How do you cut G4 filter media from the roll? Measure the inside of the frame, add about 10 mm of overlap on each side, and mark your cuts on the clean (downstream) face. Cut in a single pass with a sharp blade on a clean bench, without compressing the media. Fit each panel with the open, fluffier face towards the incoming air and the denser face towards the fan. A 2,100 mm by 20 m roll of AeroFlow Filters G4 media gives around 152 panels for a 495 mm square frame.
Buying G4 media by the roll is the cheapest way to keep a lot of filter positions stocked. It only stays cheap if the cutting goes well. Most of the waste we hear about comes from three mistakes: panels cut to the exact opening size so they slip, panels fitted back to front, and strips cut in a direction that leaves a long offcut on every pass.
This guide assumes you already know what a G4 filter is and have decided that filter media by the roll suits your site. Here is how to get the install right the first time.
What You’re Working With
AeroFlow Filters G4 filter media comes on a roll 2,100 mm wide and 20 m long, giving 42 square metres of media per roll. It is a 20 mm thick, 300 gsm polyester media rated G4, and it is fully washable. It is used for spray booth pre filter and intake positions and as a coarse first stage in HVAC systems.
Step 1: Measure the Frame, Not the Old Filter
Measure the inside of the frame or retaining grid where the media sits, width and height, in millimetres. Don’t measure the old pad, because it may have shrunk, stretched or been cut wrong in the first place. Measure each position separately, even if they look identical. Older booths and AHUs are often a few millimetres different from one opening to the next. Write each size down and label it by position.
Step 2: Add Overlap Allowance
A panel cut to the exact opening size will slip, bow or leave a gap at the edge once air pushes against it. Add around 10 mm on each side, so 20 mm on the width and 20 mm on the height, which lets the media grip the frame and seal at the edges.
For a 495 mm by 495 mm opening, cut a 515 mm by 515 mm panel. If your frame has a deep retaining lip or a clip system, you may need a little more. If it is a tight drop in the grid, a little less.
Step 3: Find the Airflow Direction
G4 synthetic media is built to load progressively. The open, fluffier face catches larger particles first, and the denser face catches finer particles on the way out. Fitted back to front, the dense face loads up quickly, resistance climbs and the panel needs changing early.
To tell the two faces apart:
- Look at it edge on. The fibres are more open on the air entry side and tighter on the air exit side.
- Run your hand across both faces. The air entry side feels lofty and soft, while the air exit side feels flatter and firmer.
- Check for markings. Some media is coloured or printed on one face to show direction.
The rule to remember is simple. Open face to the dirty air, dense face to the fan.
Step 4: Cut Cleanly
A clean cut protects the edge seal and the media’s structure.
- Roll out the media on a clean, flat bench, dense face up.
- Mark your lines on the dense face with a marker and a long straight edge.
- Cut in a single pass with a sharp blade, changing blades often.
- Don’t press down hard on the media while cutting. Compressing it flattens the fibres and reduces the dust it can hold.
- Cut all panels for one size in a batch, so they are consistent.
Step 5: Fit It So It Doesn’t Bow or Gap
Lay the panel into the frame open face towards the incoming air. Tuck the overlap evenly around all four sides, so the media seals against the frame rather than resting on it. If the frame has a support grid or mesh on the downstream side, make sure the panel sits flat against it.
Then check two things. Run a finger around the edge for any gap, since air takes the easiest path and will go around a filter rather than through it. Then look for bowing once the fan is running. A panel that bows in the middle needs better support or a tighter fit at the edges.
How Many Panels Will a Roll Give You?
Work out how many panels fit across the 2,100 mm width, then how many rows fit along the 20 m length. Multiply the two.
| Opening size | Cut size (with 10 mm overlap) | Across 2,100 mm | Rows in 20 m | Panels per roll | Width offcut per row |
| 495 x 495 mm | 515 x 515 mm | 4 | 38 | 152 | 40 mm |
| 595 x 595 mm | 615 x 615 mm | 3 | 32 | 96 | 255 mm |
Worked example: For a 495 mm square opening, four 515 mm panels fit across the roll with only 40 mm left over. Along the 20 m length you get 38 rows, so 4 x 38 = 152 panels. For a 595 mm opening, only three panels fit across and 255 mm of width is wasted on every row. That is roughly 12 percent of the roll. If you have mixed sizes, plan your cutting so a smaller size uses up that offcut strip.
Cutting Your Own vs Ordering Custom Cut
Cutting your own is the cheapest option on media cost. At current prices a 20 m roll is $296 ex GST, which works out at roughly $7 per square metre. Custom cut G4 is $18.50 per square metre ex GST.
Custom cut makes more sense when:
- You have odd sizes that leave large offcuts
- You need a large volume of panels at once
- Nobody on site has time to cut
- You need every panel identical across multiple positions
Factor in labour. If a staff member spends an afternoon cutting and fitting, the difference narrows quickly. AeroFlow Filters offers G4 filter media custom cut to your length and width.
Order the Right Amount the First Time
Count your positions, note each size and how often you change them, then use the table above to work out panels per roll. If the sums don’t add up neatly, send us your frame sizes and the team will tell you whether a roll or custom cut panels work out better. AeroFlow Filters ships from Australian stock with daily courier services to all states.
Order G4 media by the roll, or send your dimensions for custom cut panels. Call 1800 998 876 or email sales@aeroflowfilters.com.
Frequently Asked Questions
Which side of G4 filter media faces the airflow?
The open, fluffier face faces the incoming dirty air, and the denser face faces the fan or downstream side. This lets the media load progressively through its depth. Fitted back to front, the dense face clogs quickly and the filter needs changing early.
How much overlap should I leave when cutting filter media?
Around 10 mm on each side is a good starting point, so add 20 mm to both the width and height of the opening. This gives the media enough to grip the frame and seal at the edges. Deep frames or clip systems may need a little more.
How many panels can I cut from a 20 m G4 filter media roll?
It depends on panel size and how well it fits across the 2,100 mm width. A 495 mm square opening gives about 152 panels per roll. A 595 mm square opening gives about 96 panels, with more waste across the width.
What is the best tool for cutting filter media?
A sharp utility knife or long bladed cutter with a metal straight edge, used on a clean, flat bench. Cut in a single pass without pressing hard, and change blades often. Blunt blades drag and fray the edge.
Is G4 filter media washable?
AeroFlow Filters G4 media is fully washable. Washing extends its life, but check the media for damage, thinning or loss of loft before refitting it.
Is it cheaper to cut my own filter media or buy a custom cut?
Media cost is lower when you cut your own, at roughly $7 per square metre from a roll compared with $18.50 per square metre custom cut. Once you add labour time and offcut waste, custom cut often wins for odd sizes or large one off orders.
