Paint Booth CFM Calculator — Spray Booth & Fume Hood Airflow

Pick the booth style, measure the opening the air moves through, and multiply by the design face velocity. A cross-draft booth that is 10 ft wide with a 7 ft opening moves 10 × 7 × 100 = 7,000 CFM. A downdraft booth is sized on its floor footprint instead — 10 × 20 ft at 60 FPM wants 12,000 CFM. A fume hood is sized on the sash opening: a 4 ft sash opened to 18 inches at 90 FPM needs 540 CFM. Same physics in all three: velocity × open area = CFM.

Cross-draft: CFM = booth width × height × 100 FPM — the classic face-velocity rule that keeps overspray and solvent vapor moving away from the operator at a pace that captures but does not turbulently stir the mist. Downdraft: CFM = floor length × width × 50–75 FPM, applied across the pit or filter floor because air travels straight down rather than across. After the raw CFM, add the exhaust-path losses — booth filters (0.3–0.6 in. w.c. clean, up to 1.5 as they load) and duct — then pick the fan for the CFM at that resistance.

Because geometry does part of the work. In cross-draft, 100 FPM is the speed at which the airstream carries overspray past the part without eddying back — slow enough to keep the fog controlled, fast enough to sweep it. Downdraft air only has to fall: gravity assists, so 50–75 FPM achieves the same containment with less energy and less turbulence at the part. Pushing downdraft booths to 100+ FPM wastes fan power and can actually disturb the coating by driving air past the surface too briskly.

CFM = sash opening area × face velocity, with 80–100 FPM the standard containment band for general chemistry hoods. A 4 ft hood with the sash at 12 in. (1 ft) open is 4 sq ft; at 90 FPM that is 360 CFM. The critical detail: the sash area changes every time someone moves it, so constant-volume hoods are sized at full open — a 4 ft sash fully raised to 30 in. needs 900 CFM continuously — while VAV systems modulate exhaust to hold velocity as the sash moves, cutting lab energy use by half or more.

Size the fan for the booth's CFM at its true static pressure, not free air. A cross-draft booth's exhaust path includes intake filters, exhaust pads and duct: budget roughly 1.0–2.0 in. w.c. total. A 7,000 CFM booth at 1.5 in. w.c. needs an air horsepower of 7,000 × 1.5 ÷ 6,356 ≈ 1.65, so a real fan with 60–65% efficiency wants about a 2.5–3 HP motor. Booth fans must also be spark-resistant construction with the motor out of the airstream — a listing requirement, not a preference.

Every CFM exhausted must be replaced, or the booth goes negative and starts starving. An un-maked-up booth depressurizes the building, drags cold winter air through every door and crack, stratifies the booth itself, and can stall the fan's delivered CFM well below rating. The makeup-air unit supplies 90–100% of exhaust CFM, tempered in cold climates — a 7,000 CFM booth in a Michigan winter is also a 7,000 CFM heater problem, which is why makeup-air sizing is part of the booth decision, not an afterthought.

Continuously, in one direction: filters load, resistance rises, delivered CFM falls. Intake pads and exhaust pads start near 0.3–0.6 in. w.c. clean and climb toward 1.0–1.5 loaded. A fan chosen with no margin reads low within weeks; a manometer across the filter bank is the cheap instrument that turns guesswork into a schedule — when the delta doubles, swap the pads. This is the same loading physics a dust collector's baghouse shows, and the same fix: monitor the resistance, not the calendar.

Booth exhaust moves at moderate velocity — 1,500–2,500 FPM is the usual design band, because paint overspray that reaches the duct is either captured at filters or is vapor, and there is no chip-transport duty. A 7,000 CFM booth at 2,000 FPM needs 3.5 sq ft of duct — about a 26-inch round, or a box plenum of equivalent area. Fume hood exhaust is similar at 1,000–2,000 FPM, sized by lab design and corrosion/condensate considerations rather than transport velocity.

The physics overlaps — both move air through a capture opening — but the sizing inputs are entirely different. Kitchen hoods are sized by cooktop linear feet or by kitchen air changes, with grease handling and makeup air as the governing constraints; spray booths and fume hoods are sized by face velocity over an opening. If you landed here while planning a kitchen, the range hood CFM calculator handles that job with the right rules; this page stays with finishing and lab exhaust.

Yes — deliberately. Both devices are 'velocity × open area' machines, and the shared formula is the insight most sizing guides bury: a cross-draft booth at 100 FPM, a downdraft at 50–75, and a hood sash at 80–100 are three settings of one dial. The card you select changes which area you measure and which velocity band applies; everything downstream — CFM, fan, static budget, makeup air — follows identically. One page, two vocabularies, zero repeated math.