.
- Seal gaps in the filter housing; EPA says high-efficiency filters require well-sealed frames to prevent leaks.
- For a permanent solution and higher MERV, consider a media cabinet sized for your duct. See the best whole-house air filters.
Why fit beats MERV
A MERV 13 filter that leaves a gap can let enough air around it to perform worse than a well-fitted MERV 11. EPA's technical summary shows an example of large bypass around a filter caused by improper installation combined with heavy loading. Get the fit right first, then choose the MERV. See MERV ratings explained, the best HVAC filters and signs your HVAC filter is dirty.
Frequently asked questions
Why is my 16x25x1 filter not actually 16 by 25 inches?
Sizes are nominal. Filtrete's 16x25x1 measures 15.62 by 24.62 by 0.78 inches and Nordic Pure's 15.5 by 24.5 by 0.75, according to their listings.
How do I measure for an HVAC filter?
Measure the slot or rack, not the old filter, to the nearest eighth of an inch, then compare with the actual sizes brands publish. The filter should slide in without bending and without gaps.
Does a loose filter matter?
Yes. EPA says air leaking around a filter significantly decreases its effectiveness, and that filters should fit snugly.
Can I cut a filter to size?
It is not recommended. Cutting leaves an unframed edge that air can bypass. Order a custom size with a finished frame instead.
Do all brands use the same actual size?
No. Among 16x25x1 filters, the brands we checked ranged from 15.5 to 15.75 inches wide. Compare actual dimensions, not labels.
Can I use a 1-inch filter in a 2-inch slot?
Avoid it. The gap lets air bypass the filter. EPA notes filters commonly range from 1 to 5 inches thick; match the slot depth.
Sources
- Guide to Air Cleaners in the Home, US Environmental Protection Agency
- Residential Air Cleaners: A Technical Summary (3rd edition), US Environmental Protection Agency
- What is a MERV rating?, US Environmental Protection Agency
Related guides
Are Washable HVAC Filters Worth It? The Trade-Offs in Numbers
Washable filters save waste but most are MERV 8, which catches large dust and pollen and little smoke-size particulate. For allergies or smoke, a disposable MERV 13 is the better buy.
Best HVAC Filters for Allergies: MERV 12 and 13 Picks
Pollen, dust mite debris and dander are mostly large particles, which MERV 12 and 13 filters catch at 90% or more. Our top allergy pick is the Filtrete MPR 1500 (MERV 12).
Best HVAC Filters of 2026: MERV 13 and Alternatives for Every System
EPA suggests MERV 13 or the highest your system can handle. Our top pick is the Filtrete MPR 1900, a MERV 13 1-inch filter; here are five more for different systems and budgets.