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SMACNA Duct Gage & Weight Estimator — What it Does and How to Use It
This calculator turns your duct dimensions into a required gage (thickness) using SMACNA’s selection tables and then computes weight and cost. It works for rectangular ducts (unreinforced wall thickness by longest side and pressure class) and round ducts (spiral vs longitudinal seam by diameter). You can create multiple segments, switch SI/Imperial on the fly, add wastage/safety, and export a one-click HTML report.
Heads-up: For return/exhaust (negative pressure), flat-oval, reinforcements, hangers, or joints, refer to the relevant SMACNA tables. This tool focuses on positive pressure thickness picks and weight/cost estimation.
How It Works (Under the Hood)
- Pick thickness from SMACNA:
- Rectangular — Table 2-47/2-47M: find the band where the longest side fits (by pressure class, e.g., 250/500/750 Pa…), read wall thickness (mm).
- Round — Table 3-5/3-5M: use the diameter and seam type (spiral or longitudinal) to pick thickness.
- Compute surface area (sheet area that makes the shell):
Rect:
Area = 2·(W+H)·LRound:Area = π·D·L - Weight = Area × thickness(m) × density × (1 + wastage% + safety%).
- Cost = Weight × Rate/kg × (1 + markup%).
- MSG gage badge is shown by matching the chosen mm thickness to the nearest Manufacturers’ Standard Gage (handy when submittals prefer gage).
Methodology & Formulas (expand)
- Densities: GI 7850 kg/m³; SS304 8000 kg/m³; AL 2700 kg/m³.
- Conversions: 1 in = 25.4 mm; 1 ft = 0.3048 m; 1 in. w.g. = 249.08891 Pa.
- Reverse mode: Given a target weight, the required length is
L = Target / [Perimeter · t · density · (1+wastage+safety)].
“Values derived from SMACNA 2020 tables (user must verify).” When you switch to IS 655 / DW/144, defaults are mapped conservatively for guidance only—check the source standard before issuing.
How to Use the Calculator
- Choose Units (SI or Imperial). The display and inputs switch instantly.
- Select Standard (SMACNA by default). A note shows “Values derived from <Standard> (user must verify).”
- Pick Region & Currency (India/USA/UK) and set your Rate/kg, Wastage, Safety, and Markup.
- Enter Segment:
- Choose Rectangular (W × H) or Round (Diameter + seam).
- Optionally enter Design static to get a suggested pressure class.
- Set Length, Quantity, and Material (GI/SS304/AL).
- Pick Mode: Forward (default) or Reverse (target weight → required length).
- Click Add Segment.
- Repeat for more segments. The KPI cards and bar chart update live. Export a one-click HTML Report when you’re done.
Use the Preset Gallery to prefill common scenarios (supply, kitchen exhaust, cleanroom, etc.). You can add a whole group at once for quick take-offs.
Sample Project — Full Walkthrough
You’re estimating a supply main and a round drop for a commercial floor. Settings: India • INR • Rate 95 ₹/kg • Wastage 5% • Safety 0% • Markup 10% • Material GI (for supply) and GI (for round).
Segment A — Rectangular Supply Main
- Inputs: 900 × 450 mm, Length 18 m, Pressure class 750 Pa, Material GI.
- SMACNA pick (2-47M): Longest side = 900 mm @ 750 Pa → thickness = 1.61 mm.
Show the math
| Perimeter (Rect) | 2·(W+H) = 2·(0.90 + 0.45) = 2.70 m |
|---|---|
| Sheet Area | Perimeter · L = 2.70 · 18 = 48.60 m² |
| Thickness (m) | 1.61 mm = 0.00161 m |
| Volume | Area · t = 48.60 · 0.00161 = 0.078246 m³ |
| Weight (bare) | Volume · 7850 = 0.078246 · 7850 = 614.23 kg |
| Apply Wastage/Safety | 614.23 · (1 + 0.05 + 0) = 644.94 kg |
| Cost | Weight · Rate · (1+markup) = 644.94 · 95 · 1.10 = ₹67,396.51 |
Segment B — Round Drop (Spiral)
- Inputs: Ø600 mm, Length 10 m, Spiral seam, Material GI, Pressure class 500–750 Pa range (thickness governed by diameter).
- SMACNA pick (3-5M Spiral): Ø600 → thickness = 0.55 mm.
Show the math
| Perimeter (Round) | π · D = 3.14159 · 0.60 = 1.88496 m |
|---|---|
| Sheet Area | Perimeter · L = 1.88496 · 10 = 18.84956 m² |
| Thickness (m) | 0.55 mm = 0.00055 m |
| Volume | Area · t = 18.84956 · 0.00055 = 0.010367 m³ |
| Weight (bare) | 0.010367 · 7850 = 81.38 kg |
| Apply Wastage/Safety | 81.38 · (1 + 0.05 + 0) = 85.45 kg |
| Cost | 85.45 · 95 · 1.10 = ₹8,929.75 |
Optional: Reverse Mode Example
Given a target shipment limit of 500 kg for a rectangular 1000 × 500 mm section (GI, wastage 5%, safety 0%, thickness from SMACNA at this size/class say 1.31 mm), what length can we ship?
Show the math
L = Target / [Perimeter · t · density · (1+wastage+safety)]
- Perimeter = 2·(1.00 + 0.50) = 3.00 m
- t = 1.31 mm = 0.00131 m; density = 7850; factor = 1.05
- L = 500 / (3.00 · 0.00131 · 7850 · 1.05) ≈ 15.44 m
Jargon Buster
- Pressure Class: Standardized rating (e.g., 125, 250, 500, 750, 1000, 1500, 2500 Pa) used by SMACNA to select wall thickness.
- MSG Gage: Manufacturers’ Standard Gage; a legacy thickness descriptor (e.g., 24 ga ≈ 0.607 mm). The calculator shows the nearest MSG for the picked mm thickness.
- Spiral vs Longitudinal: Spiral seam round ducts typically allow slightly thinner walls at smaller diameters compared to longitudinal-seam round ducts.
- Wastage / Safety: Extra percentage applied to weight to cover offcuts, handling, or client safety margins.
- Markup: Commercial uplift applied to cost after raw rate/kg.
- IS 655 / DW/144: Indian and UK guidance documents often referenced by local specs. The tool includes dropdowns that auto-populate conservative defaults—verify against the actual clause.
- in. w.g. to Pa: 1 in. w.g. = 249.08891 Pa.
Tips & Good Practice
- Use the Design Static field to auto-suggest pressure class, then verify against your system’s sectioning.
- Switch SI/Imperial before typing sizes to avoid mis-keying conversions.
- For return/exhaust, confirm negative-pressure tables (thickness can differ). Add reinforcements/hangers per SMACNA when required.
- Keep Rate/kg live to market—steel prices move. Region toggles (India/USA/UK) simply prefill currency and a starting rate.
- Export the HTML report to drop directly into emails or submittals; it lists your assumptions, formulas, and a change log.
FAQ
Does this include reinforcements, joints, or hangers?
Not yet. The calculator focuses on unreinforced wall thickness (positive pressure) and weight/cost. For supports and hangers, consult the appropriate SMACNA tables (e.g., minimum hanger sizes and spacing) and add allowances in your BOQ.
Can I use stainless steel or aluminum?
Yes—switch material to SS304 or AL. Densities are built-in; your rate/kg and markup will still control the cost.
What about flat-oval?
Not included; use the standard’s dedicated tables or convert via equivalent perimeter where appropriate, then verify.
