Updated 28 July 2026 A ½ inch pipe is a "4 soot" pipe (15 mm), ¾ inch is "6 soot" (20 mm) and 1 inch is "8 soot" (25 mm) — because 1 inch = 8 soot, so the soot number is simply the inch fraction × 8. On every Indian site and at every hardware counter, plumbers call out water pipe by its soot size, while the box is labelled in inches or millimetres. This page is the chart that ties the three together — for GI, CPVC, UPVC and PVC pipe — plus the one caveat that trips up almost everyone who tries to convert soot to mm the obvious way. 1 inch = 8 soot ½" = 4 soot = 15 mm The commonest pipe ¾" = 6 soot = 20 mm Riser / branch size 1" = 8 soot = 25 mm Pipe size chart — soot, inch and mm This is the table people mean when they search "pipe size in soot". The mm column is the nominal bore (NB) — the trade size the pipe is sold and specified by, which is what plumbers, drawings and hardware shops all use. These are nominal-bore trade sizes, not the literal soot × 3.175 mm value — see the caveat below. The soot number tracks the inch size exactly; the mm number is a rounded designation. The one caveat: pipe soot is nominal, not literal Here is the trap. On a steel bar , 4 soot really is close to 12.7 mm, because 1 soot = 3.175 mm and you can just multiply. On a pipe , that does not hold. A ½ inch pipe is called "4 soot" because ½ inch = 4 soot — the soot label describes the inch fraction, not a measured diameter. But its nominal bore is 15 mm , not 4 × 3.175 = 12.7 mm, and its actual inside and outside diameters are different again, because a pipe has a wall thickness and the "size" is only a designation. Nominal bore is a name for a size class, agreed by standard — it is not something you can measure across the pipe with a scale and expect to match. So two different rules are in play, and mixing them is the single commonest mistake: Steel bar: soot × 3.175 = mm (a real conversion). Use the soot to inch chart. Pipe: soot ÷ 8 = inch, and the mm is the standard nominal bore from the table above (a trade designation). • Quote the soot size and confirm the inch: "4 soot, half inch" — the two always agree for pipe • Match the fitting system to the pipe: ½" CPVC elbow for ½" CPVC pipe • For drainage, switch to the mm/OD system — 110 mm soil pipe, not "4 soot" • Check the pressure class (SDR / schedule), not just the bore, for pumped lines • Converting pipe soot to mm as soot × 3.175 — that is the bar rule, not the pipe rule • Assuming the nominal bore is the measured inside diameter — it is a size class, not a measurement • Calling a 110 mm soil pipe a "4 inch pipe" and ordering a 100 mm fitting • Mixing CPVC and UPVC fittings because both say "¾ inch" Water pipe by material — GI, CPVC and UPVC The soot ↔ inch designation is the same across water-supply materials; what changes is how each one is actually marked and sold. For the material differences themselves — where CPVC, UPVC, PVC, PPR and GI each belong — see types of pipes in plumbing . Drainage & soil pipe is a different system Water pipe uses nominal bore. Drainage, waste and soil pipe (PVC) uses outside diameter in millimetres , and people rarely call it in soot at all — they call it by mm, or loosely by inch. Note the mismatch: a 110 mm soil pipe is universally called a "4 inch pipe" even though 4 inch nominal is 100 mm — 110 mm is its outside diameter. Order fittings by the mm size , not the inch nickname, or the coupling will not fit.

How to read any soot pipe size The rule is one line, and it runs both ways: 1 Soot to inch Divide the soot by 8. A 6 soot pipe is 6 ÷ 8 = ¾ inch. Divide the soot by 8. A 6 soot pipe is 6 ÷ 8 = ¾ inch. 2 Inch to soot Multiply the inch by 8. A 1½ inch pipe is 1.5 × 8 = 12 soot. Multiply the inch by 8. A 1½ inch pipe is 1.5 × 8 = 12 soot. 3 To millimetres Read the nominal bore from the chart — ¾ inch = 20 mm. Do not multiply soot by 3.175 for pipe. Read the nominal bore from the chart — ¾ inch = 20 mm. Do not multiply soot by 3.175 for pipe. The soot and inch always agree because 1 inch = 8 soot by definition. The mm is a standard trade size, read from the table, not calculated. Worked example. A plumber asks for "12 soot pipe for the pump line." 12 ÷ 8 = 1½ inch, which is 40 mm nominal bore. If the same plumber needs a soil pipe "4 inch," that is the 110 mm PVC size, not 100 mm — a different system entirely. Which size goes where A quick sizing guide for an ordinary Indian house, so the chart connects to real decisions: Undersizing the supply line is a false economy: a ½ inch main starves two bathrooms running at once, and it cannot be fixed without re-chasing the wall. The riser and the main are the two places to size up, not down. Frequently asked questions How many soot is a half-inch pipe? A half-inch pipe is 4 soot , because 1 inch = 8 soot, so ½ inch = 4 soot. Its nominal bore is 15 mm. What is a 6 soot pipe in mm? A 6 soot pipe is ¾ inch , which is 20 mm nominal bore. Do not compute it as 6 × 3.175 mm — that formula is for steel bar diameter, not pipe. Why is a ½ inch pipe called 15 mm and not 12.7 mm? Because pipe is specified by nominal bore , a standard trade size, not by a literal measurement. ½ inch is 12.7 mm as a length, but the pipe size class for it is designated 15 mm. Nominal bore is a name for a size, agreed by standard, not the diameter you would measure across the pipe. Is pipe soot the same as steel bar soot? The unit is the same — 1 inch = 8 soot — but the conversion to mm is not. For a bar , multiply soot by 3.175 to get the real diameter. For a pipe , read the nominal bore from the chart. Same soot number, different millimetre value. What is a 4 inch pipe in mm? For water/GI pipe, 4 inch nominal bore is 100 mm. But the PVC soil pipe everyone calls a "4 inch pipe" is actually 110 mm outside diameter — order the fitting by 110 mm, not 100 mm. What size pipe for a house main line? A 1 inch (8 soot, 25 mm) supply line is standard for the main into an ordinary Indian house, with ¾ inch (6 soot) risers and ½ inch (4 soot) final connections to each fixture. How do I convert soot to inch for pipe? Divide the soot by 8. So 4 soot = ½ inch, 6 soot = ¾ inch, 8 soot = 1 inch, 12 soot = 1½ inch. To go the other way, multiply the inch by 8. Soot to Inch Converter Soot to MM Converter Types of Pipes in Plumbing Plumbing Work Rate Standard Height of Plumbing Fixtures All converters → Found this useful? Send it to someone building right now The arrow button opens your phone's own share sheet — use it for Instagram, Messages or any other app you have installed. CivilSite Editorial Team ✓ Engineer reviewed Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.