Welding Fume Extraction Calculator
On this page
Calculate
Width of the effective hood opening in feet
Height of the effective hood opening in feet
Average air velocity through the effective hood opening in fpm. Use a project-, hood-, or manufacturer-specific design value.
Preliminary airflow allowance factor applied to the base hood-face airflow, greater than 0 and no more than 100. For example, 80% increases the base airflow by 25%. This is a design allowance, not a measured welding-fume capture efficiency.
Overview
The Welding Fume Extraction Calculator estimates a preliminary exhaust airflow for a welding hood from the effective hood opening area and an assumed average face velocity, with a design allowance applied on top. It is a first-pass hood airflow sizing model, not a distance-based prediction of capture velocity at the weld. 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 treats source capture as the most effective control approach.
Hood face velocity and capture velocity at the welding source are different quantities. Face velocity is the average air speed through the hood opening itself. Capture velocity is the air speed at the point where the fume is generated, some distance in front of the hood, and OSHA 29 CFR 1910.252(c)(3)(i) sets that source-point value at 100 feet per minute in the zone of welding for the freely movable hood arrangement it describes. Entering a source-point capture velocity into a hood-face calculation mixes the two.
Because the calculation works from the hood face only, it does not include the working distance between the hood and the weld, the hood geometry, flanging, cross-drafts, or the thermal plume. Those factors decide whether the air passing through the hood opening actually captures fume at the arc, and they have to be checked against manufacturer data and applicable LEV design guidance.
This calculator is a preliminary sizing tool. It helps estimate an exhaust airflow target before final hood, duct, fan, and pressure-loss design, and before any assessment of whether an installed system controls exposure.
How to Use This Calculator
Enter hood width and height — the effective dimensions of the hood opening.
Enter hood face velocity — the average design velocity through the hood opening (fpm or m/s).
Enter the design derating factor — a preliminary airflow allowance; 100% means no additional derating.
Select Imperial or Metric — CFM for Imperial, m³/h for Metric.
Click "Calculate" — review the preliminary hood exhaust airflow, screening category, and ventilation guidance.
Use the result as a first-pass hood airflow target before detailed hood, duct, and fan design. It is not a prediction of capture velocity at the weld.
Inputs & Outputs
Inputs
Outputs
Formula
Calculator Formula
The calculation is written separately for each unit system. In Imperial, ft² × fpm already gives ft³/min, so no time conversion is applied. In Metric, m² × m/s gives m³/s, which is then converted to m³/h.
Imperial:
Area_ft² = Hood_Width_ft × Hood_Height_ft
Base_CFM = Area_ft² × Hood_Face_Velocity_fpm
Required_CFM = Base_CFM / (Design_Derating_Factor / 100)
Metric:
Area_m² = Hood_Width_m × Hood_Height_m
Base_m³/s = Area_m² × Hood_Face_Velocity_m/s
Base_m³/h = Base_m³/s × 3600
Required_m³/h = Base_m³/h / (Design_Derating_Factor / 100)
This estimates the airflow passing through the hood opening at the assumed average face velocity, with a preliminary design allowance applied. It does not predict the capture velocity at a weld located away from the hood face.
Step-by-Step Formula Derivation
Step 1 — Hood area:
Area = Hood Width × Hood Height
Imperial: Area in ft² | Metric: Area in m²
Step 2 — Base hood-face airflow:
Imperial:
Base_CFM = Area_ft² × Hood_Face_Velocity_fpm
Metric:
Base_m³/s = Area_m² × Hood_Face_Velocity_m/s
Base_m³/h = Base_m³/s × 3600
Step 3 — Design allowance correction:
Required Extraction = Base Extraction / (Design Derating Factor / 100)
The Design Derating Factor is entered as a percentage greater than 0 and no more than 100 (e.g. 80 for 80%). A factor of 80% gives 1 / 0.80 = 1.25, which is 25% above the base airflow.
This correction is a preliminary sizing allowance only. It does not predict the fraction of welding fume captured by the hood. Actual LEV performance must be verified from hood placement, working distance, airflow measurements, exposure control, and system testing.
Calculator Variables
| Variable | Meaning | Units |
|---|---|---|
| Hood Width | Width of the effective hood opening | m / ft |
| Hood Height | Height of the effective hood opening | m / ft |
| Hood Face Velocity | Average air velocity through the effective hood opening | m/s / fpm |
| Design Derating Factor | User-selected preliminary airflow allowance / derating factor | % |
| Extraction Rate | Preliminary hood 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 the velocity reaching the source
- Capture velocity — the air velocity at the point of fume generation must be sufficient to draw fumes into the hood. This source-point capture velocity is not the same as the Hood Face Velocity input used by this calculator
- Capture efficiency — real-world systems rarely achieve 100% capture; cross-drafts, poor hood geometry, and incorrect placement reduce effectiveness. This is a general LEV principle, not the Design Derating Factor entered here
- System performance — duct losses, fan curve matching, and filter loading all affect whether the designed airflow is actually delivered
Why This Calculator Uses a Hood-Face Airflow Model
Many ventilation tools use room air-change rates (ACH) as the primary basis for HVAC sizing. Welding fume control needs a different approach, because the relevant question is whether the exhaust system removes fume at the weld source rather than whether the room air is turned over a certain number of times per hour.
This calculator estimates preliminary exhaust airflow from the effective hood opening area and an assumed average face velocity, with a design allowance applied. It is a first-pass hood airflow sizing model, not a distance-based prediction of capture velocity at the weld.
Q = A × V gives the volumetric flow through a defined plane whenever V is the average velocity through that plane, and the hood face is exactly such a plane. What the same product does not give is the airflow an exterior hood needs in order to reach a target velocity at a source some distance away. That depends additionally on the source-to-hood distance, hood geometry, flange or enclosure configuration, cross-drafts, the thermal plume of the arc, and the orientation of the workpiece.
Actual source capture must therefore be checked using the hood geometry, working distance, surrounding air movement, manufacturer data, and applicable LEV design guidance. The number this page returns is the airflow through the hood opening at the face velocity you assumed.
Capture Velocity at the Source Is a Different Quantity
Capture velocity at the welding source is not hood face velocity. OSHA 29 CFR 1910.252(c)(3)(i) requires 100 feet per minute in the direction of the hood in the zone of welding for the freely movable hood arrangement it specifies, measured with the hood at its most remote distance from the point of welding. That is a source-point capture velocity and should not be entered automatically as a hood face velocity.
NIOSH Health Hazard Evaluation Report 2019-0215-3371 shows the distinction in the field. Evaluating portable welding fume extractors, it measured face velocity at the inlet hood and capture velocity separately, the latter at the working distance roughly 12 inches in front of the hood, at the point of fume generation, and found that an extractor failed to produce the minimum capture air velocity recommended by ACGIH.
The same OSHA paragraph shows how strongly working distance drives the airflow needed. For a 3-inch wide flanged suction opening maintaining 100 fpm in the welding zone, the tabulated ventilation rate is 150 CFM at 4 to 6 inches from the arc, 275 CFM at 6 to 8 inches, 425 CFM at 8 to 10 inches, and 600 CFM at 10 to 12 inches. The target velocity at the weld is identical in every row. Only the distance changes, and the required airflow quadruples.
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. The badge indicates the scale of the airflow, nothing more. It is not a statement of OSHA compliance, exposure compliance, adequate source capture, or a pass/fail safety determination.
Engineering Applications
This calculator can be used across a range of welding fume LEV applications. Welding booth designers use it to estimate preliminary hood exhaust airflow before sizing fans, filters, and ductwork. Maintenance engineers use it to check whether existing extraction arms or snorkel hoods are rated in the right region for the hood opening and face velocity they are working with.
Safety professionals use it as a screening tool to see whether a station's local exhaust system is in a plausible airflow range for the hood configuration, ahead of the measurements that actually establish control. Contractors use it to generate a first-pass hood airflow target 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. Whether that airflow captures fume at the weld still depends on hood distance, hood geometry, 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 face area is the effective opening the air passes through, and using incorrect dimensions will produce misleading results.
For hood face velocity, take the design value from the extraction equipment manufacturer, from LEV design documentation for the installation, or from a qualified engineering basis for the hood type you are sizing. Published capture-velocity guidance for welding describes the velocity at the point of fume generation, out in front of the hood, and dropping that number into the face-velocity field silently changes what is being calculated.
The design derating factor is the softest input on the page. It is an allowance you choose, not a measurement, and a lower value simply raises the airflow the calculation returns. Using 80% asks for 25% more air than the bare face-area product; that is a sizing margin, and it says nothing about how much fume the hood will actually collect. When in doubt, use a lower factor so the system is not undersized, and treat any statement about capture effectiveness as something to be established by testing.
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 airflow at the actual system static pressure. Then confirm the installation controls exposure by measurement, because airflow through a hood face is not evidence of capture at the arc.
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.
- Hood face velocity and source capture velocity are different quantities: the first is the average speed through the hood opening, the second is the speed at the point where the fume is generated.
- OSHA 29 CFR 1910.252(c)(3)(i) requires airflow sufficient to maintain 100 feet per minute toward the hood in the zone of welding, with the hood at its most remote distance from the point of welding.
- The same OSHA paragraph tabulates how much airflow that takes for a 3-inch flanged suction opening: 150 CFM at 4–6 in from the arc, rising to 600 CFM at 10–12 in.
- 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 hood airflow sizing.
- Extraction arm airflow estimation.
- Snorkel hood airflow planning.
- Portable fume extractor sizing checks.
- Local exhaust ventilation pre-design.
- Welding station upgrade planning.
- First-pass hood airflow screening for LEV projects.
- Duct and fan airflow target estimation.
Example Calculation
Imperial Example
Inputs:
- Hood Width = 2.0 ft
- Hood Height = 1.0 ft
- Hood Face Velocity = 150 fpm
- Design Derating Factor = 80%
Step 1 — Hood area:
Area = 2.0 × 1.0 = 2.0 ft²
Step 2 — Base hood-face airflow:
Base_CFM = 2.0 × 150 = 300 CFM
Step 3 — Design allowance correction:
Required_CFM = 300 / 0.80 = 375 CFM
Result: Preliminary Extraction Rate = 375 CFM → MODERATE
Metric Example
Inputs:
- Hood Width = 0.60 m
- Hood Height = 0.40 m
- Hood Face Velocity = 0.75 m/s
- Design Derating Factor = 75%
Step 1 — Hood area:
Area = 0.60 × 0.40 = 0.24 m²
Step 2 — Base hood-face airflow:
Base_m³/s = 0.24 × 0.75 = 0.18 m³/s
Base_m³/h = 0.18 × 3600 = 648 m³/h
Step 3 — Design allowance correction:
Required_m³/h = 648 / 0.75 = 864 m³/h
Result: Preliminary Extraction Rate = 864 m³/h → MODERATE
Standards & References
- OSHA 29 CFR 1910.252(c) — Welding, cutting, and brazing: ventilation and protection — Primary US General Industry welding ventilation rule. Paragraph (c)(3)(i) requires a freely movable hood placed as near as practicable to the work, with a rate of airflow sufficient to maintain 100 linear feet (30 m) per minute in the direction of the hood in the zone of welding when the hood is at its most remote distance from the point of welding, and tabulates the ventilation rate needed at each working distance for a 3-inch (7.6 cm) wide flanged suction opening.
- 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 — Controlling airborne contaminants at work: a guide to local exhaust ventilation (LEV) — Core practical reference for LEV design, commissioning, thorough examination, and testing.
- NIOSH Health Hazard Evaluation Report 2019-0215-3371 — Methner, 2020. Evaluation of metals exposure in an architectural metal fabrication shop. Measures face velocity at the fume extractor inlet hood separately from capture velocity at the working distance roughly 12 inches in front of the hood, at the point of fume generation, and reports that the extractor failed to produce the minimum capture air velocity recommended by ACGIH. A practical illustration that the two velocities are not interchangeable.
- NIOSH welding fume guidance — Recommends controlling welding fume to the lowest feasible concentration and controlling specific constituents.
- OSHA 29 CFR 1926.353 — Ventilation and protection in welding, cutting, and heating — Construction scope. 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 — Construction scope. Requires local exhaust systems to prevent dispersion of harmful fumes through the work area.
These documents establish that welding fume has to be controlled at source and set out how local exhaust ventilation is designed, commissioned, examined, and tested. None of them prescribes the hood area × face velocity ÷ derating factor arithmetic used on this page. That arithmetic is a preliminary sizing convenience for the airflow through a hood opening, and the OSHA airflow-versus-distance table is the reminder that a target velocity at the weld itself is a separate calculation.
Limitations
- This calculator is a preliminary airflow sizing tool only.
- The calculator does not include the distance between the hood and the welding source and therefore does not calculate source-point capture velocity. The result is a preliminary airflow through the hood opening based on assumed face velocity.
- For an exterior hood located away from the weld, required airflow depends strongly on working distance, hood geometry, flanging, cross-drafts, and the welding plume. OSHA 29 CFR 1910.252(c)(3)(i) shows the scale of the effect for a 3-inch flanged suction opening holding 100 fpm in the welding zone: 150 CFM at 4–6 in from the arc, 275 CFM at 6–8 in, 425 CFM at 8–10 in, and 600 CFM at 10–12 in. The target velocity is identical in every row; only the distance changes.
- The Design Derating Factor is an airflow allowance chosen by the user. It does not predict the fraction of welding fume the hood actually captures.
- 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.
- Entering a welding-source capture velocity as if it were a hood face velocity.
- Reading the design derating factor as a measured welding-fume capture efficiency.
- 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.
- Failing to maintain or test LEV performance in practice.
Frequently Asked Questions
What does this calculator estimate?
What is the difference between hood face velocity and capture velocity?
What hood face velocity should I use?
Is this the same as room ventilation rate?
Why does hood distance matter so much?
What does the design derating factor actually do?
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
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Calculate
Width of the effective hood opening in feet
Height of the effective hood opening in feet
Average air velocity through the effective hood opening in fpm. Use a project-, hood-, or manufacturer-specific design value.
Preliminary airflow allowance factor applied to the base hood-face airflow, greater than 0 and no more than 100. For example, 80% increases the base airflow by 25%. This is a design allowance, not a measured welding-fume capture efficiency.