Smoke Exhaust Rate Calculator

Inputs

Enter project values, then calculate the required exhaust rate.

Reset

Input units

Imperial inputs are converted internally to metric.

Sizing method

Choose the path that matches your smoke strategy.

Safety factor

Typical range: 1.1 to 1.5.

Length

Room length in meters or feet.

Width

Room width in meters or feet.

Height

Clear height in meters or feet.

Clearing time (minutes)

Base flow uses Volume ÷ time.

System efficiency

Represents losses and real duty point (0.10–1.00).

Density correction factor

Use 1.00 if not applying temperature/density correction.

Make-up air percentage

Typical planning value: 70% to 90% of exhaust.

Fan capacity

Capacity per fan at the intended duty point.

Fan capacity unit

Used only to estimate the number of fans.

Result appears above this form after calculation.

Example data table

Sample scenarios show how method choice affects required exhaust rate.

Scenario	Method	Dimensions	Key input	Exhaust rate	Fans @ 15,000 m³/h
Small plant room	Clearance	10×8×3 m	12 min	~1,200 m³/h	1
Basement car park	ACH	60×35×3 m	8 ACH	~50,400 m³/h	4
Service corridor	ACH	40×3×3 m	10 ACH	~3,600 m³/h	1
Smoke shaft intake	Face velocity	—	2.0 m² @ 3.0 m/s	~21,600 m³/h	2
Atrium exhaust opening	Face velocity	—	3.5 m² @ 4.0 m/s	~50,400 m³/h	4

Formula used

Space volume: V = L × W × H
Time-based clearance: Q₀ = V / t (t in seconds)
Air changes per hour: Q₀ = (ACH × V) / 3600
Opening face-velocity: Q₀ = A × v
Adjusted requirement: Q = (Q₀ × ρf × SF) / η

Where Q is required exhaust flow, η is efficiency, SF is safety factor, and ρf is the density correction factor.

How to use this calculator

Select input units and a sizing method aligned with your smoke strategy.
Enter space dimensions, then provide the method-specific input (time, ACH, or area and velocity).
Set efficiency and safety factor to reflect losses and project uncertainty.
Choose a make-up air percentage to support stable exhaust performance.
Press Submit to view results above the form and download reports.

Smoke control objectives

Smoke exhaust design aims to maintain tenable conditions for evacuation and firefighting. Engineers typically target visibility above 10 m, radiant heat below 2.5 kW/m², and gas temperatures that limit structural and human exposure. A practical approach is controlling the smoke layer interface height so egress routes remain clear for the required safe egress time.

Selecting a design method

This calculator supports three common sizing paths: volume clearance time, air changes per hour, and face-velocity through an exhaust opening or duct. Volume clearance is useful for post-fire purge or small compartments. ACH-based sizing is common for car parks and large open areas where mixing is assumed. Face-velocity sizing is often used at shafts, atria, or dedicated smoke vents.

For performance-based smoke-layer control, link the exhaust rate to the design fire heat release rate and plume entrainment. When detailed modelling is unavailable, conservative flow margins and staged fan operation can reduce over-extraction during early growth to save energy.

Key inputs and typical ranges

Space volume drives the base flow, so verify length, width, and height against drawings. For purge, 10–20 minutes is a frequent clearing window for operational planning. For car parks, 6–10 ACH is common for smoke management concepts, while some projects adopt higher values depending on authority requirements. Face-velocity targets often fall between 2 and 6 m/s at openings, balancing capture against noise and pressure losses.

Interpreting results and fan selection

The computed exhaust rate is adjusted by efficiency and a safety factor. Efficiency accounts for leakage, system losses, and real operating points; values such as 0.6–0.85 are typical. Safety factors of 1.1–1.5 help cover uncertainties in geometry and control strategy. Convert m³/s to m³/h for fan schedules, then divide by a single fan’s rated capacity to estimate quantity, allowing redundancy where required.

Verification and commissioning checkpoints

Check that make-up air is provided to prevent excessive negative pressure, door pulling forces, and reduced exhaust flow. Validate duct velocities, pressure drops, and fan motor power at the selected duty point. During commissioning, measure flow at multiple grilles, confirm damper actuation, and test control sequences under power-loss scenarios. Document results and update the design flow setpoints used for maintenance.

FAQs

1. What is smoke exhaust rate?

It is the airflow removed by an exhaust system to manage smoke during a fire or purge after an event. It is commonly expressed in m³/s, m³/h, or CFM and depends on the selected design method.

2. Which sizing method should I use?

Use volume clearance for post-event purge or small rooms with a target clearing time. Use ACH for large, mixed spaces like car parks. Use face-velocity when sizing a vent, shaft, or opening capture requirement.

3. How do safety factor and efficiency change the result?

The base flow is multiplied by the safety factor to cover uncertainties. It is then divided by efficiency to reflect real system losses and operating points. Lower efficiency or higher safety factors increase the required fan duty.

4. Why is make-up air included?

Exhaust without adequate make-up air can create excessive negative pressure, reduce actual exhaust flow, and increase door opening forces. Providing controlled make-up air improves stability and helps the system reach the calculated duty point.

5. How is the number of fans estimated?

The calculator converts the required exhaust flow to m³/h and divides it by the rated capacity of one fan at its design duty. It rounds up to the next whole fan, so you can add redundancy if needed.

6. Can I rely on this for final compliance?

It is a planning and checking tool, not a substitute for code-required smoke control analysis. Final design should consider pressure losses, temperature effects, control logic, redundancy, and the applicable authority or standard requirements.