Welding Fume Extraction Calculator | CFM, m³/h
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Calculate
Width of the hood opening or capture face in feet
Height of the hood opening or capture face in feet
Target air velocity at the hood face or capture plane in fpm (typical welding: 100–170 fpm)
Expected fraction of fumes captured by the hood (e.g. 80 for 80%)
Overview
The Welding Fume Extraction Calculator estimates the airflow required to capture welding fumes at the source using local exhaust ventilation principles. Instead of using generic room-air-change logic, this calculator uses a fixed source-capture model based on hood opening area, target capture velocity, and effective capture efficiency. OSHA's welding ventilation rule requires local exhaust hoods to be placed as close as practicable and to remove fumes and smoke at the source, and HSE likewise emphasizes source capture as the most effective control approach.
Welding fume control is fundamentally a local exhaust ventilation (LEV) problem, not a comfort ventilation problem. Required extraction flow also depends on how effectively the hood captures fumes at the source, which is strongly influenced by hood type, hood distance, and surrounding air movement. HSE guidance also stresses that LEV must work in practice, not just on paper.
This calculator is a preliminary sizing tool. It helps estimate required extraction flow before final hood, duct, fan, and pressure-loss design.
How to Use This Calculator
Enter hood width and height — or measure the effective hood face area.
Enter capture velocity — target velocity at the hood face or capture plane (fpm or m/s).
Enter capture efficiency — expected capture effectiveness as a percentage (e.g. 80%).
Select Imperial or Metric — CFM for Imperial, m³/h for Metric.
Click "Calculate" — review required extraction rate, category badge, and ventilation guidance.
Use the result as a first-pass extraction airflow target before detailed hood, duct, and fan design.
Inputs & Outputs
Inputs
- •Hood Width (m / ft)
- •Hood Height (m / ft)
- •Capture Velocity (m/s / fpm)
- •Capture Efficiency (%)
Outputs
- •Required Extraction Rate (m³/h / CFM)
Formula
Calculator Formula
Required Extraction Rate = (Hood Width × Hood Height × Capture Velocity × 3600) / (Capture Efficiency / 100)
This formula estimates the airflow required to achieve the target capture velocity across the hood face, adjusted for capture effectiveness.
Step-by-Step Formula Derivation
Step 1 — Hood area:
Area = Width × Height
Imperial: Area in ft² | Metric: Area in m²
Step 2 — Base extraction rate:
Imperial:
Base_CFM = Area_ft² × Capture_Velocity_fpm
Metric:
Base_m³/s = Area_m² × Capture_Velocity_m/s
Base_m³/h = Base_m³/s × 3600
Step 3 — Capture efficiency correction:
Required Extraction = Base Extraction / (Capture Efficiency / 100)
Where Capture Efficiency is entered as a percentage (e.g. 80 for 80%).
Calculator Variables
| Variable | Meaning | Units |
|---|---|---|
| Hood Width | Width of the hood opening | m / ft |
| Hood Height | Height of the hood opening | m / ft |
| Capture Velocity | Target air velocity at the hood face | m/s / fpm |
| Capture Efficiency | Fraction of fumes captured by the hood | % |
| Extraction Rate | Required local exhaust airflow (output) | m³/h / CFM |
Unit Conversions
| Conversion | Factor |
|---|---|
| 1 ft → m | × 0.3048 |
| 1 fpm → m/s | × 0.00508 |
| 1 m³/s → m³/h | × 3600 |
| 1 CFM → m³/h | × 1.699 |
| 1 m³/h → CFM | × 0.5886 |
What is Welding Fume Extraction Rate
Welding fume extraction rate is the airflow required to remove welding fumes from the source before they spread into the worker's breathing zone or the surrounding space. In industrial ventilation terms, it is usually the required local exhaust flow for a hood, extraction arm, capture nozzle, or enclosure.
This is different from general room ventilation. For welding fumes, the preferred engineering control is source capture using local exhaust ventilation positioned as close as practicable to the weld. OSHA requires local exhaust hoods to be placed as close as practicable and to have sufficient capacity to remove fumes at the source, while HSE states that suitable LEV should be used wherever possible to control welding fume exposure.
Key Principles of Welding Fume Control
The following principles form the basis of effective welding fume LEV:
- Source capture first — capturing fumes at the weld point is far more effective than dilution ventilation
- Hood placement — OSHA requires hoods as close as practicable; distance significantly affects capture velocity at the source
- Capture velocity — the air velocity at the point of fume generation must be sufficient to draw fumes into the hood
- Capture efficiency — real-world systems rarely achieve 100% capture; cross-drafts, poor hood geometry, and incorrect placement reduce effectiveness
- System performance — duct losses, fan curve matching, and filter loading all affect whether the designed airflow is actually delivered
Why This Calculator Uses a Source-Capture Model
Many ventilation tools use room air-change rates (ACH) as the primary basis for HVAC sizing. Welding fume control requires a fundamentally different approach. The relevant metric is whether the exhaust system can capture fumes at the weld source — not whether the room air is turned over a certain number of times per hour.
This calculator uses a fixed source-capture model: hood opening area multiplied by target capture velocity, divided by capture efficiency. This directly reflects the industrial ventilation design logic described in OSHA, HSE, and ACGIH guidance for local exhaust systems.
Extraction Rate Categories
The calculator maps the result to a fixed decision model using four categories:
| Category | Imperial (CFM) | Metric (m³/h) |
|---|---|---|
| LOW | < 300 | < 510 |
| MODERATE | 300 – 799 | 510 – 1359 |
| HIGH | 800 – 1499 | 1360 – 2549 |
| VERY HIGH | ≥ 1500 | ≥ 2550 |
These are practical screening thresholds for preliminary sizing only — not regulatory limits or universal welding standards.
Engineering Applications
This calculator can be used across a range of welding fume LEV applications. Welding booth designers use it to estimate the minimum extraction airflow before sizing fans, filters, and ductwork. Maintenance engineers use it to check whether existing extraction arms or snorkel hoods are rated appropriately for the welding task.
Safety professionals use it as a screening tool to identify whether a station's local exhaust system appears adequate for the process intensity and hood configuration. Contractors use it to generate first-pass airflow targets for new installations before engaging a full LEV design review.
The extraction rate from this calculator is a starting point for system selection in all applications. Actual performance still depends on hood distance, duct losses, fan performance, and maintenance condition — factors that require a full LEV engineering review to address properly.
Practical Tips
When using this calculator, always enter the actual hood opening dimensions — not the overall extraction arm or unit dimensions. The capture face is the effective area where fumes are drawn in, and using incorrect dimensions will produce misleading results.
For capture velocity, published LEV guidance for welding fumes typically recommends 100–170 fpm (0.51–0.87 m/s) for exterior hoods, based on ACGIH industrial ventilation guidance. The appropriate value depends on hood type, hood distance from the weld, and whether there are significant cross-drafts in the work area.
Capture efficiency is the most uncertain input. A well-positioned, properly sized hood in a controlled environment might achieve 80–90% capture. A poorly placed hood, or one operating near air-movement from doors or fans, may achieve far less. When in doubt, use a conservative (lower) capture efficiency to ensure the system is not undersized.
Always verify the result against manufacturer airflow specifications for the extraction equipment being used, and check that the fan, filter, and ductwork are all rated to deliver the required extraction rate at the actual system static pressure.
Key Facts
- Welding fume is best controlled by capturing it at source, not by relying on general dilution alone.
- Hood position matters: OSHA requires local exhaust hoods to be placed as close as practicable to the weld.
- Published welding LEV literature cites recommended exterior-hood capture velocities around 100–170 fpm (0.51–0.87 m/s).
- Even good airflow is not enough if hood distance, cross-drafts, or duct losses are poor.
- NIOSH recommends controlling welding fumes to the lowest feasible concentration.
Applications
- Welding booth exhaust sizing.
- Extraction arm airflow estimation.
- Snorkel hood airflow planning.
- Portable fume extractor sizing checks.
- Local exhaust ventilation pre-design.
- Welding station upgrade planning.
- Source-capture feasibility review.
- Duct and fan airflow target estimation.
Example Calculation
Imperial Example
Inputs:
- Hood Width = 2.0 ft
- Hood Height = 1.0 ft
- Capture Velocity = 150 fpm
- Capture Efficiency = 80%
Step 1 — Hood area:
Area = 2.0 × 1.0 = 2.0 ft²
Step 2 — Base extraction rate:
Base_CFM = 2.0 × 150 = 300 CFM
Step 3 — Efficiency correction:
Required_CFM = 300 / 0.80 = 375 CFM
Result: Extraction Rate = 375 CFM → MODERATE
Metric Example
Inputs:
- Hood Width = 0.60 m
- Hood Height = 0.40 m
- Capture Velocity = 0.75 m/s
- Capture Efficiency = 75%
Step 1 — Hood area:
Area = 0.60 × 0.40 = 0.24 m²
Step 2 — Base extraction rate:
Base_m³/s = 0.24 × 0.75 = 0.18 m³/s
Base_m³/h = 0.18 × 3600 = 648 m³/h
Step 3 — Efficiency correction:
Required_m³/h = 648 / 0.75 = 864 m³/h
Result: Extraction Rate = 864 m³/h → MODERATE
Standards & References
- OSHA 29 CFR 1926.353 — Ventilation and protection in welding, cutting, and heating — Local exhaust hoods must be placed as close as practicable and have sufficient capacity to remove fumes at the source.
- OSHA 29 CFR 1926.57 — Ventilation. Requires local exhaust systems to prevent dispersion of harmful fumes through the work area.
- HSE — Controlling the risks from welding — States that the most effective way to reduce welding fume is to capture it at source and use suitable LEV where possible.
- HSE HSG258 — A guide to local exhaust ventilation (LEV) — Core practical reference for LEV design, commissioning, and control of airborne contaminants.
- NIOSH welding fume guidance — Recommends controlling welding fume to the lowest feasible concentration and controlling specific constituents.
- Published welding LEV research (ACGIH-cited) — Reports recommended exterior-hood welding-fume capture velocities around 100–170 fpm (0.51–0.87 m/s).
Limitations
- This calculator is a preliminary airflow sizing tool only.
- It does not model: contaminant-specific exposure limits, toxic constituent composition, full hood-entry loss behavior, detailed duct static pressure loss, or fan curve matching.
- Actual control performance depends on hood location, hood type, duct losses, fan performance, maintenance condition, and process variability.
- OSHA, HSE, and NIOSH all make clear that effective control depends on practical source capture and proper system performance, not airflow alone.
- This calculator does not prove regulatory compliance for any jurisdiction.
Common Mistakes to Avoid
- Treating welding fume control like general room ventilation.
- Placing the hood too far from the weld source.
- Ignoring cross-drafts around the work area.
- Assuming airflow alone guarantees acceptable exposure control.
- Ignoring duct losses and fan capability.
- Using a capture velocity target without considering hood geometry.
- Failing to maintain or test LEV performance in practice.
Frequently Asked Questions
What does this calculator estimate?
Is this the same as room ventilation rate?
Why does hood distance matter so much?
What capture velocity should I use?
Does a higher CFM always mean better control?
Does this calculator prove regulatory compliance?
Can this calculator be used for portable extraction arms?
What happens if the result is zero or negative?
Frequently Used Together
Engineers often use these calculators in combination for complete project workflows:
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Calculate
Width of the hood opening or capture face in feet
Height of the hood opening or capture face in feet
Target air velocity at the hood face or capture plane in fpm (typical welding: 100–170 fpm)
Expected fraction of fumes captured by the hood (e.g. 80 for 80%)