Furnace Filter Size Chart & Calculator
Wondering what size furnace filter your system needs? This furnace filter size chart and calculator estimates your system's airflow from its tonnage, then matches it to the right standard filter size based on a safe face velocity for your filter type — so the blower gets enough air and the filter doesn't clog prematurely.
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How to use this calculator
Enter your system's cooling capacity in tons — check the model number on the outdoor unit's nameplate or the label on your air handler. Most residential systems fall between 1.5 and 5 tons. Then pick your filter type: cheap 1-inch fiberglass disposables, the standard 1-inch pleated filter, or a deep 4-5-inch media filter.
Results update instantly as you change inputs. The headline number is the nominal filter size to shop for — the size printed on the box, like 20×25×1. Below it you'll see the estimated system airflow, the target face velocity for your filter type, and how often that filter should be changed.
Treat the recommendation as a planning estimate, then confirm it fits your actual filter slot or rack — the results table shows nearby standard sizes with their actual face velocities so you can compare tradeoffs if your slot is a different size.
How it works
Step 1 — estimate airflow. Residential HVAC moves about 400 CFM per ton of cooling capacity, so a 3-ton system moves roughly 3 × 400 = 1,200 CFM. This is a standard rule of thumb; actual airflow varies with ductwork, blower speed setting, and how dirty the filter is.
Step 2 — set a safe face velocity. Face velocity is how fast air passes through the filter face, measured in feet per minute (FPM): velocity = CFM ÷ filter face area in sq ft. Denser filters need slower air to keep pressure drop low. This calculator uses about 300 FPM for 1-inch pleated filters, and about 500 FPM for fiberglass disposables and deep 4-5-inch media filters — fiberglass because it barely resists air, and media filters because their deep pleats provide far more actual media surface area.
Step 3 — convert to filter area and match a standard size. Required area in square inches = (CFM ÷ target FPM) × 144. For a 3-ton system with a pleated 1-inch filter: 1,200 ÷ 300 = 4 sq ft, × 144 = 576 sq in. Standard nominal sizes are spaced widely apart, so the calculator picks the smallest standard size covering at least ~85% of the calculated area — here that's 20×25 (500 sq in), the industry-standard recommendation for 3 tons. Its actual face velocity is 1,200 ÷ (500 ÷ 144) ≈ 346 FPM, a bit above the 300 FPM design target but well within common practice.
If no single standard size covers the requirement (typical at 4+ tons with pleated filters), the calculator recommends two filter locations — for example two 16×25×1 filters — which is exactly how larger systems are built in practice.
All airflow and sizing figures here are estimates for planning, not engineering design. Real systems differ in duct static pressure, blower settings, and filter loading. When in doubt, confirm sizing with an HVAC technician and always measure your actual filter slot before buying.
Frequently asked questions
What size furnace filter do I need?
If you already have a filter slot, measure it first. For planning, multiply your system tonnage by 400 CFM per ton, divide by a safe face velocity (about 300 FPM for pleated 1-inch filters, about 500 FPM for fiberglass or 4-5-inch media filters), and match the result to the closest standard size. A 3-ton system with a pleated 1-inch filter, for example, typically uses a 20x25x1 filter.
Does the 400 CFM per ton rule apply to heat pumps?
Yes, as a planning estimate. Most residential heat pumps move about 400 CFM per ton in cooling mode, the same as air conditioners. Some run slightly higher, up to about 450 CFM per ton, for heating performance. If you have a variable-speed system, check the manufacturer's blower table for the exact airflow.
Can I use a smaller furnace filter than the chart recommends?
Not without a tradeoff. A smaller filter forces the same airflow through less area, which raises face velocity and pressure drop. That makes the blower work harder, can starve the system of air, and lets a pleated filter clog faster. Sizing up one standard size is usually safer than sizing down.
How often should I change my furnace filter?
Change a 1-inch fiberglass disposable filter about every 30 days, a 1-inch pleated filter every 60-90 days, and a 4-5-inch media filter every 6-12 months. Homes with pets, smokers, or recent renovation dust should check monthly regardless of filter type, and replace when the filter looks dirty rather than waiting for the calendar.
Are nominal filter sizes the same as actual sizes?
No. The nominal size printed on the filter, for example 20x25x1, is rounded, and the actual filter is usually about 1/4 to 1/2 inch smaller in each dimension so it fits the slot. Always measure your existing slot or filter rack before ordering, and buy by nominal size.