Key takeaways
NPS and DN identify a size; OD is the measurement
NPS means nominal pipe size. DN is the corresponding nominal designation used in the metric system. Neither gives the measured diameter for the small and medium pipe sizes in the table below. NPS 2 corresponds to DN 50, while its nominal outside diameter is 2.375 in., or 60.3 mm. Ordering a 50 mm outside-diameter tube therefore produces a different item from ordering DN 50 pipe.
A pipe schedule adds the nominal wall thickness to that size designation. The complete dimensional description needs both size and wall: NPS 2 Schedule 40 is more precise than a request for 2-inch pipe. On a factory order, we keep the original NPS or DN designation alongside OD and wall so a buyer's material list and the production dimensions refer to the same item. The dimensional standard and its edition belong on that list as well.
Schedule 40 and 80 keep the same outside diameter
Read the Four Common-Size Examples
At each size shown here, Schedule 80 has a thicker nominal wall than Schedule 40. The outside diameter stays the same. These are carbon steel pipe dimensions in the ASME B36.10 size system, checked against Wheatland Tube's published standard-pipe tables. The table covers four common sizes; it is not a dimensional chart for every pipe or tube product.
Use the inch or metric column consistently when checking a drawing. Published metric dimensions are rounded, so converting a rounded millimeter number back into inches can introduce a small difference. Keep the specified unit system as the inspection reference, rather than setting two conflicting acceptance limits from independently rounded values.
Swipe to see every column.
| NPS | DN | OD (in. / mm) | Schedule 40 wall (in. / mm) | Schedule 80 wall (in. / mm) | Plain-end reference mass: Sch 40 / Sch 80 (kg/m) |
|---|---|---|---|---|---|
| 1/2 | 15 | 0.840 / 21.3 | 0.109 / 2.77 | 0.147 / 3.73 | 1.27 / 1.62 |
| 1 | 25 | 1.315 / 33.4 | 0.133 / 3.38 | 0.179 / 4.55 | 2.50 / 3.24 |
| 2 | 50 | 2.375 / 60.3 | 0.154 / 3.91 | 0.218 / 5.54 | 5.44 / 7.48 |
| 4 | 100 | 4.500 / 114.3 | 0.237 / 6.02 | 0.337 / 8.56 | 16.07 / 22.32 |
A heavier schedule reduces the bore
For a circular pipe with a uniform wall, nominal inside diameter equals outside diameter minus twice the nominal wall. Using the inch dimensions, NPS 2 Schedule 40 has a calculated bore of 2.067 in.; Schedule 80 has a calculated bore of 1.939 in. The fitting size name remains NPS 2, but the available internal passage changes. That matters when comparing flow calculations, lined pipe or a connection that needs a particular internal transition.
A schedule number does not state the pipe's allowable working pressure. Material, temperature, wall tolerance, corrosion allowance, joint design and the governing piping code also enter the pressure design. Retain the engineer's specified schedule when requesting a quote. Replacing it with a heavier or lighter wall changes more than the price and shipment weight. Our black steel pipe range separates the material specification, dimensions and end processing for this reason.
Calculate plain-pipe weight from OD and wall
Use the Original Dimension and Unit Basis
For a circular carbon steel pipe, theoretical mass per meter is approximately 0.02466 × (D − t) × t. Enter outside diameter D and wall thickness t in millimeters; the result is kilograms per meter. The factor comes from the metal's annular cross-sectional area and an assumed density of 7.85 g/cm³, equivalent to 7,850 kg/m³. Do not enter nominal size in place of actual outside diameter.
Multiply kilograms per meter by the total ordered length in meters to estimate plain-pipe mass. Couplings, flanges, end caps, coatings and packing need separate allowances. This calculation also uses nominal wall, whereas delivered pipe has permitted dimensional variation. A quotation based on theoretical mass and one settled by actual weighing therefore need their price bases stated explicitly.
Wheatland's plain-end tables list NPS 4 Schedule 40 at 16.07 kg/m and Schedule 80 at 22.32 kg/m. The thicker schedule adds mass without changing the nominal 114.3 mm OD. These reference values help check an estimate; they do not establish the gross shipping weight of a packed order.
Keep cut lengths and quantity units together
A pipe order expressed in pieces needs a length for each piece. A purchase by linear feet or meters needs a length range or cut-length instruction so the delivery can be used without an unexpected cutting allowance. Random lengths, fixed lengths and a cutting list produce different receiving checks, even when the total footage is identical.
The international foot is exactly 0.3048 m: 20 ft equals 6.096 m and 21 ft equals 6.4008 m. A 6 m length is consequently not an exact 20 ft length. Keep the required length, its tolerance and the measurement basis on the order. Threaded and coupled products also need a clear statement of whether the stated length includes the fitted coupling. Separate short cut pieces from full lengths in the packing list so quantity can be reconciled by item.
Turn Length and kg/m Into an Order Estimate
A Worked Quantity Using the Table
For a hypothetical NPS 2 Schedule 40 order of 80 pieces at 20 ft each, total length is 1,600 ft. Using 0.3048 m/ft gives 487.68 m. The published reference mass of 5.44 kg/m gives approximately 2,653 kg of plain pipe. Keep the rounded reference mass and estimate basis visible.
This estimates plain-end pipe, rather than packed shipment weight or an acceptance tolerance. Count couplings, flanges, coatings and packing separately where present. For sizes outside the four examples, use the project’s designated dimensional table and product data, then the same calculation method.
Pipe, structural tube and conduit use different size systems
A round structural tube can be ordered by its outside diameter and wall thickness, while square and rectangular hollow sections use outside width, height and wall. An HSS designation does not automatically refer to a pressure-pipe schedule. Compare the dimensions in our round HSS range against the drawing's section designation before treating it as interchangeable with a nominal pipe size.
Electrical conduit has its own dimensional and connection requirements. A trade-size 2 EMT is not NPS 2 pressure pipe simply because both names contain the number 2. Our EMT steel conduit page lists the conduit dimensions separately. Stainless pipe also needs its applicable dimensional standard and schedule designation, including any S suffix. Use the stainless steel pipe specification rather than extending this carbon steel table to every stainless order.
Turn the drawing into a usable size schedule
Give each pipe item its own line with material standard, grade, NPS or DN, outside diameter, schedule or nominal wall, length and quantity. Where a drawing specifies a nonstandard OD or wall, state those actual dimensions and the acceptance tolerance instead of assigning the nearest schedule name. Keep mixed sizes separated so one material description cannot conceal a wall change further down the list.
End preparation and finish complete the dimensional request. A thread, groove, bevel or internal lining can change the connection checks and the usable bore. The ends, connections and coatings guide covers those interfaces. Before comparing quotations, reconcile the size list with the quantity and weighing basis in the buying and inspection guide. The result is a material schedule that production, shipping and receiving can all follow.
- Material standard, grade and specified edition
- NPS or DN together with actual OD and nominal wall
- Fixed or random length, tolerance and piece count
- Plain, beveled, threaded or grooved ends
- Coating, quantity unit and theoretical or actual mass basis
The RFQ checklist carries these dimensions and quantity units into an enquiry.
Frequently asked questions
Is DN 50 the same as a tube with a 50 mm outside diameter?
No. DN 50 corresponds to NPS 2, whose nominal OD is 60.3 mm. A 50 mm outside-diameter tube is a different dimensional item.
Does Schedule 80 change the outside diameter?
For the carbon steel sizes shown in this guide, the nominal OD stays the same as Schedule 40. The thicker wall reduces the bore and increases the plain-pipe mass.
Can I use calculated pipe mass as the shipment's gross weight?
No. The formula estimates plain-pipe mass from nominal OD, wall and length. Couplings, flanges, coatings and packing need separate allowances, and the quotation should identify theoretical mass or actual weighing as its price basis.
How is the area inside a pipe calculated?
Using nominal dimensions in one unit system, bore diameter is OD minus twice the wall. Internal cross-sectional area is π × bore diameter squared ÷ 4, separate from the outside area or steel wall area.
Is one gauge number a complete pipe-wall specification?
Use the exact wall in inches or millimetres with the product and tolerance basis. A gauge label depends on the applicable product system and does not replace the decimal wall value on the order.
Can a 6 m pipe be counted as a 20 ft finished length?
A 20 ft length is 6.096 m. The two lengths differ by 96 mm per piece, so the original finished-length requirement remains on the order rather than being rounded to 6 m.
Technical Basis and Source Scope
Dimension and reference-mass basis: Wheatland Tube, Standard Pipe brochure, page 5, selected plain-end Schedule 40/80 rows. Only the four shown nominal geometries and reference masses are used. The weight formula uses the explicitly assumed steel density of 7,850 kg/m³.
NIST: international foot and metric conversion — The international foot is exactly 0.3048 m.
