Ductwork Carbon Calculator

Decarbonising ductwork: why the metal matters

Ductwork is one of the heaviest material packages in a mechanical installation, and almost all of that weight is galvanised steel sheet. As buildings become more energy efficient in use, the carbon locked into materials before a building opens forms a growing share of its whole-life footprint. For HVAC contractors and MEP designers, that makes sheet metal a sensible place to start asking questions.

This calculator gives an early, indicative figure for the rectangular ductwork on your schedule, so you can see where the mass sits and which lines drive the total. It uses the product factors reported for Smart Vent ductwork and your entered dimensions. It does not calculate operational energy, fan power or refrigerant impacts.

Embodied carbon and operational carbon

Operational carbon comes from running the building: the electricity and fuel consumed by fans, heating, cooling and controls. Embodied carbon comes from making, transporting, maintaining and disposing of the physical products. A well-designed duct system influences both. Generous sizing and smooth routes cut fan energy over decades, while lighter, leaner fabrication reduces the steel bought on day one.

The two should be weighed together. Downsizing a duct to save steel can raise velocity, pressure drop and noise, so treat this estimate as one input alongside your airflow design, never as a reason to override it. Our circular to rectangular duct calculator helps check velocity when you change a profile.

What is CIBSE TM65?

CIBSE TM65 sets out a methodology for estimating the embodied carbon of building services equipment where manufacturer-specific data such as an EPD is limited. It is widely referenced by consultants preparing embodied carbon assessments for mechanical, electrical and public health packages. You can read about it on the CIBSE TM65 page.

To be clear: this tool is not a TM65 calculation and is not certified. It applies product per-kg factors to an estimated or entered finished sheet mass. If your project requires a TM65-based assessment, share this schedule with your sustainability consultant as supporting data rather than a substitute.

Scope 1, 2 and 3 explained

People often search for "stage 1, 2 and 3" carbon when they mean Scopes. Scopes are a corporate greenhouse gas accounting framework, not project stages and not lifecycle modules. Under the GHG Protocol, Scope 1 covers direct emissions from sources a company owns or controls, Scope 2 covers indirect emissions from purchased energy, and Scope 3 covers other upstream and downstream emissions in the value chain.

A product footprint is different from a corporate scope inventory. The factors here span raw materials, manufacturing, distribution and end of life, so the full figure should not simply be booked as your procurement Scope 3. Agree with your assessor which parts of the boundary map to which category for your organisation.

Net zero and mechanical procurement

Clients with net zero commitments increasingly ask the mechanical supply chain for product carbon data at tender. For ductwork, that usually means knowing the mass of steel, the source of the factor used and its boundary. A single number without those three facts is hard to use. This tool keeps all three visible and in the PDF report, so the figure can be traced. It is not a net zero certification for any building or project.

For the wider picture of how we approach material choice, see our low carbon ductwork page and the galvanised vs stainless steel ductwork guide.

A practical workflow from design to delivery

During early design, enter indicative sizes and lengths to see which runs carry the most steel. When specifying, fix sheet thickness to the pressure class and size, and list report-covered curved rectangular bends by their finished sheet mass from your takeoff or fabrication schedule. Add reducers, sets and other fittings needing individual assessment as separate other-fitting lines so nothing is silently left out.

At tender, download the PDF and attach it to your enquiry so we can price the same schedule. Before delivery, confirm final quantities; the guide to measuring ductwork and lead times guide may help. You can price parts in the shop or contact our team for a detailed assessment.

Frequently asked questions

Does this calculator show a saving against standard ductwork?

It shows a conditional 47% reduction for valid straight ductwork only against an assumed baseline of 5.0377 kgCO₂e/kg, derived from the supplied calculation 2.67 ÷ (1 − 0.47). This is not a verified saving or an independently established TM65 or standard-ductwork factor. Bends and unassessed items are excluded from the comparison.

Is this a CIBSE TM65 assessment?

No. It applies product per-kg factors to an estimated or entered sheet mass. It is not a full TM65 calculation and is not certified.

Are the results Scope 3 emissions?

Not automatically. The figure is a product footprint covering raw materials, manufacturing, distribution and end of life. Map the relevant parts of that boundary to your corporate inventory with your assessor.

Does the estimate include operational energy?

No. It covers embodied product carbon for the entered ductwork only. Fan energy and other operational emissions are not calculated.

What is not included in the total?

Separate flanges, supports, insulation, controls, site installation and fittings not yet assessed by Smart Vent are excluded. The 3.00 factor applies only to report-covered curved rectangular bends, not reducers or sets; add those as other fittings for assessment. Only 0.8, 1.0 and 1.2 mm sheet thicknesses are covered.

Where do the factors come from?

From the Carbon Trust, Smart Ventilation Systems Ltd Rectangular ducts Product Emissions Report, August 2026, using company data for 1 July 2025 to 30 June 2026.