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FAQ
Quick answer: These are the questions people ask most about furnace and air conditioner filters: how to find the right size, why the actual size is smaller than the label, which MERV to choose, how often to change it, and which way the arrow points. Each answer links to a deeper guide or calculator on FilterSizer.
Sizing questions
How do I find my filter size?
Read the nominal size on the old filter's frame. If it is unreadable, measure and round each side up to the next inch with the filter size finder.
Why is my filter smaller than its label?
Nominal sizes are rounded; actual sizes are about half an inch smaller so the filter slides into its slot. See nominal vs actual.
Does the order of dimensions matter?
No for the two face dimensions; depth always comes last.
What if I have no old filter?
Measure the slot or grille frame and round down to the nearest inch. See how to measure.
Can I find my size from the furnace model number?
Sometimes the furnace manual or label lists it. Our model number page explains where to look.
Efficiency questions
What does MERV mean?
Minimum Efficiency Reporting Value, a 1-to-16 scale from ASHRAE Standard 52.2 for particle capture. See MERV explained.
Which MERV should I choose?
Many homes use MERV 8 to 11 in 1-inch filters, and MERV 13 in deep media filters where airflow allows. The MERV picker gives an educational starting point.
Is higher MERV always better?
No. It can restrict airflow in systems not designed for it.
Will a filter help my allergies?
We do not make medical claims. Ask a medical professional about health concerns; the EPA and Health Canada publish indoor air guidance.
Changing filters
How often should I change my filter?
Commonly one to three months for 1-inch and six to twelve months for deep media, shorter with pets or heavy use. Use the replacement schedule and check monthly.
Which way does the arrow point?
Toward the furnace or air handler, in the direction of airflow.
Is it OK to run without a filter briefly?
Avoid it. Dust can coat the blower and coil.
Can I wash a disposable filter?
No. Washing damages the media. Only filters sold as washable should be cleaned.
Airflow and equipment
Can a dirty filter cause problems?
Yes. It reduces airflow, which can lower efficiency and trigger safety shut-offs. See airflow and pressure drop.
Should I have two filters, one at the grille and one at the furnace?
Usually only one location is intended. Ask an HVAC technician.
Is a 4-inch filter better?
It has more media and lower resistance, but needs a media cabinet. See 1-inch vs 4-inch.
How big should my return grille be?
It depends on airflow. The return grille sizer estimates face area.
Costs
How much do filters cost a year?
It depends on price and interval; use cost per year.
Are subscriptions worth it?
They add convenience, often at a higher per-filter cost. See subscriptions compared.
Special situations
What about apartments?
Filters are usually behind a ceiling or wall return grille; see apartment filters.
Mobile homes?
Many use downflow furnaces with the filter on top; see mobile home filters.
Box fans?
A 20-inch box fan fits a 20x20x1; see box fan filters.
Are Canadian sizes different?
No; see Canada filter sizes.
Storage and buying questions
How should I store spare filters?
Flat or on edge, in their wrap, somewhere dry. Crushed corners and damp frames leak once installed.
Do filters expire?
Not strictly, but electrostatic charge can fade in long storage and cardboard can soften in humidity. Use spares within a reasonable time.
Should I buy multi-packs?
For common sizes they usually lower the per-filter price and make it more likely you change on time.
Can I trim a bigger filter to fit?
No. Cutting breaks the frame and seal. Buy the correct or a custom size.
Do filters come in metric sizes?
Residential HVAC filters across North America are sold in inches, even in Canada, though some packages print a centimetre conversion. Convert centimetres to inches and round up.
Does filter brand matter?
Compare MERV, actual size and published resistance rather than brand names. We do not rank brands.
What FilterSizer covers
FilterSizer covers HVAC air filters for furnaces, air handlers, heat pumps and central air systems, plus related DIY uses such as box fan builds. It does not cover vehicle cabin filters, vacuum filters, standalone purifier cartridges or any non-HVAC filter type.
Worked example of using the tools together
A new homeowner opens the furnace slot and finds a filter labelled 15.5 × 24.5 × 0.75. The size finder confirms 16x25x1. They want better dust capture with a dog, so the MERV picker suggests a MERV 8–11 starting range for a 1-inch slot. The schedule tool suggests about 60 days with a pet. With a hypothetical $12 MERV 11 filter, the cost tool estimates about $73 a year. They add six calendar reminders and buy a three-pack.
Illustrative note (2026-10-29): This example was prepared on 2026-10-29 for illustration. The household and prices are hypothetical.
When to call a professional
Call a qualified HVAC technician if airflow seems weak after a filter change, the furnace short-cycles or shuts off, the coil ices in summer, you smell burning, or you want to add a media cabinet or return grille. Filters are a homeowner task; diagnosing equipment is not.
How we write these answers
Each answer draws on general HVAC practice and the ASHRAE and EPA material on MERV. We keep claims conservative, label every example as illustrative and do not rank brands. If something here conflicts with your equipment manual, follow the manual.
Didn't find your question?
Email hello@charlandandco.com with your question and we will consider adding it. We cannot diagnose individual systems by email.