Free Air Door Tool

Air Curtain Sizing Calculator — Velocity, Airflow & Heater Power by Door Size

An air curtain only works if the jet still has enough velocity when it reaches the floor to push back the wind and the stack effect. This tool turns door height, width, wind exposure and ΔT into the outlet velocity, airflow per metre and total airflow the unit must deliver — plus heater power for heated curtains and the cross-flow fan length that produces the jet.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Air Curtain Requirements

Jet must reach the floor at ≥ 2 m/s (people doors) / ≥ 3 m/s (industrial). Velocity decays ≈ 1/√(height ÷ nozzle width).

Quick preset

Mounting height (floor to outlet)

Door width

Application

Wind exposure

Indoor–outdoor temperature difference (worst case)

Heated curtain Heated (electric or hot-water coil) — size the heater

Door open time

% of hours

For the energy-saving estimate.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How air curtains are sized

The curtain is a plane jet. As it travels down, it entrains room and outdoor air, widens and slows. What matters is the velocity left when it hits the floor: it must exceed the velocity of the wind and of the "stack" flow created by the indoor–outdoor temperature difference, otherwise the jet bends and the door is open in all but name.

Floor velocity target v_floor = 2.0 m/s (comfort) · 3.0 m/s (industrial / cold store) · 2.5 m/s (insect) × wind factor (1.0–1.6) × ΔT factor
Jet decay (plane jet) v(x) ≈ 2.5 × v₀ × √(b ÷ x) // b = nozzle width, x = distance from outlet
Outlet velocity v₀ = v_floor ÷ (2.5 × √(b ÷ H))
Airflow per metre q = v₀ × b × 3600 [m³/h per m]
Jet angle 10–25° toward the outside, more for windy sites
Open-door infiltration Q ≈ 0.2 × A × √(H × ΔT) [m³/s] — a curtain stops 60–80 % of it

Typical air-curtain classes

Class	Max mounting height	Outlet velocity	Airflow per metre	Fan inside
Residential / small shop	2.3 m	6–9 m/s	600–1,000 m³/h·m	Cross-flow 60–80 mm Ø
Commercial	3.0–3.5 m	9–13 m/s	1,000–2,000 m³/h·m	Cross-flow 90–120 mm Ø, or centrifugal
Industrial	4–6 m	13–20 m/s	2,000–4,000 m³/h·m	Centrifugal / EC plug fans
Cold store	3–4 m	8–12 m/s (wide nozzle)	1,500–3,000 m³/h·m	Cross-flow or centrifugal, low-temp motor
Recessed / ceiling cassette	3.0 m	8–11 m/s	900–1,600 m³/h·m	Cross-flow

Installation rules that matter as much as sizing: mount as close to the door head as possible (every 100 mm of gap costs velocity), cover the full width with 100–200 mm overhang, tilt 10–20° outward, and interlock with the door so the fan runs only while the door is open (or at low speed when closed). EC cross-flow fans allow door-switch speed control and cut energy by 40–60 %.

Frequently Asked Questions

What size air curtain do I need for my door?

Choose a unit at least as long as the door width (plus 100–200 mm), and of a class rated for your mounting height: residential to 2.3 m, commercial to 3.5 m, industrial to 6 m. Then check the airflow per metre — roughly 1,000 m³/h per metre for a sheltered 2.2 m shop door, 2,000+ for windy or 3 m+ doors.

What outlet velocity should an air curtain have?

Enough that the jet still moves ≥ 2 m/s (comfort) or ≥ 3 m/s (industrial) at the floor. For a 2.2 m door that means 7–9 m/s at the outlet; for 3 m about 10–13 m/s; for a 4.5 m dock door 15–20 m/s with a wide nozzle.

Do air curtains really save energy?

Yes — field studies show 60–80 % reduction in infiltration through open doors, which in a heated shop with a frequently open door is worth several thousand kWh per winter. The fan itself uses 0.2–1 kW per metre; heated curtains add 3–10 kW per metre but only run while the door is open.

Heated or unheated air curtain?

Unheated curtains block infiltration; heated ones also warm the entrance zone, which customers notice. In cold climates with staff near the door, heated (water or electric) is typical; in mild climates, cold stores, insect barriers and industrial doors, unheated.

Which fan is inside an air curtain?

Most units up to 3.5 m use a cross-flow (tangential) fan — a long drum impeller that delivers a uniform sheet of air from a slim housing. Industrial and high-velocity units use one or more centrifugal blowers. EC versions allow speed control from a door switch or thermostat.

Link to or Cite This Tool

This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:

<a href="https://www.longwellfans.com/resources/air-curtain-sizing-calculator/">Air Curtain Sizing Calculator</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). Air Curtain Sizing Calculator. https://www.longwellfans.com/resources/air-curtain-sizing-calculator/

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