HVAC Filter Size Guide

1. Find Your Filter Size

Enter your current filter dimensions or pick a common size:

2. MERV Rating Comparison

MERV (Minimum Efficiency Reporting Value) rates a filter's ability to capture particles. Higher = better filtration but more airflow resistance.

MERVCapturesBest ForPrice RangeLevel

3. Replacement Schedule Calculator

Answer these questions to get a personalized filter replacement schedule.

4. How to Measure Your Filter

  1. Check the existing filter first. Most filters have the size printed on the frame (e.g., 20x25x1). This is the nominal size.
  2. Actual vs. nominal size. The actual size is typically 0.25" to 0.5" smaller than the nominal size. Always buy by nominal size.
  3. Measure the slot, not the filter. If no size is printed, measure the length, width, and depth of the filter slot in your HVAC unit.
  4. Round up to the nearest inch. If your slot measures 19.5" x 24.5", the nominal size is 20x25.
  5. Depth matters. 1" filters need frequent replacement. 4-5" filters last much longer and provide better filtration.
  6. Direction arrow. Install with the arrow pointing toward the blower/furnace (toward airflow direction).

Further reading

Filter Replacement Interval by Depth and Household

Every figure comes from the schedule calculator above. Find the row that matches your household and read across for the filter depth your system takes. Depth does most of the work — a 4-inch filter holds far more before airflow suffers — so if a row leaves you changing a 1-inch filter every couple of months, a deeper slot is worth asking about.

Household1 inch2 inch4 inch5 inch+
No pets, suburban, normal use90 days120 days270 days365 days
One pet75 days105 days255 days350 days
Two pets60 days90 days240 days335 days
Allergies or asthma70 days100 days250 days345 days
Smoking indoors65 days95 days245 days340 days
Urban location75 days105 days255 days350 days
Heavy year-round use70 days100 days250 days345 days
Rural, light seasonal use115 days145 days295 days390 days

ν‘œμ˜ λͺ¨λ“  값은 이 νŽ˜μ΄μ§€μ˜ 계산기가 직접 κ³„μ‚°ν•œ κ²°κ³Όμž…λ‹ˆλ‹€ Β· one factor per row — they stack in the calculator, so a household with pets and allergies lands lower than either row alone · nothing is ever shorter than 14 days · a dirty filter is the one to change, whatever the schedule says

Find the right HVAC filter size, compare MERV ratings, and calculate your ideal replacement schedule based on your home environment.

Related tools

Sources

Before choosing a higher rating, check that your system can handle the added resistance β€” the EPA's guidance covers the trade-off.

Links checked 2026-09-09.

Frequently asked questions

How do I measure my filter if the size is not printed on it?

Measure the slot, not the filter, and round up to the nearest inch. This page's own worked example is a slot measuring 19.5 by 24.5 inches, which is a 20x25 filter. The printed number is the nominal size and the actual filter runs about a quarter to half an inch smaller, so buying by the nominal size is what gives you a filter that fits.

How often should I actually change it?

Depth sets the baseline: this page's schedule calculator starts at about 90 days for a 1-inch filter, 120 for 2-inch, 270 for 4-inch and a year for 5-inch or deeper. That is why a thicker filter is worth the slot if your system takes one — more media means more to fill before airflow suffers.

What shortens the interval most?

Smoking indoors takes the most off in this calculator, then allergies or asthma in the household, then heavy year-round HVAC use, then each pet, then an urban location. A rural location and light seasonal use push the interval the other way. Whatever the total comes to, the calculator never recommends longer than the depth baseline allows or shorter than about two weeks.

Which way does the filter go in?

Arrow toward the furnace or air handler — that is, pointing in the direction the air travels. Installing it backwards puts the weaker face of the media against the incoming air and lets the filter collapse toward the blower.