Pipes are one of the most essential components in construction, plumbing, oil & gas, HVAC, irrigation, and countless other industries. Whether you are a professional engineer, a plumber, or simply someone working on a DIY project at home, knowing the right pipe size is crucial for efficiency, safety, and cost savings.
But here’s the challenge: pipes don’t always measure the way you’d expect. For example, a “1-inch pipe” doesn’t always have an outer diameter (OD) of exactly 1 inch. Instead, pipe sizes follow standardized systems like Nominal Pipe Size (NPS), Nominal Bore (NB), or DN (Diameter Nominal). That’s where a pipe size chart comes in handy — it helps you compare inside diameters, outside diameters, wall thicknesses, and schedules.
In this detailed guide, we’ll cover everything you need to know about pipe sizing, including:
What pipe size really means
The difference between NPS, DN, and NB
Pipe schedule and wall thickness
Pipe size chart for steel, PVC, and copper pipes
Why choosing the correct size matters
Practical applications and tips
Let’s dive in.
What Does Pipe Size Actually Mean?
When someone says “I need a 2-inch pipe,” it might sound simple, but it’s not as straightforward as grabbing a pipe with a 2-inch diameter. Pipe sizing is based on nominal values, which are reference numbers and not exact physical measurements.
There are two main measurements for pipes:
Outside Diameter (OD): The total width across the pipe from edge to edge.
Inside Diameter (ID): The usable space inside the pipe where fluid, gas, or material flows.
The confusion arises because nominal size often refers to the approximate inside diameter, while the actual OD remains constant. As the pipe wall thickness (schedule) increases, the inside diameter becomes smaller, even though the OD doesn’t change.
For example:
A 2-inch Schedule 40 steel pipe has an OD of 2.375 inches and an ID of 2.067 inches.
A 2-inch Schedule 80 steel pipe also has an OD of 2.375 inches, but the ID reduces to 1.939 inches due to thicker walls.
That’s why referencing a pipe size chart is critical before selecting materials.
Pipe Sizing Systems: NPS, NB, and DN
Different industries and countries use different standards. Here are the three common systems you’ll encounter:
NPS (Nominal Pipe Size): Used in North America, it defines pipe size using a nominal number (like ½”, 1″, 2″) and corresponding OD.
NB (Nominal Bore): Common in the UK and some other regions, similar to NPS.
DN (Diameter Nominal): The metric equivalent used internationally, expressed in millimeters (DN 25 ≈ 1″).
So, a pipe marked NPS 2” in the U.S. is approximately DN 50 in Europe.
Pipe Schedule and Wall Thickness
Apart from diameter, the wall thickness of a pipe is equally important. This is defined by the schedule number (SCH).
Lower schedules (e.g., SCH 10, SCH 20, SCH 40) = Thinner walls.
Higher schedules (e.g., SCH 80, SCH 160) = Thicker walls, stronger pressure resistance.
For instance:
A 1-inch Schedule 40 pipe has a wall thickness of 0.133 inches.
A 1-inch Schedule 80 pipe has a wall thickness of 0.179 inches.
This means pipe size charts usually include columns for OD, ID, and wall thickness across different schedules.
Standard Pipe Size Chart (Steel Pipes – ASME B36.10/36.19)
Here’s a simplified version of the steel pipe size chart:
| Nominal Pipe Size (NPS) | Outside Diameter (OD) | Schedule 40 ID | Schedule 80 ID |
|---|---|---|---|
| ½ inch | 0.84 in (21.3 mm) | 0.622 in | 0.546 in |
| 1 inch | 1.315 in (33.4 mm) | 1.049 in | 0.957 in |
| 2 inch | 2.375 in (60.3 mm) | 2.067 in | 1.939 in |
| 4 inch | 4.500 in (114.3 mm) | 4.026 in | 3.826 in |
| 6 inch | 6.625 in (168.3 mm) | 6.065 in | 5.761 in |
| 8 inch | 8.625 in (219.1 mm) | 7.981 in | 7.625 in |
| 10 inch | 10.750 in (273 mm) | 10.020 in | 9.564 in |
This chart allows engineers and plumbers to quickly select the correct pipe for pressure and flow requirements.
PVC Pipe Size Chart
PVC (Polyvinyl Chloride) pipes are widely used in plumbing, irrigation, and drainage systems. The sizing for PVC is slightly different because it often follows Nominal Pipe Size (NPS) but is standardized under ASTM.
| PVC Pipe Size | Outside Diameter (OD) | Schedule 40 ID | Schedule 80 ID |
|---|---|---|---|
| ½ inch | 0.84 in | 0.622 in | 0.526 in |
| 1 inch | 1.315 in | 1.029 in | 0.936 in |
| 2 inch | 2.375 in | 1.939 in | 1.709 in |
| 3 inch | 3.500 in | 2.939 in | 2.709 in |
| 4 inch | 4.500 in | 3.961 in | 3.826 in |
PVC schedules are similar to metal pipes: higher schedules mean thicker walls and more pressure resistance.
Copper Pipe Size Chart
Copper pipes are common in residential water supply systems. Unlike steel or PVC, copper pipes are measured by nominal inside diameter, but classifications are based on wall thickness:
Type K (thickest, green) – used for underground or high-pressure systems.
Type L (medium, blue) – common for residential water systems.
Type M (thin, red) – for low-pressure applications.
| Copper Pipe Size | Outside Diameter (OD) | Type K ID | Type L ID | Type M ID |
|---|---|---|---|---|
| ½ inch | 0.625 in | 0.527 in | 0.545 in | 0.569 in |
| 1 inch | 1.125 in | 1.049 in | 1.065 in | 1.093 in |
| 2 inch | 2.125 in | 2.009 in | 2.027 in | 2.051 in |
Why Choosing the Correct Pipe Size Matters
Selecting the right pipe size isn’t just about fit — it impacts safety, performance, and cost.
Flow Rate Efficiency – If the pipe is too small, it restricts flow, increases pressure drop, and causes inefficiency. Too large, and it may waste material and cost.
Pressure Handling – Thicker walls and correct sizing ensure pipes withstand pressure without bursting.
System Compatibility – Different systems (plumbing, HVAC, oil & gas) follow strict standards for safety compliance.
Cost Savings – Choosing the right schedule prevents overspending on unnecessary material thickness.
Longevity & Safety – Properly sized pipes reduce stress, prevent leaks, and extend system life.
Practical Applications of Pipe Size Charts
Pipe size charts are used across industries:
Plumbing: Ensuring water supply lines and drainage are balanced.
Construction: HVAC, fire sprinkler systems, and structural piping.
Oil & Gas: High-pressure pipelines need accurate scheduling.
Manufacturing: Process piping for chemical plants, food processing, etc.
DIY Projects: Garden irrigation, aquarium piping, or home repairs.
Tips for Using Pipe Size Charts Effectively
Always check both OD and ID before purchasing.
Match the pipe schedule with system pressure requirements.
Remember that metric and imperial units differ — double-check DN vs NPS.
When replacing old pipes, measure the OD directly to avoid confusion.
Use manufacturer’s charts for specific materials since tolerances may vary.
Final Thoughts
Pipe sizing can feel confusing at first because a “1-inch” pipe isn’t really 1 inch in every way. But with the help of a pipe size chart, you can easily navigate between nominal sizes, actual diameters, schedules, and wall thicknesses.
Whether you’re working with steel, PVC, or copper, understanding these charts ensures better planning, safer systems, and cost efficiency. In industrial projects, incorrect pipe sizing can lead to system failures or massive repair costs. In home projects, it can cause leaks, low water pressure, or wasted money.
By referring to the right pipe size chart, you take the guesswork out of the process and make sure every fitting, valve, and joint connects seamlessly.
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