Pipe Schedule Chart: How to Read Wall Thickness by NPS

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What a Schedule Number Is

A pipe schedule is an index into a wall-thickness table. When ASME B36.10 lists NPS 6 Schedule 40, it is pointing at one specific combination: an outside diameter of 168.3 mm and a wall thickness of 7.11 mm. Change the NPS and the same schedule number points at a different wall — NPS 2 Schedule 40 is 3.91 mm, NPS 12 Schedule 40 is 10.31 mm. The schedule number travels; the thickness does not. That is why an order that says only "Schedule 40 pipe" without the size is meaningless, and why fittings must be ordered as size plus schedule: the fitting's bore and bevel are machined for exactly one row of this chart.

Historically the schedule series grew out of a pressure formula — roughly, schedule number ≈ 1000 × P/S, working pressure over allowable stress — which is why heavier schedules exist in the first place. In modern practice nobody calculates with it; the number is simply the row label in B36.10 (carbon and alloy pipe) and B36.19 (stainless pipe). What matters on an order is that both sides of every welded joint are read off the same row.

The Chart: Wall Thickness in Millimetres (ASME B36.10)

Read one row across and the system shows itself: NPS 6 grows from a 3.40 mm wall in Schedule 10 to 18.24 mm in Schedule 160, while the outside diameter never changes. All the extra metal goes inward — the bore shrinks. That single fact drives three practical consequences. First, flow area drops sharply at heavy schedules, which the process engineer must allow for. Second, weight per metre climbs fast, which the estimator must allow for. Third, a fitting ordered in the wrong schedule physically cannot be welded flush to the pipe: the bores step at the joint, the bevels mismatch, and the weld is rejected or the fitting is returned.

STD, XS and XXS: The Old Names Inside the Chart

Three traditional designations survive inside the schedule system. STD (standard weight) coincides with Schedule 40 for sizes up to NPS 10 — above that, STD stays at 9.53 mm while Schedule 40 keeps growing, so NPS 12 STD (9.53 mm) and NPS 12 Schedule 40 (10.31 mm) are different pipes. XS (extra strong) coincides with Schedule 80 up to NPS 8, then diverges the same way. XXS (double extra strong) has no schedule number at all — it is its own wall series, heavier than Schedule 160 in some sizes and lighter in others. When an old drawing says XS, translate it to a wall thickness in millimetres before ordering, especially in NPS 10 and above where the names and numbers part company.

The Full Schedule Ladder

Between the four columns of the chart sit the intermediate schedules: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160, plus the named STD / XS / XXS walls. Not every schedule exists in every size — the table is a grid with gaps, filled where industry demand created a standard wall. Stainless pipe follows ASME B36.19 with its own series marked with an S (5S, 10S, 40S, 80S); the numbers look familiar but the walls are a different series from the carbon numbers — check the B36.19 table rather than assuming the carbon wall, as our stainless steel fittings page explains. Never order stainless by a carbon schedule number, or carbon by an S schedule.

How Schedules Drive Fitting and Flange Orders

Every welded component on a line keys off the schedule. Butt weld fittings are manufactured with the wall of the schedule they are marked with; a Schedule 80 elbow has the Schedule 80 bore. Weld neck flanges are bored to the pipe ID of the stated schedule. Socket weld flanges and forged fittings translate the schedule into their class system instead. A complete order line therefore always carries four elements: size, schedule (or exact wall), material grade, and the component standard — for example: elbow 90° LR, NPS 6, Sch 80, A234 WPB, ASME B16.9. Leave the schedule out and the supplier must either ask (delay) or guess (risk).

When the Chart Is Not Enough: Ordering by Wall Thickness

Sometimes the right answer is not in the chart at all. Corrosion allowances, non-standard design pressures and replacement work on existing lines regularly produce walls between schedule rows. In those cases, state the order by the exact wall thickness in millimetres or inches and let the schedule fall where it may; a wall between schedule rows is assessed for the specific product before order. State it as "NPS 8 x 14.27 mm wall" rather than hunting for a schedule that almost matches — almost is how bore steps happen.

A Worked Example

Take a common order line: NPS 6, Schedule 80, A234 WPB elbow. From the chart, NPS 6 has an OD of 168.3 mm; Schedule 80 sets the wall at 10.97 mm, so the bore is 168.3 minus twice 10.97 — 146.4 mm. The elbow is formed with that wall and bore; the weld neck flange on the same line is bored to 146.4 mm; the pipe is rolled to the same row. If someone "saves money" by ordering the elbow in Schedule 40 (7.11 mm wall, 154.1 mm bore), the joint gets a 3.85 mm step at its inside — a stress concentration, a turbulence edge and a rejected weld, discovered at the worst possible moment. The chart is not reference decoration; it is the contract between every component on the line.

Summary for the RFQ

• Schedule = a row in B36.10, valid only together with the NPS.
• OD is constant per NPS; heavier schedule = smaller bore, heavier pipe.
• STD = Sch 40 only up to NPS 10; XS = Sch 80 only up to NPS 8; XXS stands alone.
• Stainless uses the B36.19 S-series — different walls, similar numbers.
• Pipe, fittings and weld neck flange bores must all be ordered on the same schedule or the same exact wall.

Send us your line list with sizes and schedules stated, and the pipe, fittings and flanges will be quoted as one matched set. If a wall on your list is not a schedule row, give the millimetres — state the exact wall on the enquiry and it will be assessed for the specific product before order.