🔬 Face Velocity
📈 Negative Pressure Offset
Design Breakdown
Full Calculation Table
| Parameter | Value | Result |
|---|
How to Use the Laboratory Ventilation Calculator
Enter how many hoods the lab has, the sash width and opening height, and the target face velocity for each hood.
Enter length, width and ceiling height to calculate room volume for the supply air change check.
Choose the minimum air change rate that matches the lab's hazard level and the percentage by which supply sits below exhaust.
Click Calculate Exhaust to see design exhaust, recommended supply, the supply air change rate and the negative pressure offset.
Sizing Laboratory Exhaust and Supply Together
Two separate requirements set a laboratory's airflow. Exhaust has to cover every fume hood at its design face velocity, and supply has to deliver the room's minimum air changes. This calculator sets total exhaust as the larger of the hood total and the minimum-ACH supply divided by one minus the offset fraction, then takes supply as exhaust less the offset that keeps the lab negative. With hoods running, the hoods usually govern and supply clears the air change minimum easily. A lab with few hoods falls back on the air changes, and the extra exhaust comes from general room exhaust. ANSI/ASSP Z9.5-2022 gives typical dilution rates of 4 to 10 air changes per hour depending on exposure risk and says generic assignments are not appropriate, so the presets here are starting points for a risk assessment rather than a rule. Take two hoods 1.2 m wide with a 0.5 m sash opening at 0.5 m/s. Each moves 0.5 × 1.2 × 0.5 = 0.30 m³/s, or 300 L/s, so the hoods need 600 L/s. An 8 × 6 × 3 m room is 144 m³, and 6 ACH needs 240 L/s of supply. Because 240 ÷ 0.9 = 267 L/s is well under 600, the hoods govern: exhaust 600 L/s, supply 540 L/s (13.5 ACH) and an offset of 60 L/s.
Face Velocity Is a Range, Not a Number to Maximize
Z9.5-2022 treats design face velocities of 80 to 100 fpm (0.4 to 0.5 m/s) as suitable for most applications and advises against running below 60 fpm (0.3 m/s). The older 0.4 to 0.6 m/s figure from the NFPA 45 appendix is still widely quoted. Too little velocity lets fumes drift into the room, and too much creates turbulence at the opening that pulls contaminants back out, so raising exhaust is not an automatic cure for a poorly performing hood. Some high-efficiency hoods are designed to contain at 60 fpm, which cuts their exhaust by 40 percent compared with a 100 fpm design. Containment is checked at a working sash opening, commonly 18 in (460 mm), and the air flow measurement calculator helps turn a grid of face velocity readings into an average.
Hood Performance Is More Than Face Velocity
In Canada the governing fume hood standard is CSA Z316.5, which covers design, placement, performance testing, selection, maintenance and commissioning. Containment is judged by tracer gas testing under ASHRAE 110 rather than by face velocity alone, and a hood that passes at the factory can fail in place because of cross-drafts, door swings or a supply diffuser aimed at the sash. Treat the face velocity here as a design basis for airflow and let the as-installed containment test decide whether the hood works.
Multiple Hoods Add Up Fast
Every fume hood exhausts independently, so a lab with several hoods needs total exhaust capacity equal to the sum of every hood at design face velocity with its sash fully open, even if that combination rarely happens in practice. Variable air volume hood controls cut the average airflow by closing the sash opening or throttling the exhaust valve, and the VAV system calculator shows how that flexibility is sized, but the worst-case capacity still has to be there for the moments when it is needed.
Negative Pressure Needs a Flow Offset
NFPA 45 requires laboratory work areas to be negative to corridors and non-laboratory spaces, so air flows into the lab and any fumes that escape stay inside. The offset is the difference between exhaust and supply, and the tool takes it as a percentage of exhaust, 10 percent by default. That is a design choice and not a code figure. The right value depends on door gaps and the pressure difference your safety officer specifies. Check the resulting cascade with the space pressurization calculator, and size the makeup air the building must bring in with the makeup air calculator.
Related Controlled Environments
Lab exhaust design shares some logic with the dilution ventilation approach used for general contaminant control, covered in the dilution ventilation calculator, and with the air-change-driven design used in hospital isolation rooms and clean rooms, covered in the hospital ventilation calculator and clean room calculator.
Frequently Asked Questions
ANSI/ASSP Z9.5-2022 treats design face velocities of 80 to 100 fpm (0.4 to 0.5 m/s) as suitable for most hoods and advises against operating below 60 fpm (0.3 m/s). Older guidance in the NFPA 45 appendix cites 0.4 to 0.6 m/s (80 to 120 fpm) as generally containing. Too little velocity lets fumes escape and too much creates turbulence that pulls contaminants back out, and the real test of a hood is containment measured at the sash opening, not velocity alone.
There is no single number. Z9.5-2022 gives typical dilution rates of 4 to 10 air changes per hour depending on exposure risk, and says generic air change assignments are not appropriate. Many labs start at 6 ACH while occupied, and NFPA 45 has long cited 4 ACH as the minimum when a lab is unoccupied. Higher hazard work moves toward the top of the range, and a risk assessment by your safety officer should set the final value.
Chemical labs are held negative to the corridor and other non-lab areas so that air flows into the lab, as NFPA 45 requires for lab work areas. Supply is set below exhaust by an offset. This calculator uses 10 percent of exhaust by default and lets you change it, because the right figure depends on door gaps and the pressure difference your safety officer specifies. Door openings and sash movement cause brief swings that the standards tolerate.
Each hood exhausts on its own, so the design exhaust is the sum of every hood at its design face velocity with its sash fully open. Two 1.2 m hoods with a 0.5 m opening at 0.5 m/s move 300 L/s each, which is 600 L/s. Supply then has to follow at about 540 L/s with a 10 percent offset, and the system has to deliver both even if every sash is open at once.