Spray Booth Airflow and Pressure Guide
Two numbers tell you most of what you need to know about a spray booth: how much air it moves, and what pressure it holds while it moves it. Get the airflow right and overspray leaves the booth. Get the pressure right and dirt stays outside.
- Airflow calculator, CFM and m³/h
- Australian face velocities
- Negative vs positive pressure
- Reading your manometer
Minimum air movement a booth should be capable of, by booth type.
Equals one CFM. Same airflow, different units, and the calculator gives both.
Set on the gauge at commissioning. Change filters when the reading reaches it.
Weekly manometer readings show your filter interval inside two or three cycles.
Work out the airflow your booth needs
Booth airflow is a volume. Multiply the cross section the air passes through by how fast you want it moving, and you have the extraction volume the fan has to deliver.
Measure the cross section. Width times height of the booth opening or working section.
Pick a face velocity. 0.3, 0.4 or 0.5 m/s depending on booth type, or your commissioning figure.
Multiply. Square feet times feet per minute gives CFM. Square metres times m/s times 3,600 gives m³/h.
Optional: add filter face area to get the velocity through the filter media, which governs how quickly filters load up.
A 4 m by 2.7 m working section is 10.8 m². At 0.5 m/s that is 19,440 m³/h, or about 11,440 CFM.
Spray booth airflow calculator
Enter your booth opening or working section to get the extraction volume it needs.
Figures from AS/NZS 4114.1 as summarised in Australian work health and safety guidance. Confirm the target for your own booth with your compliance test report.
Leave blank if you only need the extraction volume.
Extraction volume
Filter face velocity
General guidance only, not a standard.
What face velocity should a spray booth run at
Australian spray booths are designed, built and installed to AS/NZS 4114.1 and AS/NZS 4114.2. The SafeWork NSW Code of Practice for spray painting and powder coating sets the minimum air movement a booth ventilation system should be capable of producing.
These are minimum capability figures, not targets to aim just above and forget. A booth that measured 0.52 m/s at commissioning and reads lower now has lost airflow somewhere, usually in the filters.
| Booth type | Minimum | Imperial |
|---|---|---|
| Full down draught booth | 0.3 m/s | about 59 ft/min |
| Electrostatic spraying | 0.4 m/s | about 79 ft/min |
| Any other booth | 0.5 m/s | about 98 ft/min |
Booths should be inspected at regular intervals and maintained to the manufacturer’s specifications, which is where a periodic airflow test earns its keep.
Negative, positive and balanced booth pressure
Booth pressure is the relationship between how much air the supply fan pushes in and how much the exhaust fan pulls out. These three states are standard booth engineering practice, not a figure written into a standard. Confirm the target for your booth against its commissioning report.
Negative
Exhaust moves more air than supplyThe booth sits below workshop pressure, so air leaks inward through door seals and gaps.
- Upside
- Fumes and overspray stay contained.
- Cost
- Inward leakage brings workshop dust, and dust ends up in the clear coat.
Balanced or slightly positive
How most refinish booths are set upClean inside without pushing much air back into the shop. Whatever the booth was commissioned at, it should still be there.
- Upside
- Clean finish, little air pushed out.
- Cost
- Drifts as filters load, so watch the gauge.
Positive
Supply moves more air than exhaustAir leaks outward through the same gaps, so unfiltered workshop air cannot get in.
- Upside
- Finish quality.
- Cost
- Push it too far and solvent laden air vents into the workshop through the door seals.
Exhaust filters loading up cuts extraction and pushes the booth positive.
Inlet filters loading up cuts supply and pulls the booth negative.
What the manometer is actually reading
A booth manometer does not read airflow. It reads a pressure difference, usually across the exhaust filters: the higher pressure in front of them against the lower pressure behind. The scale is normally inches of water column, and the installer marks two points on most gauges: the clean filter reading and the changeout point.
Gauge unreadable, cracked, or the fluid gone? A booth without a working gauge is changing filters on guesswork.
Why the reading climbs
- Step 1Filter media catches overspray.
- Step 2As it fills, the air has less open area to pass through and resistance goes up.
- Step 3The fan spins at the same speed, so the pressure drop across the filter rises.
- Step 4The volume of air moving through the booth falls. A rising reading is falling airflow in disguise.
The same physics runs on the intake side. If the reading has moved and the exhaust filters look clean, check the roof inlet filters before you suspect the fan.
Using the reading to time filter changes
The gauge does two jobs. It tells you when a change is due, and it tells you how long the last set lasted, which is the only honest way to compare one filter against another.
Use the manufacturer’s mark
The number to change at is the one your booth manufacturer set, marked on the gauge at commissioning.
Mark lost? About half an inch
Common industry practice is to change when the reading has risen by around half an inch of water column above the clean baseline. Some jurisdictions that regulate booth emissions set tighter thresholds.
What Australian codes require
Filter medium cleaned or replaced often enough to stop deposits blocking the air flow, and never spraying without filter medium in place.
Floor exhaust filters are where most of that spend sits. The Paint Pocket vs EU3 comparison covers how the two media types differ on holding capacity and change intervals. Log the reading weekly and the pattern shows up inside a couple of cycles.
When the airflow problem is not the fan
Each of these cuts measured velocity while the fan sounds perfectly healthy. Rule out these cheaper causes before anyone quotes you a new motor.
- Loaded exhaust or inlet filters
- Damper settings nudged and never put back
- Blocked or partially collapsed ductwork
- A slipping or worn drive belt
- Doors or seals that no longer close properly
A booth that has slowly got slower over two years is almost never a fan problem. If the numbers still do not add up, an airflow test tells you what you are actually pulling rather than what the nameplate says.
Frequently asked questions
How do I calculate the CFM for a paint booth?
Multiply the booth cross section by the face velocity you want through it. In imperial that is square feet multiplied by feet per minute, which gives CFM directly. In metric it is square metres multiplied by metres per second multiplied by 3,600, which gives cubic metres per hour. One CFM equals 1.699 cubic metres per hour. The calculator on this page does both at once.
What air flow does a spray booth need in Australia?
The SafeWork NSW Code of Practice for spray painting and powder coating states that a booth ventilation system should be capable of a minimum air movement of 0.3 m/s for a full down draught booth, 0.4 m/s for electrostatic spraying, and 0.5 m/s for any other booth. Booths themselves are designed and installed to AS/NZS 4114.1 and AS/NZS 4114.2. Your own booth’s commissioning report is the figure to measure against.
Should a paint booth be positive or negative pressure?
Negative pressure keeps fumes and overspray contained but draws workshop dust in through every gap. Positive pressure keeps dust out but pushes booth air into the workshop. Most refinish booths are commissioned to run balanced or slightly positive while spraying. The setting that matters is the one your booth was commissioned at, and holding it there as filters load.
What does a rising manometer reading mean?
It means the exhaust filters are loading up. The manometer reads the pressure difference across the filters, so as overspray fills the media the resistance climbs and the reading rises with it. Airflow through the booth is dropping at the same time. When the reading reaches the changeout mark set at commissioning, the filters are done.
What units does a spray booth manometer use?
Most booth manometers are scaled in inches of water column. Some gauges are marked in pascals instead. Either way the number on its own means nothing without the clean baseline reading and the changeout mark for your booth, which is why both are normally marked on the gauge face.
How often should spray booth filters be changed?
By the gauge, not the calendar. Australian guidance requires the filter medium to be cleaned or replaced often enough to stop deposits blocking the air flow, and there is no single interval that suits every booth because throughput, paint type and booth design all change how fast media loads. Log the manometer reading weekly and the interval for your booth becomes obvious after two or three cycles.
Have your booth’s airflow measured
Total Spraybooth Care has serviced spray booths across Australia for more than 25 years, from 511 Hammond Road, Dandenong. Airflow measured, gauge set up properly, or a straight answer on which filters suit your booth: call 03 9797 5555 and talk to someone who works on these every week.
The full filter range is in the spray booth filters and parts shop.