Schedule 20 vs STD Pipe: Key Differences

Schedule 20 and STD (Standard Weight) are both used to describe steel pipe wall thickness, but they are not interchangeable terms for every pipe size.

The important point is that Schedule 20 is a numbered pipe schedule, while STD is a standard-wall designation. Their wall thicknesses can be identical at certain NPS sizes, but they can also be different. For this reason, pipe size, outside diameter, wall thickness, and applicable standard should always be checked before ordering.

NPS 20 Schedule 20 pipe has a 0.375 in. (9.53 mm) wall, which is also designated STD at that size. By contrast, NPS 12 Schedule 20 has a 0.250 in. (6.35 mm) wall, while NPS 12 STD is 0.375 in. (9.52 mm).

Schedule 20 vs STD Steel Pipe at a Glance

Item	Schedule 20	STD Steel Pipe
Meaning	Numbered wall-thickness designation	Standard Weight designation
Main reference	ASME B36.10M	ASME B36.10M
Wall thickness	Depends on NPS	Depends on NPS
Same as each other?	Only at certain sizes	Only at certain sizes
Outside diameter	Determined by NPS	Determined by NPS
Inside diameter	Depends on wall thickness	Depends on wall thickness
Typical use	Large-diameter, relatively light-wall piping	Standard-wall piping
Ordering method	Specify NPS + Schedule 20	Specify NPS + STD
Main selection factor	Required wall and service conditions	Required wall and service conditions

What Is Schedule 20 Steel Pipe?

Schedule 20 is a numbered wall-thickness designation used for steel pipe dimensions covered by ASME B36.10M.For a given NPS, the outside diameter remains fixed while the wall thickness changes with the schedule. As the wall becomes thicker, the inside diameter becomes smaller and the pipe becomes heavier.For example, NPS 8 Schedule 20 has an outside diameter of 8.625 in. (219.1 mm) and a nominal wall thickness of 0.250 in. (6.35 mm). Its approximate inside diameter is 8.125 in. (206.4 mm).
For larger sizes, Schedule 20 is commonly encountered in applications where a relatively large flow passage is required without using a heavier wall such as Schedule 40 or Schedule 80.

Schedule 20 Does Not Mean 20 mm Wall Thickness

The number 20 does not represent wall thickness in millimeters.
Schedule numbers are designations rather than direct measurements. The actual wall thickness must be obtained from the applicable dimensional standard for the selected NPS.

For this reason, a pipe should not be specified simply as "20 mm schedule pipe" or assumed to have a 20 mm wall.

For carbon steel pipe, theoretical weight can be estimated from the pipe dimensions:
Weight ≈ 0.02466 × (OD − WT) × WT
where OD and WT are expressed in millimeters, producing an approximate weight in kg/m.
Actual published pipe weights should be checked against the applicable dimensional standard and material density.
For large-diameter piping, even a small difference in wall thickness can significantly affect the total weight and project cost.

What Does STD Mean on Steel Pipe?

STD stands for Standard Weight.It is an older wall-thickness designation that remains part of the dimensional terminology used for steel pipe. STD appears in ASME B36.10M alongside numbered schedules and other traditional designations such as XS (Extra Strong) and XXS (Double Extra Strong).
STD is therefore not a material grade and does not indicate whether the pipe is carbon steel, alloy steel, or another material.
It describes the pipe's dimensional wall category.The actual material specification must be separately identified, such as ASTM A53, ASTM A106, API 5L, or another applicable product specification.

Is Schedule 20 the Same as STD?

Sometimes, but not universally.This is the most important distinction when comparing Schedule 20 and STD pipe.
ASME B36.10M assigns wall thicknesses according to NPS. At some large NPS sizes, the Schedule 20 wall thickness is exactly the same as the STD wall thickness. At other sizes, they are different.
Example: NPS 20
For NPS 20:
Outside diameter: 20.000 in. (508.0 mm)
Schedule 20 wall: 0.375 in. (9.52 mm)
STD wall: 0.375 in. (9.52 mm)
Approximate ID: 19.250 in. (488.95 mm)
Therefore:
NPS 20 Schedule 20 = NPS 20 STD dimensionally.
The same relationship can be seen at NPS 22, where Schedule 20 and STD are both 0.375 in. (9.52 mm).
However, this does not mean Schedule 20 and STD are interchangeable for all NPS sizes.

NPS 20 Standard (STD) Pipe
20.000 in (508.0 mm) outside diameter on a 0.375 in (9.5 mm) wall, giving a 19.250 in (488.9 mm) bore and 78.60 lb/ft (116.97 kg/m) empty. ASME B36.10M.

Schedule 20 vs STD Pipe Thickness

The Schedule 20 vs STD pipe thickness comparison changes according to NPS.
For a particular NPS, the outside diameter remains fixed. If the wall thickness changes, the inside diameter and pipe weight change accordingly.
Some representative comparisons are shown below.

NPS	Outside Diameter	Schedule 20 Wall	STD Wall	Difference
12	12.750 in. (323.8 mm)	0.250 in. (6.35 mm)	0.375 in. (9.53 mm)	Different
14	14.000 in. (355.6 mm)	0.312 in. (7.92 mm)	0.375 in. (9.53 mm)	Different
16	16.000 in. (406.4 mm)	0.312 in. (7.92 mm)	0.375 in. (9.53 mm)	Different
18	18.000 in. (457.2 mm)	0.312 in. (7.92 mm)	0.375 in. (9.53 mm)	Different
20	20.000 in. (508.0 mm)	0.375 in. (9.53 mm)	0.375 in. (9.53 mm)	Same
22	22.000 in. (558.8 mm)	0.375 in. (9.53 mm)	0.375 in. (9.53 mm)	Same
24	24.000 in. (609.6 mm)	0.375 in. (9.53 mm)	0.375 in. (9.53 mm)	Same

This comparison shows why it is not correct to say simply that Schedule 20 is thinner than STD.At NPS 12, STD is thicker. At NPS 20, the two designations have the same nominal wall thickness.
For procurement and engineering work, the actual NPS-specific dimensions should be checked against the applicable edition of ASME B36.10M.

Schedule 20 vs Standard Weight Pipe

Schedule 20 vs Standard Weight pipe is essentially a comparison between two different ways of designating pipe wall thickness.Schedule 20 uses a numbered schedule designation. STD, or Standard Weight, is a traditional designation that is still used in steel pipe dimensional terminology.
The difference is therefore not that one is a type of steel and the other is a different type of steel.Both designations can be applied to pipes manufactured from different materials, provided the pipe meets the applicable product specification.
For example, a project may specify a particular carbon steel grade together with a pipe dimension such as Schedule 20 or STD.
A specification might therefore contain separate requirements for:
NPS
Schedule or wall designation
Material specification
Material grade
Manufacturing method
Pipe length
End finish
Inspection and testing

This distinction is important because STD does not mean a particular carbon steel grade, and Schedule 20 does not identify the material either.

Schedule 20 vs STD Dimensions

The difference between Schedule 20 vs STD dimensions mainly comes down to wall thickness. For the same NPS, the outside diameter (OD) is fixed, while the wall thickness (WT) determines the approximate inside diameter (ID).
The basic relationship is:
ID ≈ OD − 2 × WT
This means a thicker pipe wall produces a smaller internal bore when the outside diameter remains the same.
NPS 12: Schedule 20 vs STD
For NPS 12, both Schedule 20 and STD pipe have an outside diameter of 12.750 in. However, their wall thicknesses are different.

NPS	Pipe	OD (in.)	WT (in.)	Approx. ID (in.)
12	Schedule 20	12.75	0.25	12.25
12	STD	12.75	0.375	12

The calculation is:
Schedule 20: 12.750 − (2 × 0.250) = 12.250 in.
STD: 12.750 − (2 × 0.375) = 12.000 in.
Therefore, NPS 12 Schedule 20 and STD have the same OD but different wall thicknesses and internal diameters. The STD pipe has a 0.125 in. thicker wall and a 0.250 in. smaller approximate ID.

NPS 20: Schedule 20 vs STD
NPS 20 shows a different relationship. At this size, Schedule 20 and STD have the same listed wall thickness.

NPS	Pipe	OD (in.)	WT (in.)	Approx. ID (in.)
20	Schedule 20	20	0.375	19.25
20	STD	20	0.375	19.25

The calculation is:
20.000 − (2 × 0.375) = 19.250 in.
Therefore, NPS 20 Schedule 20 and NPS 20 STD are dimensionally equivalent in terms of OD, nominal wall thickness, and calculated ID.

Is Schedule 20 the Same as STD?

Not always.Schedule 20 and STD are separate pipe designations, and their dimensions can differ depending on the nominal pipe size. For NPS 12, they have different wall thicknesses. For NPS 20, the listed wall thickness is the same.
Therefore, Schedule 20 should not be treated as universally interchangeable with STD. Always check the applicable dimensional standard and the specific NPS before selecting or ordering pipe.
The key relationship can be summarized as follows:

NPS	Schedule 20 vs STD	OD	WT	ID
NPS 12	Different	Same	Different	Different
NPS 20	Equivalent	Same	Same	Same

For procurement, fabrication, and engineering calculations, use the specified NPS, pipe designation, wall thickness, and applicable standard rather than assuming that Schedule 20 and STD mean the same thing.

Conclusion

Schedule 20 and STD are different pipe wall-thickness designations. Schedule 20 is part of the pipe schedule system, while STD means Standard Weight.They may have comparable dimensions for certain pipe sizes, but they are not universally interchangeable.For accurate selection, always compare the actual wall thickness, inside diameter, weight, material grade, and applicable standard. This prevents dimensional errors and ensures the pipe meets the project's mechanical and pressure requirements.