A schedule number is not a thickness. It is a pressure-to-stress ratio that gets converted to a thickness, which is why Schedule 40 is 3.68 mm at DN50 and 9.53 mm at DN250. Stainless S schedules are a separate series and are not interchangeable with carbon steel schedules of the same number. Schedule is not the same as weight, but they do align on some diameters. Inside diameter is always calculated from OD minus twice the wall, never read off a chart as a primary figure. Do NOT get caught out thinking Schedule and Weight always align.
Technical Resource
Pipe Fitter’s Mate
Pick the application, then select size, get the dimension, fittings included.
Note: always double check current batch of fittings for mill tolerance variation.
Carbon steel wall thickness chart
ASME B36.10, all values in mm. Selection highlighted. Scroll sideways for the heavier schedules.
Stainless wall thickness chart
ASME B36.19, all values in mm. Wall thickness with inside diameter underneath in lighter type.
Elbows, return bends and caps
ASME B16.9 long radius and caps, ASME B16.28 short radius. All values in mm. Click a row to load it above.
Tees and reducers
ASME B16.9. Equal ends are the straight tee. All values in mm. Click a row to load it above.
Lap joint stub ends
ASME B16.9, long and short pattern. All values in mm. Click a row to load it above.
What the dimensions mean
Centre lines dashed, dimensions in copper. Faces are the weld prep ends, so every dimension runs to the end of the fitting. On a return bend, O is the centre to centre spacing of the two legs.
Elbows, return bends and caps
ASME B16.9 long radius and caps, ASME B16.28 short radius. All values in mm.
Tees and reducers
ASME B16.9. Equal ends are the straight tee. All values in mm.
Source. Nominal dimensions to ASME B36.10M for carbon steel pipe, ASME B36.19M for stainless pipe, ASME B16.9 for buttwelding fittings, and ASME B16.28 for short radius elbows and return bends. Carbon steel mass is (OD − t) × t × 0.02466 kg/m. Stainless mass uses 0.02491 kg/m, based on 7.93 kg/dm3 for austenitic grades. A dash means the size is not made in that schedule or the dimension does not apply.
These are nominal values. Mill tolerance, commonly −12.5% on wall thickness, and any corrosion allowance both come off before you check a wall against design pressure to AS 4041 or ASME B31.3. STD, XS and XXS overlap the numbered schedules in the smaller sizes, which is why the tool tells you when the wall you have selected is identical to another schedule. Order to the schedule, not to the thickness.
Stainless S schedules are not the carbon steel schedules. Sch 40S and Sch 40 agree up to DN 250, then part company. Sch 80S is capped at 12.70 mm from DN 250 up, where Sch 80 keeps climbing. Two flagged cells: at DN 250 and DN 300 the source marks Sch 80S with an asterisk for this reason.
Inside diameter is calculated, not transcribed. The stainless chart works out inside diameter as OD minus twice the wall, so it can sit 0.01 mm off a printed figure through rounding. One published cell is properly out: DN 300 Sch 40S prints as 304.08 mm where the arithmetic gives 304.81 mm, so the chart shows the calculated value.
Note: always double check the current batch of fittings for mill tolerance variation.
What a schedule number actually is
Outside diameter is fixed for a given nominal size. That is the whole basis of the system: a DN50 pipe is 60.3 mm OD whether it is Schedule 10 or Schedule 160, because the fittings, flanges and tooling all reference the OD. Change the schedule and the wall thickness changes inwards. The bore gets smaller, the outside stays put.
The number itself comes from a pressure over stress ratio, roughly:
Schedule number = 1000 x P / S
Where P is the working pressure and S is the allowable stress in the same units. It descends from Barlow’s formula. Which is why the numbers run 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160 rather than anything that looks like a thickness, and why the same schedule number gives you wildly different walls at different diameters.
The older designations still turn up on drawings:
- STD (standard) matches Schedule 40 up to DN250. Above that it stays at 9.53 mm while Schedule 40 keeps climbing.
- XS (extra strong) matches Schedule 80 up to DN200. Above that it holds at 12.7 mm.
- XXS (double extra strong) follows no schedule series at all. It is its own set of numbers.
If a drawing says STD at DN300, it means 9.53 mm, not Schedule 40’s 10.31 mm. That divergence above DN250 has caught out more than one detailer.
Carbon steel and stainless are different series
This is the single most common error on this subject, so it gets its own heading.
Stainless S schedules are not the carbon steel schedules. 5S, 10S, 40S and 80S are a separate series under ASME B36.19M, and they are generally thinner than the carbon steel equivalents. Stainless has higher allowable stress and better corrosion resistance, so it does not need the same corrosion allowance built into the wall.
At the smaller sizes 40S and 40 happen to coincide, and at some sizes 80S and 80 do too. That coincidence is exactly what makes the error easy to make and hard to spot. Above DN250 they diverge and the assumption breaks.
Practical rule: never carry a schedule number across from a carbon steel line to a stainless line without checking the actual wall dimension. Two lines specified “Schedule 40” in the same document may be different thicknesses if one is carbon and one is stainless.
Mill tolerance, and why the chart is not the pipe
The chart gives you nominal values. The pipe in the rack is not the chart.
Wall thickness tolerance is commonly minus 12.5 per cent on seamless and welded pipe. On a DN100 Schedule 40 pipe, nominal 6.02 mm, that permits 5.27 mm and it is still compliant product.
Three things follow.
For pressure design, the minimum wall after tolerance is what you calculate on, not the nominal. ASME B31.3 is explicit about this and it is a routine finding when someone has used the nominal figure.
For welding, the joint you actually cut may be thinner than the drawing implies. On thin-wall stainless in particular, a 12.5 per cent under-tolerance changes the root technique, and it can move you outside the thickness range your procedure was qualified for. That is the connection between a dimension chart and a non-conformance, and it is not a hypothetical.
For fit-up, two lengths of the same nominal spec from different heats can differ in bore by enough to produce a genuine internal misalignment at the root. Which is a hi-lo problem, and hi-lo is a lack of fusion problem waiting to happen.
Inside diameter on this tool is calculated, not transcribed. ID = OD minus twice the wall. Charts that list ID as a primary figure have already rounded it once, and you inherit their rounding.
The fittings
Buttweld fitting dimensions are to ASME B16.9, with short radius elbows and return bends to ASME B16.28.
The dimensional letters on this tool follow the standards’ own notation:
- A – centre to end, long radius elbows (1.5 x DN)
- B – centre to end, short radius elbows (1.0 x DN)
- C – length through the hub, caps
- E – centre to end, run, tees
- M – centre to end, outlet, tees
- H – end to end, reducers
- K and O – back to face and outside diameter, return bends
- F, G, R – lap joint stub end dimensions
Long radius versus short radius. LR is 1.5 times the nominal diameter to the centreline, SR is 1.0. LR is the default and SR exists for congested runs. SR costs you pressure drop and it costs you erosion life in abrasive service, so it is a fit-up decision with a process consequence attached.
Lap joint stub ends are worth knowing properly. The stub end takes the weld and the loose backing flange takes the bolting, so the flange never sees the process fluid and can be a cheaper material than the pipe. On stainless and exotic lines that is a real saving. The catch is the stub end lap thickness and radius have to match the flange, and mixing manufacturers here produces a joint that will not pull up square.
Why a fitter’s chart belongs on a welding site
Because wall thickness drives most of the welding decisions before anyone strikes an arc.
Root technique. Under about 3 mm you are into a different set of options than at 10 mm. Open root GTAW, consumable insert, backing gas requirements all follow from the wall.
Joint preparation. Land and included angle scale with wall thickness. So does whether you need a compound bevel.
Number of runs, and therefore heat input. A 3.68 mm wall is a root and a cap. A 15 mm wall is a filled prep with interpass control to think about. See the heat input calculator.
Procedure qualification thickness range. ISO 15614-1 and ASME IX both set the qualified thickness range off the test piece thickness. Get the actual wall wrong and the range you are working to may not cover the job.
Preheat. Combined thickness at a pipe joint is a function of the wall. See the preheat calculator.
The chart is upstream of all of it.
Standards and editions
- ASME B36.10M, Welded and Seamless Wrought Steel Pipe. Carbon and alloy steel pipe dimensions.
- ASME B36.19M, Stainless Steel Pipe. The S schedule series.
- ASME B16.9, Factory-Made Wrought Buttwelding Fittings.
- ASME B16.28, Wrought Steel Buttwelding Short Radius Elbows and Returns.
For the welding and pressure design side: ASME B31.3 for process piping, AS 4041 for pressure piping in Australia, AS/NZS 1554.1 for structural. The dimensional standards above tell you what the pipe is. They do not tell you what you are allowed to do with it.
What is verified, what is derived, what is a modelling choice
Verified: all dimensional values are nominal figures from ASME B36.10M, B36.19M, B16.9 and B16.28 as cited on the tool. The schedule number derivation from the pressure over stress ratio. The STD, XS and XXS divergence points.
Derived: inside diameter, calculated as OD minus twice the nominal wall rather than transcribed from a secondary chart.
Modelling choice: the tool presents nominal dimensions only. It does not apply mill tolerance, because tolerance depends on the product specification, the manufacturing route and sometimes the purchase order. Applying an average tolerance would give a number that is wrong in a more confident-looking way.
Not included: pressure ratings, temperature derating and allowable stress. Those are design calculations, they belong to the applicable code, and putting them on a dimension tool would invite exactly the shortcut this note exists to prevent.
Limitations
Nominal dimensions. Not a substitute for the standard, the mill certificate or the pipe in front of you.
Check the actual batch. Mill tolerance is real, it is commonly minus 12.5 per cent on the wall, and it applies to compliant product. For any pressure-retaining calculation, work to the minimum wall permitted by the specification, not the nominal figure on a chart.
Related tools
- Preheat Calculator – AS/NZS 1554.1 method
- Heat Input Calculator – arc energy and heat input in kJ/mm
- Shielding Gas Waste Calculator
- Volts, Amps and the GMAW Arc
- GTAW Electrode Angle and Penetration
Talk to Graham about welding procedures for piping, or about a fit-up problem that is producing root defects nobody can explain. Get in touch.