You are designing a piping system. The engineer specifies a 2-inch Schedule 40S pipe. What does that mean? How do you know if the thickness and diameter are right for your application?

Stainless steel pipe is measured by Nominal Pipe Size (NPS) – which is neither ID nor OD for small sizes – and schedule (wall thickness). For NPS 1/8" to 12", the OD is fixed and larger than the NPS. For example, 2-inch pipe has an OD of 60.3mm, not 50.8mm. Choose thickness based on pressure rating, corrosion allowance, and cost. Nominal Pipe Size explained What do pipe schedules mean? ANSI / ASME B36.19M Schedule 40S dimensions

I have helped customers select pipe thickness and diameter for many projects. A water line needed Sch 10S (thin wall). A high-pressure line needed Sch 80S (thick wall). Let me walk you through the selection process. How to choose stainless steel pipe thickness Stainless steel pipe sizing guide

Is stainless pipe measured ID or OD?

Stainless pipe is measured by Nominal Pipe Size (NPS), which is neither ID nor OD for small sizes. For NPS 1/8" to 12", the OD is fixed and larger than the NPS. For NPS 14" and above, the OD equals the NPS. NPS (Nominal Pipe Size) guide

For example, a 1-inch pipe has an OD of 33.4mm, not 25.4mm. The ID varies depending on the schedule (wall thickness). Fittings are made to fit the OD, not the NPS. Always refer to a pipe size chart for exact dimensions. Pipe size chart – NPS, OD, wall thickness Stainless steel pipe OD and wall thickness explained

NPS, OD, and ID Explained

Let me clarify how stainless steel pipe is measured.

NPS vs OD vs ID

NPS	OD (mm)	ID varies by schedule
1/2"	21.3	15.8-17.5
3/4"	26.7	20.9-22.9
1"	33.4	24.3-29.5
1-1/2"	48.3	35.0-42.9
2"	60.3	44.8-55.0
3"	88.9	77.9-83.3
4"	114.3	97.2-107.0
6"	168.3	154.1-160.2

Key Points

• NPS is a nominal size, not an actual measurement
• OD is fixed for each NPS
• ID varies by schedule (wall thickness)
• Fittings are based on OD, not NPS

My Experience
A customer thought 1-inch pipe had 1-inch OD. I explained the OD is 33.4mm (1.315 inches). They checked the pipe size chart and ordered correctly. Pipe OD vs ID vs NPS Stainless steel pipe dimensions guide

How thick is 304 stainless steel pipe?

304 stainless steel pipe thickness varies by schedule. Common thicknesses for 1-inch pipe: Schedule 10S = 2.77mm, Schedule 40S = 3.38mm, Schedule 80S = 4.55mm. For 2-inch pipe: Schedule 10S = 2.77mm, Schedule 40S = 3.91mm, Schedule 80S = 5.54mm. Nominal Pipe Size and schedule overview Stainless steel pipe size and schedule chart

The thicker the wall, the higher the pressure rating and weight, but the smaller the inside diameter. Always check the size chart for your specific NPS and schedule. Stainless steel pipe dimensions and tolerances

304 Pipe Thickness by Schedule

Let me provide thickness information for 304 stainless steel pipe.

1-inch NPS (OD 33.4mm)

Schedule	Wall (mm)	ID (mm)	Weight (kg/m)	Pressure Rating (psi)
10S	2.77	27.9	2.11	750
40S	3.38	26.6	2.54	1,700
80S	4.55	24.3	3.24	2,800

2-inch NPS (OD 60.3mm)

Schedule	Wall (mm)	ID (mm)	Weight (kg/m)	Pressure Rating (psi)
10S	2.77	54.8	4.06	510
40S	3.91	52.5	5.44	1,100
80S	5.54	49.2	7.48	1,800

Schedule Selection Guide

Application	Recommended Schedule
Drainage, low pressure	Sch 10S
Water, air, general	Sch 40S
High pressure, threaded	Sch 80S
High corrosion allowance	Sch 80S or thicker

My Experience
For a compressed air line (150 psi), we used Sch 40S. For a hydraulic line (1,800 psi), we used Sch 80S. How to select stainless steel pipe thickness Stainless steel pipe sizing guide

How to choose a pipe diameter?

To choose a pipe diameter, consider the required flow rate, allowable pressure drop, velocity limits, and connection to existing equipment. Use flow calculations (e.g., Darcy‑Weisbach) to determine the minimum ID needed. Then select the NPS that provides that ID. Pipe sizing overview Darcy‑Weisbach pipe sizing

For example, a 2-inch pipe has an ID of about 52.5mm (Sch 40S). If you need 100 GPM water flow, 2-inch may be adequate. For 500 GPM, you would need 4-inch or larger. Always consult a pipe sizing chart or engineer. Pipe diameter and flow rate calculator Understanding pipe sizes and schedules

Pipe Diameter Selection Factors

Let me explain the factors for choosing pipe diameter.

Key Factors

Factor	Consideration
Flow rate (GPM or m³/h)	Higher flow needs larger diameter
Velocity limit	Too high causes erosion, noise
Pressure drop	Larger diameter reduces pressure drop
Connection size	Must match existing equipment
Cost	Larger diameter costs more

Recommended Velocity Limits (304 Stainless)

Service	Velocity (m/s)
Water	1.5-3.0
Air / gas	10-20
Steam	20-40

Simple Diameter Selection (Water)

Flow Rate (GPM)	Recommended NPS (Sch 40S)
Up to 20	1"
20-50	1-1/2"
50-150	2"
150-300	3"
300-600	4"
600-1,200	6"

My Experience
For a 200 GPM water line, we selected 3-inch pipe. The velocity was 2.5 m/s, within the recommended range. Pipe sizing calculator for liquids and gases

What is the difference between 304 and 316 stainless steel pipe?

304 stainless steel pipe is standard grade for general applications. 316 stainless steel pipe contains molybdenum (2-3%) for superior corrosion resistance in chloride (salt) environments. 304 vs 316 stainless steel differences Difference between 316 and 304

304 is suitable for freshwater, indoor, and general outdoor use. 316 is required for coastal, marine, seawater, and high-chloride applications. 316 costs 30-50% more than 304. Corrosion resistance of 316 vs 304 Cost comparison 304 vs 316

304 vs 316 Comparison

Let me compare 304 and 316 stainless steel pipe.

304 Stainless Steel Pipe

Property	Value
Corrosion resistance	Good (general)
Chloride resistance	Moderate
Strength	205 MPa yield
Cost	Lower
Best for	Freshwater, indoor, general outdoor

316 Stainless Steel Pipe

Property	Value
Corrosion resistance	Excellent (marine)
Chloride resistance	High
Strength	205 MPa yield
Cost	Higher (30-50%)
Best for	Coastal, marine, seawater, high salt

Selection Guide

Application	Recommended Grade
Indoor water line	304
Outdoor water line (inland)	304
Outdoor water line (coastal)	316
Marine / seawater	316
Chemical (chlorides)	316

My Experience
For a freshwater line in Chicago, we used 304. For a seawater cooling line, we used 316. Chloride attack on stainless steel (304 vs 316) Guidelines for stainless steel under chloride conditions

Conclusion

Stainless pipe is measured by NPS (OD fixed, ID varies by schedule). Nominal Pipe Size explained 304 pipe thickness ranges from Sch 10S (thin) to Sch 80S (thick). Stainless steel pipe size and schedule chart Choose pipe diameter based on flow rate and velocity. Pipe sizing fundamentals 316 is better than 304 for saltwater and coastal applications. 316 vs 304 in marine environments