Stainless Steel Pipe Roughness: Absolute and Relative Values Chart

For friction-loss calculations, new stainless steel pipe is usually given an absolute roughness (ε) of about 0.0015–0.015 mm (0.000005–0.00005 ft, or 0.00006–0.0006 in). Smooth drawn or polished tube sits at the low end. Ordinary welded or seamless process pipe is commonly taken as 0.015 mm, which is a safe working value. That is about three times smoother than commercial carbon steel (0.045 mm). The value is the same for 304, 316 and other grades, because roughness comes from how the pipe is made and finished, not from its chemistry. This guide gives the values in every common unit, relative roughness (ε/D) for standard pipe sizes, a worked pressure-drop example, and the difference between hydraulic roughness and the Ra surface finish used for sanitary tube.

Absolute roughness of stainless steel pipe in mm, m, in and ft

Absolute roughness, written ε (epsilon) or k, is the “equivalent sand-grain roughness” used in the Darcy–Weisbach method. It is a hydraulic number: a surface with roughness ε causes the same friction as a pipe lined with uniform sand grains of height ε. It is not measured directly with a gauge. These are the values usually used for stainless steel:

Stainless pipe or tube condition	mm	µm	m	in	ft
Drawn, bright-annealed or polished tube (smooth)	0.0015	1.5	1.5 × 10⁻⁶	0.00006	0.000005
New welded or seamless process pipe (typical design value)	0.015	15	1.5 × 10⁻⁵	0.0006	0.00005
Conservative allowance (same as commercial steel)	0.045	45	4.5 × 10⁻⁵	0.0018	0.00015

Published tables don’t all agree, because “stainless steel pipe” covers everything from electropolished pharmaceutical tube to hot-finished heavy-wall pipe. Most give a figure between 0.0015 and 0.015 mm. If your software has a “stainless steel” entry, it will almost always be in that range. When in doubt, 0.015 mm is the usual middle-of-the-road choice. If you need the bore first, the stainless steel pipe guide covers sizes and schedules.

How stainless compares with other pipe materials

These are the classic Moody-chart values for new pipe. They are typical figures. Real pipes vary.

Material (new)	Typical ε (mm)	Typical ε (ft)	Rougher than smooth stainless tube by about
Drawn tubing: stainless, copper, brass, glass	0.0015	0.000005	1×
Plastic pipe (PVC, PE)	0.0015–0.007	0.000005–0.00002	1–5×
Stainless process pipe (design value)	0.015	0.00005	10×
Commercial carbon steel, wrought iron	0.045	0.00015	30×
Galvanized steel	0.15	0.0005	100×
Cast iron (uncoated)	0.26	0.00085	170×
Concrete	0.3–3	0.001–0.01	200–2,000×

The bigger difference shows up over time. Carbon steel and cast iron rust, scale and build up tubercles, so their roughness can grow many times over in water service. Stainless doesn’t form rust scale in normal water, so it stays close to its new value for much longer. See does stainless steel rust in water for the conditions where it can.

Relative roughness (ε/D) for common stainless pipe sizes

The Moody chart and the Colebrook equation use relative roughness: ε divided by the inside diameter, in the same units. Smaller pipes have a higher relative roughness for the same surface. The table uses ASME B36.19M bores for Schedule 10S and 40S, the two most common stainless schedules.

NPS (DN)	Schedule	Inside diameter	ε/D at 0.0015 mm	ε/D at 0.015 mm	ε/D at 0.045 mm
½” (DN15)	10S	0.674 in (17.1 mm)	0.000088	0.00088	0.0026
½” (DN15)	40S	0.622 in (15.8 mm)	0.000095	0.00095	0.0028
¾” (DN20)	40S	0.824 in (20.9 mm)	0.000072	0.00072	0.0022
1″ (DN25)	10S	1.097 in (27.9 mm)	0.000054	0.00054	0.0016
1″ (DN25)	40S	1.049 in (26.6 mm)	0.000056	0.00056	0.0017
1½” (DN40)	40S	1.610 in (40.9 mm)	0.000037	0.00037	0.0011
2″ (DN50)	10S	2.157 in (54.8 mm)	0.000027	0.00027	0.00082
2″ (DN50)	40S	2.067 in (52.5 mm)	0.000029	0.00029	0.00086
3″ (DN80)	40S	3.068 in (77.9 mm)	0.000019	0.00019	0.00058
4″ (DN100)	10S	4.260 in (108.2 mm)	0.000014	0.00014	0.00042
4″ (DN100)	40S	4.026 in (102.3 mm)	0.000015	0.00015	0.00044
6″ (DN150)	40S	6.065 in (154.1 mm)	0.0000097	0.000097	0.00029
8″ (DN200)	40S	7.981 in (202.7 mm)	0.0000074	0.000074	0.00022

The bores are the same for 304 and 316 pipe. Only the schedule changes them. For full wall thicknesses, see 316 stainless steel pipe dimensions. Tube is different: it is sized by actual outside diameter and wall, so work out the bore from the tube specification (see stainless steel tube).

Does the roughness value actually matter? A worked example

Often less than people expect. Stainless pipe is so smooth that, at normal flow rates, it behaves close to a “hydraulically smooth” pipe. The surface bumps are buried in the thin viscous layer next to the wall. Here is water at 20 °C flowing at 2 m/s (6.6 ft/s) through 2″ Schedule 40S pipe (52.5 mm bore). The Reynolds number is about 105,000, and the friction factor comes from the Colebrook equation:

Roughness used	Darcy friction factor f	Pressure drop per 100 m of straight pipe	vs. perfectly smooth
Perfectly smooth (ε = 0)	0.0178	68 kPa (0.68 bar, 9.8 psi)	n/a
0.0015 mm (smooth tube)	0.0180	68 kPa (0.68 bar, 9.9 psi)	+1%
0.015 mm (stainless design value)	0.0193	73 kPa (0.73 bar, 10.6 psi)	+8%
0.045 mm (commercial steel)	0.0216	82 kPa (0.82 bar, 11.9 psi)	+21%
0.15 mm (galvanized, or badly fouled pipe)	0.0271	103 kPa (1.03 bar, 15.0 psi)	+52%

Two lessons follow from this:

• Picking 0.0015 or 0.015 mm changes the result by only a few percent. It isn’t worth agonizing over. Fittings, valves and the exact flow rate usually matter more.
• The penalty grows in larger pipes and at higher velocities, where flow moves toward the fully rough regime. It also grows if the pipe fouls. Scale, biofilm or deposits can make an old pipe behave like a much rougher one, whatever it is made of.

The calculation uses Darcy–Weisbach: Δp = f × (L/D) × ρv²/2. With ρ = 998 kg/m³, L = 100 m and D = 0.0525 m, a friction factor of 0.0193 gives 0.0193 × 1,905 × 1,996 ≈ 73,300 Pa. If you prefer an explicit formula to iterating Colebrook, the Swamee–Jain approximation, f = 0.25 / [log₁₀(ε/3.7D + 5.74/Re^0.9)]², gives the same answer to within about 1% here.

How to use stainless roughness in a pressure-drop calculation

1. Find the real bore. Use the inside diameter for your size and schedule (or tube OD minus two walls). Don’t use the nominal size.
2. Choose ε. Use 0.015 mm (0.00005 ft) for ordinary stainless pipe and 0.0015 mm for drawn, polished or electropolished tube. Use 0.045 mm if you want a margin for deposits or an older system.
3. Work out relative roughness ε/D, keeping both in the same unit.
4. Work out the Reynolds number, Re = vD/ν. Water at 20 °C has ν ≈ 1.0 × 10⁻⁶ m²/s. Below about 2,300 the flow is laminar, and f = 64/Re regardless of roughness.
5. Get the friction factor from the Moody chart, the Colebrook equation or Swamee–Jain.
6. Calculate the straight-pipe loss with Darcy–Weisbach, then add the losses for fittings, valves and equipment separately. For short stainless runs, these often outweigh the pipe friction.

If you work with the Hazen–Williams formula instead (common for water and fire systems in the US), stainless steel is usually given a C factor of about 140–150. Hazen–Williams is only valid for water at ordinary temperatures. For other fluids or for accurate work, use Darcy–Weisbach. Pipe schedule also sets the pressure the pipe can hold; see schedule 10 pipe pressure rating and the 304 stainless pipe pressure rating chart.

Hydraulic roughness vs surface finish (Ra)

Two different numbers are both called “roughness”, and they are often confused:

	Absolute roughness ε (k)	Surface finish Ra
What it describes	Equivalent sand-grain height that gives the same friction	Arithmetic mean height of the surface profile, measured with a profilometer
Used for	Friction factor, pressure drop, pump sizing	Cleanability, hygiene, product contact, appearance
Standards	Moody chart, Colebrook–White equation	ISO 4287 / ISO 21920, ASME B46.1; ASME BPE and 3-A for hygienic tube
Typical stainless values	0.0015–0.015 mm (1.5–15 µm)	0.38–0.8 µm (15–32 µin) for polished sanitary tube

There is no exact conversion between the two. A surface with a given Ra usually behaves hydraulically as if its equivalent sand roughness were somewhat larger, because ε also reflects waviness, weld seams and the shape of the peaks. For very smooth stainless tube the difference doesn’t change the friction factor much, as the example above shows. Use ε values in flow calculations and Ra values in hygiene specifications.

Sanitary and high-purity finishes

In food, dairy, beverage and pharmaceutical systems, the inside finish is specified as a maximum Ra, because smoother surfaces are easier to clean and give bacteria fewer places to hold on. The usual reference points are:

Specification	Max Ra (µin)	Max Ra (µm)	Finish
3-A Sanitary Standards (product contact)	32	0.8	Mechanically polished or equivalent
ASME BPE SF1	20	0.51	Mechanically polished
ASME BPE SF2	25	0.64	Mechanically polished
ASME BPE SF3	30	0.76	Mechanically polished
ASME BPE SF4	15	0.38	Mechanically polished and electropolished
ASME BPE SF5	20	0.51	Mechanically polished and electropolished
ASME BPE SF6	25	0.64	Mechanically polished and electropolished

Sanitary tube is usually ordered to ASTM A270 with the ID finish stated on the order. Ordinary ASTM A312 pipe has no bore finish requirement. It is normally supplied annealed and pickled, with a matt surface that is clean but noticeably rougher than polished tube. For how finishes look and compare on the outside of the metal, see brushed vs polished stainless steel.

What makes one stainless pipe rougher than another

Factor	Effect on roughness
Manufacturing route	Cold-drawn and bright-annealed tube is the smoothest. Welded pipe made from cold-rolled strip often has a smoother bore than hot-finished seamless pipe, apart from the weld seam.
Heat treatment and pickling	Annealing in air followed by pickling leaves a matt, slightly etched surface. Bright annealing in a protective atmosphere keeps the surface smooth and shiny.
Internal polishing and electropolishing	Mechanical polishing cuts the peaks down. Electropolishing then smooths and cleans the surface further and is used for the lowest Ra classes.
Weld beads and joints	Internal weld protrusions and misaligned joints add local losses. In hygienic systems they are also places where product can collect, which is why orbital welding with full penetration and a flush bead is specified.
Grade (304, 316, duplex)	No meaningful effect on roughness. The grade matters for corrosion resistance; see 304 vs 316 stainless steel and the stainless steel grades guide.
Service history	Hard-water scale, biofilm, product deposits or corrosion pitting all raise the effective roughness over time.

Welding heat tint inside a pipe should be removed by pickling or another suitable cleaning method, because it weakens corrosion resistance and any later pitting roughens the surface. See the stainless steel weld color chart and acid cleaning stainless steel after welding.

Roughness of old stainless steel pipe

A clean stainless pipe in water service stays close to its new roughness for decades, because it doesn’t rust or form tubercles the way carbon steel and cast iron do. The effective roughness of an older stainless line can still rise for other reasons:

• Scale from hard water or heated water (calcium carbonate deposits).
• Biofilm and slime in untreated or warm water.
• Product deposits in process lines, such as milkstone, sugar or fats, if cleaning-in-place isn’t effective.
• Corrosion pitting from chlorides, stagnant water or crevices, especially at welds that weren’t cleaned and passivated.

There is no standard “aged stainless” figure. For an existing system, the honest approach is to measure the pressure drop at a known flow and back-calculate the friction factor. If you have to estimate, using the commercial-steel value of 0.045 mm for a stainless line with some deposits is a common, conservative assumption. Keeping the surface passive is the best way to keep it smooth: see passivation of stainless steel pipe and why stainless steel rusts.

Measuring the surface roughness of stainless pipe

Ra and related parameters (Rz, Rmax) are measured with a contact stylus profilometer. A diamond tip is drawn along the surface over a set evaluation length, following ISO 21920 (which replaced ISO 4287) or ASME B46.1. For tube bores, small-diameter probes reach a short distance inside the cut end. For sanitary components, the reading is usually taken on samples or at accessible ends. Optical (non-contact) instruments such as white-light interferometers are used in laboratories and for very smooth electropolished surfaces. None of these methods measures ε directly. Absolute roughness is a hydraulic property inferred from flow tests.

Frequently asked questions

What roughness should I use for stainless steel pipe?

Use 0.015 mm (0.00005 ft, 0.0006 in) for new welded or seamless stainless pipe. Use 0.0015 mm (0.000005 ft) for drawn or polished tube, and 0.045 mm if you want a conservative allowance for deposits.

What is the absolute roughness of 316 stainless steel tubing?

The same as any other stainless tube of the same finish: about 0.0015 mm (1.5 µm) for drawn or bright-annealed tubing. The grade doesn’t change roughness; the manufacturing method and the bore finish do.

Is 304 pipe rougher than 316 pipe?

No. 304 and 316 pipe made the same way, to the same specification, have the same roughness and the same bore for a given schedule. The difference between them is corrosion resistance.

What is the roughness of stainless steel pipe in feet?

About 0.000005 ft for smooth tube and 0.00005 ft for typical process pipe. The commercial-steel value often used as a conservative figure is 0.00015 ft.

What is the relative roughness of 1-inch (DN25) stainless pipe?

With ε = 0.015 mm, Schedule 40S pipe (26.6 mm bore) has ε/D ≈ 0.00056, and Schedule 10S (27.9 mm bore) has ε/D ≈ 0.00054. With 0.0015 mm, both are about 0.000055.

Can I use the Ra value from a tube certificate as the pipe roughness?

Not directly. Ra is a profile average used for hygiene specifications, while ε is an equivalent sand-grain roughness for friction calculations. For polished tube, use ε ≈ 0.0015 mm. At these levels the pipe is close to hydraulically smooth, so the exact figure has little effect on the result.

What is the Hazen–Williams C factor for stainless steel?

Usually about 140–150 for new, clean stainless pipe. Hazen–Williams is an empirical formula for water only. For other fluids or temperatures, use Darcy–Weisbach with an absolute roughness value.

Last updated: November 2026. Roughness values are typical design figures from standard friction-factor practice, not guaranteed properties of any product. Pipe bores follow ASME B36.19M. The pressure-drop example is calculated with the Colebrook equation for straight pipe only. Check the current editions of ASME BPE and 3-A standards for hygienic finish requirements, and have critical systems designed by a qualified engineer.