Understanding your result
The headline is the cost of filters per year, with the number of filters and the replacement interval in the sub-line. The stats give the interval in days, the filters per year and the cost per month. The table shows the base interval for the thickness, the adjusted interval after the factors for pets, allergies and dust, the price used (typical or yours), and the cost per year and over ten years.
The notes pick up two common situations. A MERV 13 filter in a 1-inch slot gets a pressure-drop warning, because it restricts airflow on many residential blowers. A 1-inch filter costing more than about $80 a year gets a note suggesting a media cabinet, which usually pays for itself.
Filters sit in the return air path, and a clogged one is the most common reason a system cannot move its rated airflow. The duct equivalent length calculator counts the filter’s pressure drop in the static pressure budget, and the return grille calculator sizes filter grilles at the lower velocity they need. For a single room that needs more filtration than the central system gives, the air purifier CADR calculator sizes a portable unit.
How we calculate this
The base intervals are the typical guidance printed by filter and equipment manufacturers for each thickness. The adjustment factors are our judgement, written as simple multipliers so they can be checked: a shedding pet roughly halves a filter’s life, allergies call for changing before the filter is fully loaded, and dusty conditions such as renovation, smoking, wildfire smoke or a dirt road load it much faster. The typical prices are mid-range retail for pleated MERV 8 filters of each thickness, scaled up for MERV 11 and 13. Pressure drop and airflow are covered with the duct figures on the duct size chart, and the airborne particle targets on the ventilation and exhaust minimums page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Base intervals | 90 / 120 / 180 / 270 days for 1 / 2 / 4 / 5 in | Typical filter and equipment manufacturer guidance |
| Pets | × 0.67 for one, × 0.5 for two or more | Judgement consistent with manufacturer guidance |
| Allergies, dust | × 0.75, × 0.5 | Judgement |
| MERV 13 in 1 in | × 0.8 | Higher capture loads a thin filter faster |
| Typical prices | $12, $20, $35, $50 for MERV 8 | Mid-range US retail, 2025 |
| MERV scaling | × 0.4 (MERV 4), × 1.2 (MERV 11), × 1.4 (MERV 13) | Typical retail price differences |
Assumptions last reviewed October 8, 2026.
The calculator does not measure how fast your filter actually loads, which depends on the house’s air tightness, the system’s run hours and the outdoor air; the light test or a pressure gauge across the filter is the real check. It does not cover electronic or electrostatic filters, UV lights, or washable filters. The guide to MERV filters and airflow explains the trade-off between filtration and airflow, and the guide on return air covers where filters belong in the system.
Two worked examples
A standard 1 inch filter
1 in MERV 8 pleated filter, typical price, one filter, no pets, no allergies, system running all year.
- Base interval: 90 days; no adjustments
- Filters a year: 365 ÷ 90 = 4.06, rounded up to 5
- Cost: 5 × $12 = $60 a year, $5 a month, $600 over ten years
Five filters rather than four is the rounding of a 90-day schedule over 365 days; in practice most households change four or five times a year.
Two pets and allergies
1 in MERV 11, typical price, two pets, allergies, running all year.
- Interval: 90 × 0.5 × 0.75 = 34 days
- Filters a year: 365 ÷ 34 = 10.7, rounded up to 11
- Cost: 11 × $12 = $132 a year
At $132 a year, a 4-inch MERV 11 or 13 media cabinet changed every 4 to 6 months costs about the same in filters, filters better, and is far easier on the blower. The note says so.
Where to find your inputs
Thickness and size. Printed on the filter’s frame, for example 16 × 25 × 1. The last number is the nominal thickness.
MERV. Printed on the frame or package; some brands use their own scale (MPR or FPR), and the package usually gives the MERV equivalent.
Number of filters. Look at the air handler and at every return grille; count each filter slot.
Months running. Twelve for a system that heats and cools; fewer if it sits off in spring and autumn.
Common mistakes
- The 90-day rule with pets. Two shedding dogs can clog a 1 inch filter in a month.
- MERV 13 in a 1 inch slot. It can starve the blower; use a media cabinet.
- Filters in series. A grille filter and an air handler filter together add pressure drop for little gain.
- Forgetting a second return. Every filter slot needs changing.
- Fibreglass filters for allergies. MERV 1 to 4 protects the equipment, not your lungs.
- Washing disposable filters. It damages the media; replace them.
Questions people ask
- How often should I change my furnace filter?
- A 1-inch pleated filter every 90 days in a house with no pets and no allergies, every 60 days with one pet, every 30 to 45 days with several pets or allergy sufferers. Thicker filters last longer, a 2-inch about 4 months, a 4-inch about 6 months and a 5-inch about 9 months, because they have several times the media area. The surest test is to hold the filter to a light; if light barely shows through, change it.
- What MERV rating should I use?
- MERV 8 for most homes, which catches dust, pollen and mould spores; MERV 11 for pets and mild allergies; MERV 13 for allergies, asthma or wildfire smoke, which catches a large share of fine particles. MERV 13 in a 1-inch filter has a high pressure drop that many residential blowers cannot overcome without losing airflow, so it belongs in a 4- or 5-inch media cabinet. MERV 1 to 4 fibreglass filters protect the equipment but not the air.
- How much do HVAC filters cost a year?
- A typical 1-inch MERV 8 pleated filter at about $12 changed every 90 days is about $60 a year. With two pets and allergies it may need changing every five weeks, about 11 filters and $132 a year. A 4-inch MERV 13 media filter at about $45 every 6 months is about $135 a year with much better filtration and less strain on the blower. Multi-packs and subscription services cut the per-filter price by a fifth to a third.
- What happens if I do not change the filter?
- The filter loads with dust, its pressure drop rises, and the blower moves less air. Less air across the cooling coil can make it freeze in summer; less air across the furnace heat exchanger makes it overheat and trip the high-limit switch in winter. Efficiency falls and the house gets less comfortable. A badly clogged filter can also tear and let dust onto the coil, which is expensive to clean.
- Is a thicker filter better?
- Usually. A 4- or 5-inch media filter has five to ten times the pleated area of a 1-inch filter, so at the same MERV it has a much lower pressure drop and lasts several times longer. Fitting one needs a media cabinet at the air handler, which costs $100 to $300 installed and is often included with new equipment. Over a few years it pays for itself in filters and fan energy.
- Do I need more than one filter?
- If the house has filter grilles on several returns, each holds a filter and all of them must be changed; if there is a filter at the air handler too, the grilles do not need filters (two filters in series add pressure drop for little gain). Count every filter slot in the system, and change them on the same schedule.