Updated 26 August 2026 1 soot = 3.175 mm. A soot is one-eighth of an inch, and since 1 inch = 25.4 mm exactly, 1 soot = 25.4 ÷ 8 = 3.175 mm. Convert any soot value to millimetres below — then use the charts that follow, which cover every value tradespeople actually call out, from "dhai soot" (2.5) to 8 soot, with the steel bar, pipe and material size each one maps to. Enter an amount and choose your units. The answer updates instantly. These values stay included in your result when this section is closed. Complete the record details before using an inspection or approval record. 1 Soot = 3.175 mm 1 Soot = 3.175 mm 1 Soot in every unit 4 soot (≈12 mm bar) 8 soot = 1 inch Quick answers — soot to mm Multiply soot by 3.175 for millimetres. The sizes people look up most: The complete soot to mm chart Each soot adds exactly 3.175 mm — the scale is linear, so you can interpolate any in-between value by eye. On site, "4 soot bar" means the 12 mm TMT bar even though 4 soot is exactly 12.7 mm. The soot name is a traditional label carried over from imperial sizes — always order steel in mm. Bars show the true soot value (soot × 3.175 mm). India moved to metric bar sizes but kept the soot names — so the name and the steel never quite agree. Order in mm; the soot word is only for talking. Published 1 soot = 3.275 mm Actually 3.175 mm, exactly A transposed digit that has been copied between conversion pages for years. A soot is one-eighth of an inch and an inch is exactly 25.4 mm, so a soot is exactly 25.4 ÷ 8 = 3.175 mm. There is no rounding involved and no room for a second opinion. The wrong figure is 0.1 mm out per soot — nothing on one bar, 3 mm across a 30-soot dimension. Published 1 soot = 3.275 mm Actually 3.175 mm, exactly A transposed digit that has been copied between conversion pages for years. A soot is one-eighth of an inch and an inch is exactly 25.4 mm, so a soot is exactly 25.4 ÷ 8 = 3.175 mm. There is no rounding involved and no room for a second opinion. The wrong figure is 0.1 mm out per soot — nothing on one bar, 3 mm across a 30-soot dimension. Published 1 foot = 304.32 mm Actually 304.8 mm, exactly This comes from rounding a soot down to 3.17 mm and multiplying by 96. The pages that publish it also publish 1 foot = 96 soot and 1 soot = 3.175 mm — and those two multiply to 304.8, so the figure contradicts the two numbers sitting beside it. Published 1 foot = 304.32 mm Actually 304.8 mm, exactly This comes from rounding a soot down to 3.17 mm and multiplying by 96. The pages that publish it also publish 1 foot = 96 soot and 1 soot = 3.175 mm — and those two multiply to 304.8, so the figure contradicts the two numbers sitting beside it. Every one of these is currently published on pages ranking for these searches. They are listed because a reader who has already seen them needs the correction more than they need another table of correct values. The two conversions worth memorising Soot → mm: multiply by 3.175. For quick mental math multiply by 3 and remember you're about 5.5% low; for anything written down, use the exact factor. mm → soot: divide by 3.175 (or multiply by 0.315). So a 10 mm bar is 3.15 soot — which is why site language rounds it to "3 soot", and why the rounding is harmless in speech but dangerous in a cutting list. Why soot sizes don't exactly match bar sizes Steel bars in India are rolled in metric sizes (6, 8, 10, 12, 16, 20, 25, 32 mm) under IS 1786, the standard for high-strength deformed bars. The pre-metrication market sold imperial sizes in inch fractions, and each fraction was replaced by the nearest metric size when mills converted. The spoken soot names survived that changeover, so today every soot size points at a bar that is close to but not equal to its exact millimetre value: Where the gap bites: steel weight Steel is billed by weight, and bar weight rises with the square of diameter — the D²/162 rule (kg per metre). Compare the "spoken" and "sold" sizes: A true 4-soot bar (12.7 mm) would weigh 12.7²/162 = 0.995 kg/m . The 12 mm bar actually sold weighs 12²/162 = 0.888 kg/m — about 11% lighter. A true 6-soot bar (19.05 mm) would weigh 2.24 kg/m; the 20 mm bar sold weighs 2.47 kg/m — 10% heavier . So a bar schedule that treats the spoken soot name as a literal diameter can materially over- or understate weight, depending on the bar size. The rule: talk in soot if you like, but calculate and order using the metric size on the drawing , with the D²/162 weight chart . Cross-check a soot name against the metric bar Select the nominal diameter written on the structural drawing or bundle tag. The checker shows the spoken site name, its literal arithmetic value and the steel-weight error that appears if the trade name is incorrectly used as a decimal diameter. This is a translation and estimating check—not a way to identify or accept a bar from a rib-to-rib measurement. Soot Name ↔ Metric Bar Cross-check Drawing or bundle-tag size → spoken site name → weight effect of a literal conversion Select the specified nominal diameter—not a rib-to-rib caliper reading. These values stay included in your result when this section is closed. Complete the record details before using an inspection or approval record. Use a BBS total or 12 m for one full-length bar. Site trade name 4 soot = 12 mm bar The literal calculation is 4 × 3.175 = 12.7000 mm—not the product diameter. Metric bar in soot arithmetic value actual bar minus literal name Actual nominal weight 0.8889 kg/m × 100.0 m Literal-conversion error Use soot only as a spoken trade name. Put the millimetre diameter from the structural drawing on the PO, BBS and delivery record. This comparison explains a mismatch; it does not identify or accept reinforcement by measurement. Soot sizes beyond steel Pipes. Plumbing sizes remain genuinely imperial (BSP threads), so soot maps cleanly: 4 soot = ½" pipe (the standard house supply line), 6 soot = ¾" (riser/main), 8 soot = 1" (pump line). Note that a pipe's inch size is nominal bore , not outside diameter — a "½ inch" CPVC pipe is not 12.7 mm across outside. Use soot only to name the pipe, never to measure it. (More on materials: types of pipes .) Glass. Toughened glass for partitions and railings: "4 soot" = nominal 12 mm, "3 soot" = 10 mm, "2 soot" = 6 mm float glass for windows. Boards and stone. "6 soot ply" = the 18–19 mm board class ( plywood price list ); granite slabs commonly "6 soot" ≈ 19–20 mm; wall tiles ~8–10 mm are "2.5–3 soot" in trade talk. Tile joints and gaps. "Ek soot ka gap" — a 3 mm spacer joint — is probably the most spoken soot phrase on Indian sites after bar sizes. A soot is a length, not a steel term — the same eighth-inch step names plywood, pipe bore, tile spacer and glass. Each bar below is drawn at true relative thickness, so 'char soot ply' and a 'char soot bar' really are the same dimension in two trades. Worked conversions (the ones people actually search) 2 soot in mm = 6.35 mm → 6 mm bar/wire, 6 mm glass. 2.5 soot in mm = 7.94 mm → the 8 mm bar; "dhai soot" in Hindi. 3 soot in mm = 9.53 mm → the 10 mm bar; also common for pipe clamps. 4 soot in mm = 12.7 mm → 12 mm bar; ½" pipe; 12 mm glass. 5 soot in mm = 15.88 mm → 16 mm bar (main column steel in homes). 6 soot in mm = 19.05 mm → 20 mm bar; ¾" pipe; 19 mm ply/granite.

8 soot in mm = 25.4 mm → 25 mm bar; 1" pipe; exactly one inch. Fractional callouts follow the same math: "साढ़े तीन सूत" (3.5 soot) = 11.11 mm, which has no standard bar — the dealer will supply 10 or 12 mm, so ask which the design intends. Converting a whole bar schedule from soot (worked example) A contractor's slip for a small slab reads: "3 soot – 220 m, 4 soot – 160 m" . Map sizes: 3 soot → 10 mm; 4 soot → 12 mm. Weights (D²/162): 10 mm = 0.617 kg/m; 12 mm = 0.888 kg/m. Tonnage: 220 × 0.617 = 135.7 kg; 160 × 0.888 = 142.1 kg → order ≈ 278 kg plus 3–5% cutting wastage → 290 kg. Cross-check the bill: at ₹57/kg the steel line should read ≈ ₹16,500 — a slip priced off "12.7 mm weight" would read ~₹1,800 higher for the same bars. That five-minute conversion is the difference between paying for steel and paying for arithmetic. A short history of why India talks in soot but sells in mm Until the early 1960s, Indian steel, pipe and hardware followed British imperial sizing — bars in inch fractions, pipes in BSP inches, sheets in gauges. The Standards of Weights and Measures Act (1956) began official metrication, and Indian Standards progressively re-issued product specifications in metric: reinforcement moved through IS 432 (mild steel bars) to today's IS 1786 (high-strength deformed bars), whose size series — 6, 8, 10, 12, 16, 20, 25, 28, 32, 36, 40 mm — is what every mill now rolls. But language metricates slower than mills. The generation of mistris trained on ¼" and ½" bars renamed the new sizes with the old words, apprentices learned the renamed versions, and plumbing — whose BSP threads never metricated at all — kept re-teaching inch vocabulary to every new tradesperson. The result is today's stable bilingual system: spoken sizes in soot, written commerce in millimetres. Neither is going away; the skill is translating correctly at the boundary, which is exactly what this page's charts are for. Don't confuse soot with gauge or sutar Three thin-size vocabularies circulate in Indian hardware markets: Soot — a length of 1/8 inch = 3.175 mm. Bigger number = bigger size. Gauge (SWG) — a wire/sheet series where bigger number = thinner material : 8 SWG ≈ 4.06 mm but 20 SWG ≈ 0.91 mm. Binding wire ("18–20 gauge"), GI sheets and wire mesh are gauge items. Ordering "3 soot binding wire" or "20 soot sheet" marks you as convertible — and may get you the wrong material. Sutar — regional usage varies: in much of Maharashtra/Gujarat it means exactly what soot means (1/8"), elsewhere it loosely means "fine size". If a quotation uses sutar, confirm the millimetre figure once; it costs one sentence. Talking to the steel dealer — a two-minute protocol Steel purchases go wrong at the counter, not in the warehouse. A sequence that prevents it: Name sizes in mm, quantities in kg. "12 mm, 500 kg" is unambiguous; "4 soot, 45 lengths" invites both a size assumption and a length assumption (bars come in 12 m standard lengths, but shorter stock exists). Ask the per-kg rate for each diameter separately. Thin bars (8 mm) price ₹1–2/kg above thick ones; a single blended rate usually rounds against you ( current rates ). Record the commercial weight basis. A quote may use nominal schedule weight, a supplier scale or a weighbridge record. Keep the relevant slip and reconcile it with diameter-wise rod counts and actual lengths before approving the bill. The steel delivery checker makes that comparison without treating a difference as automatic acceptance or rejection. Check the manufacturer, grade and size records on bundle tags and the mill test certificate against the engineer's specification. The soot/mm question is only one way a steel order can silently change. For permissible deviation from nominal mass, the official IS 1786 product manual distinguishes batch-average limits of ±7% for bars up to 10 mm, ±5% above 10 mm through 16 mm and ±3% above 16 mm from the separate individual-sample limits of ±8%, ±6% and ±4%. Those limits are part of a defined conformity process—not a shortcut for deciding from one delivery total. See the BIS IS 1786 product manual . Printable pocket chart For the site notebook — the six values that settle almost every soot conversation: Bar weights use D²/162 with the metric diameter — the full logic is on the unit weight of steel bars page. One more worked example — interior work A modular-kitchen fabricator quotes: carcass in "6 soot BWP ply", shutters "4 soot with 1 soot laminate both sides", back panel "2 soot". Decode before signing: Carcass: 6 soot = 19.05 mm → the 18–19 mm BWP board class (fine). Shutters: 4 soot = 12.7 mm → a 12 mm core. For 600 mm-wide shutters this is on the thin side; most quality kitchens use 16–18 mm shutters. Worth questioning. "1 soot laminate" = 3.175 mm?? No laminate is 3 mm — here the fabricator means 1 mm laminate and is using "soot" loosely for "one unit". This is exactly the ambiguity that makes written mm specifications worth insisting on. Back panel: 2 soot = 6.35 mm → 6 mm ply (standard). The lesson repeats across trades: soot is precise when it maps to inherited imperial sizes (bars, pipes, glass) and sloppy when borrowed for products that were never imperial (laminates, edge bands). When the material is modern, get the millimetres in writing. Ordering across the soot–mm divide The practical problem is not conversion, it is that two people in the same conversation are using different systems and both think they are being precise. When a contractor says "char soot ka saria" he means the bar that everyone calls 4 soot — which is 12 mm, even though 4 soot is arithmetically 12.7 mm. He is naming a product, not stating a dimension. The mill rolled 12 mm; the market kept the old word. Read the demand as a name and you will be right every time; read it as a measurement and you will be 0.7 mm out. Where this genuinely bites is bar bending schedules and lap lengths , because those are calculated from diameter. A lap of 50d is 600 mm for a 12 mm bar and 635 mm if someone "converts" 4 soot to 12.7 mm first. The error is small per bar and systematic across a building — and it is entirely avoidable by working in mm from the drawing onward. Three rules that prevent the whole class of error: Order in mm, always. Put 12 mm on the purchase order even if you said "4 soot" on the phone. The invoice, the test certificate and the bar schedule then agree. Never convert a soot name into a decimal. The name is not a measurement. Look the bar up in the mm column instead. Check traceable identification. Match the manufacturer's rolled identification, grade marking, bundle tag and mill test certificate to the ordered millimetre size. Do not assume the diameter can be read from the rolled brand mark alone. The soot word is not going away — it is faster to say and everyone in the supply chain understands it. It just should not survive contact with a calculator. A soot is simply one-eighth of an inch, so a tape reading in eighths is already reading in soot — the eighth mark is 1 soot, the quarter is 2, the half is 4. The millimetre value under each is exact. Rounding every soot to 3 mm makes a full inch 1.4 mm short, so use 3.175 mm when the dimension must remain exact. Frequently asked questions How many mm is 1 soot? 1 soot equals exactly 3.175 mm (one-eighth of 25.4 mm). What is 4 soot in mm? 4 soot is 12.7 mm. On site it refers to the 12 mm TMT bar. What is 5 soot in mm? 5 soot is 15.875 mm, which corresponds to the 16 mm TMT bar. What is 2.5 soot in mm? 2.5 soot ("dhai soot") is 7.9375 mm — the size that maps to the 8 mm bar, the workhorse of slab distribution steel. How many soot make 1 inch? 8 soot make 1 inch, because a soot is defined as 1/8 inch. How do I convert mm to soot? Divide the millimetre value by 3.175: a 16 mm bar is 5.04 soot, which site language rounds to "5 soot". Is a 12 mm bar really 4 soot? Not exactly — 4 soot is 12.7 mm. The 12 mm bar inherited the name from the old ½" bar it replaced. Weight calculations must use 12 mm. Why does my steel bill differ from my soot-based estimate? Because weight grows with diameter squared, the ~0.5–1 mm gaps between soot sizes and metric bars shift weight by up to ±10% per size. Estimate with metric diameters and the D²/162 rule and the gap disappears. What is "dhai soot" and "dedh soot"? Hindi fraction names: dedh = 1.5 soot (4.76 mm) and dhai = 2.5 soot (7.94 mm — the 8 mm bar). You'll also hear saade teen (3.5 soot = 11.11 mm) and paune chaar (3.75 soot = 11.91 mm) in carpentry. Which mm sizes have no soot name? The metric-native sizes that had no imperial ancestor: 28 mm and 32 mm bars are simply called by their millimetre names even by mistris — evidence that the soot system is inherited vocabulary, not an active preference. Is soot used outside India? The 1/8-inch fraction is universal in imperial countries (US carpentry talks in eighths too), but the word soot and its use as a standalone unit are specific to South Asian construction trades, including Pakistan and Bangladesh where the same imperial history applies. Related converters Soot to Inch Conversion Millimeter to Inches Unit Weight of Steel Bar 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.