“4 steel pipe” means nominal pipe size (NPS) 4, and its outside diameter is always 4.500 inches — the inside diameter and weight depend on the schedule.
Ordering or measuring 4 steel pipe dimensions trips up more DIYers and pros than almost any other size, because “4-inch pipe” doesn’t mean what it sounds like. The pipe’s actual outside diameter is 4.500 inches (114.30 mm), not 4.000. That nominal sizing system, standardized by ASME/ANSI B36.10 and B36.19, keeps fittings and flanges interchangeable while the wall thickness — and therefore the inside diameter — shifts with each schedule.
The Standard Schedule 40 Numbers
Schedule 40 is the default reference point for 4-inch steel pipe, and it’s the spec most supplier charts and project plans start from. For NPS 4 Schedule 40, the ASME/ANSI tables give an outside diameter of 4.500 inches, a wall thickness of 0.237 inches (6.02 mm), and an inside diameter of about 4.026 inches (102.26 mm). The pipe weighs roughly 10.79 lb per foot (16.06 kg/m).
Metric conversions show up on nearly every standard table too: NPS 4 matches DN 100, and the 114.3 mm outside diameter is the metric value you’ll see on European and imported spec sheets. One important note — these figures are nominal, not exact manufactured dimensions. Standard tables state this explicitly; actual pipe can vary slightly from the published numbers within manufacturing tolerances.
Schedule 80 And Why Schedule Matters
Schedule 80 keeps the same 4.500-inch outside diameter but thickens the wall to 0.337 inches, which shrinks the inside diameter to 3.826 inches. That thicker wall is why Schedule 80 handles higher pressures — but it also means less internal volume and a heavier pipe for the same footprint.
The compatibility rule is simple: fittings, flanges, and thread specs key off the outside diameter and NPS, so a 4-inch Schedule 40 flange still fits Schedule 80 pipe. Flow calculations are a different story — they need the exact inside diameter for the schedule you’re actually using. Treating “4-inch pipe” as one fixed dimension set is the most common mistake in the trade, and it produces fittings that don’t seal or flow numbers that are simply wrong.
Because 4-inch steel pipe serves pressurized and structural applications, the material standard and schedule must match your design requirements before you buy or install. If you’re comparing options for an upcoming project, our roundup of the best 4 steel pipe choices walks through the top picks by application.
4-Inch Steel Pipe Quick Reference
Here are the key numbers for the two most common schedules at NPS 4, side by side.
| Specification | Schedule 40 | Schedule 80 |
|---|---|---|
| Outside diameter | 4.500 in (114.30 mm) | 4.500 in (114.30 mm) |
| Wall thickness | 0.237 in (6.02 mm) | 0.337 in |
| Inside diameter | 4.026 in (102.26 mm) | 3.826 in |
| Weight | 10.79 lb/ft (16.06 kg/m) | Heavier — thicker wall, same OD |
| Metric size | DN 100 | DN 100 |
One more thing to watch: different supplier spec sheets can show slightly different weight values. An ASTM A53 schedule-40 reference table, for example, lists 4-inch pipe at 0.237-inch wall and a weight of 10.88 lb/ft, while the widely cited ASME/ANSI value is 10.79 lb/ft. That small gap comes from rounding and specification context, not an error — always confirm the exact standard your project requires.
For a deeper dive into the full ASME/ANSI steel pipe sizing system, Engineering ToolBox’s steel pipes dimensions chart covers all schedules and sizes. The internal cross-sectional area for 4-inch Schedule 40 works out to about 12.73 in², which matters for flow and capacity planning.
Frequently Asked Question
FAQs
Is 4-inch pipe actually 4 inches wide?
No. The “4-inch” label refers to nominal pipe size (NPS), not the physical measurement. The outside diameter of NPS 4 steel pipe is 4.500 inches — the nominal size is a historical reference that doesn’t match any actual dimension on the pipe.
What is the inside diameter of 4-inch Schedule 40 pipe?
The inside diameter is approximately 4.026 inches (102.26 mm) for Schedule 40. For Schedule 80, the thicker 0.337-inch wall reduces the inside diameter to 3.826 inches. Always use the schedule’s specific ID for flow calculations.
Does Schedule 40 and Schedule 80 4-inch pipe use the same fittings?
Yes, for the most part. Both schedules share the same 4.500-inch outside diameter, so flanges and socket fittings that match NPS 4 will fit either. Thread engagement can differ slightly because wall thickness changes, so confirm thread specs against your exact schedule before assembly.
References & Sources
- Engineering ToolBox. “ASME/ANSI B36.10/19 Steel Pipe Dimensions.” Source for NPS 4 outside diameter, wall thickness, inside diameter, and weight figures.