Interactive Calculator · 2026
Vent Pipe Size Calculator
Get a first-cut vent (soil / drain venting) pipe size. Enter the soil stack it serves, the drainage fixture units (DFU) on that stack and the developed vent length — get an indicative recommended vent size in mm to sanity-check against the code table.
Soil stack served → indicative vent size
Your stack & vent run
The stack this vent serves.
Sum of DFU on the stack — higher means more air demand.
Total run of the vent from its connection to the open air — long runs need more open area.
Soil stack served vs recommended vent
A vent is sized relative to the stack it serves — this is a first-cut, not the authoritative table value.
The baseline vent is at least half the stack diameter and never below 50 mm. With 20 DFU over a 15 m run, this tool keeps the vent at the baseline — landing on a 50 mm vent.
This is an indicative first-cut only. The authoritative size comes from the vent-sizing table in NBC 2016 Part 9, which cross-references stack size, DFU and developed length.
How this is calculated
- Baseline = at least half the stack diameter, minimum 50 mm = max(100 ÷ 2, 50) = 50 mm, snapped up to the nearest nominal size = 50 mm (minimum vent size).
- Step-up check — DFU 20 ≤ 32; vent run 15 m ≤ 30 m. Neither is triggered, so the vent stays at the baseline.
- Recommended vent size = 50 mm (nominal sizes considered: 50 / 75 / 100 / 150 mm).
Indicative sizing for concept planning only — this is a first-cut, not a fabricated code table. The authoritative vent size is set by the vent-sizing table in NBC 2016 Part 9. Confirm with a qualified plumbing consultant before procurement.
Frequently asked questions
- How does this vent pipe size calculator work?
- It starts from the stack the vent serves. The baseline vent is at least half the soil or drain stack diameter and never below 50 mm, snapped up to the nearest nominal pipe size of 50, 75, 100 or 150 mm. It then steps the vent up one nominal size when the load is heavy or the run is long, so a busy or distant vent keeps enough open area to stay balanced.
- What inputs do I need to size a vent pipe?
- Three values. First, the soil or drain stack diameter in mm that the vent serves, typically 75, 100 or 150 mm. Second, the total drainage fixture units on that stack, a common default around 20; more fixtures mean more air demand. Third, the developed vent length in metres from its connection up to open air. If unsure, use the shown defaults for a first-cut and refine once your layout is fixed.
- How accurate is this vent size and what should I verify?
- Treat the result as an indicative first-cut for concept planning, not an authoritative figure. It uses a simple half-stack baseline plus a load and length step-up, not the full code table. The governing vent size comes from the vent-sizing table in NBC 2016 Part 9, which cross-references stack size, fixture units and developed length. Confirm the final size with a qualified plumbing consultant before you procure pipe.
Related Guides — Deep-dive reading
Air Admittance Valves (AAV) in India: How They Work, Fitting, Sizing & Code Limits
The one-way mechanical vent that lets air into a drainage system to break siphonage but seals against sewer gas — how an AAV works, where and how to fit it above the trap arm and flood level, how to size it by drainage fixture units, the rule that it can never replace every vent, and where it stands under Indian code.
PlumbingDrainage Systems Guide for Indian Homes: DWV, Traps, Slopes & Stacks
The section pillar for household drainage — the two waste streams, gravity and gradients, traps and the water seal, the soil stack and branch layout, why venting matters, keeping foul and rain separate, and how it all connects to sewer, septic or an STP at the plot boundary.
PlumbingPlumbing Ventilation Guide (India): Vent Systems, Stack, Loop & Wet Vents
A professional, India-first reference to drainage venting — why traps lose their seal, how vents equalise pressure, and how to size and route main, branch, loop and wet vents to code.
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