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Minimum wall thickness and MAWP

Straight pipe under internal pressure. Work out the thickness a design pressure needs, or the pressure a given wall can carry. You type the allowable stress and the factors from your design basis.

Inputs

Pipe size (NPS)Optional. With a schedule it fills D and the nominal wall from ASME B36.10M and B36.19M.

Schedule

Outside diameter D

Nominal wall

Design pressure

bar

Design temperature

°C

The temperature your allowable stress applies at.

Allowable stress S

MPa

Type the value from your design basis. The tool holds no material tables.

Quality factor E

Weld strength reduction W

Coefficient Y

Mill under-tolerance

Corrosion allowance

Erosion allowance

Mechanical allowance

Example values are filled in. Replace them with your own.

Minimum thickness t_m

3.911mm

Pressure thickness plus allowances (no structural minimum entered). The wall given is adequate.

The working

Design pressure in MPa (typed in bar)P in bar / 104 MPa (gauge)
Pressure design thicknesst = P x D / (2 x (S x E x W + P x Y))2.411 mm
Total allowancescorrosion + erosion + mechanical1.5 mm
Pressure thickness plus allowancest_m = t + c3.911 mm
Governing thicknesslarger of the above3.911 mm
Nominal wall required after mill under-tolerancegoverning / (1 - m / 100)4.47 mm
Wall available after mill under-tolerancet_nom x (1 - m / 100)6.221 mm
Margin over the governing thicknessavailable - governing2.31 mm

Against the nominal wall

Nominal wall needed after mill tolerance4.47 mm
Wall available after mill tolerance6.221 mm
Margin2.31 mm

For guidance only. Verify against the code edition in force and your own design basis.

This pipe wall thickness and MAWP calculator gives the minimum wall for a design pressure, or the allowable pressure (MAWP) of a given wall, by the ASME B31.3 thin-wall equation. Free, no sign-up.

The formula

Thickness: t = P x D / (2 x (S x E x W + P x Y)). With allowances: t_m = t + c.
Nominal wall needed = t_m / (1 - m / 100), where m is the mill under-tolerance in per cent.
MAWP: t = nominal wall x (1 - m / 100) - c, then P = 2 x t x S x E x W / (D - 2 x t x Y).

Worked example

InputNPS 6 schedule 40 (D = 168.3 mm, wall 7.11 mm), 40 bar, S = 138 MPa (illustrative), E = 1, W = 1, Y = 0.4, m = 12.5 %, c = 1.5 mm

Thicknesst = 4 x 168.3 / (2 x (138 + 4 x 0.4)) = 2.411 mm; t_m = 3.911 mm

Nominal wall needed3.911 / 0.875 = 4.470 mm; the 7.11 mm wall leaves 6.221 mm, a margin of 2.31 mm

MAWPt = 6.221 - 1.5 = 4.721 mm; P = 7.920 MPa = 79.20 bar

Limits

Straight pipe under internal pressure only, and thin wall only (not once the wall is D / 6 or more, or P / (S x E) is above 0.385). You type S, E, W and Y, and a line is limited by its weakest component. It is a guide and does not replace a notified or approved body's assessment, your design basis or the code edition in force.

Common questions

How do you calculate the minimum wall thickness of a pipe?

Use the ASME B31.3 thin-wall equation, t = P x D / (2 x (S x E x W + P x Y)), then add the corrosion, erosion and mechanical allowances. Divide by (1 - mill under-tolerance / 100) to find the nominal wall to order. For 40 bar on a 168.3 mm pipe with S = 138 MPa that is 2.411 mm, then 3.911 mm, then 4.470 mm.

What is MAWP and how is it worked out for a pipe?

Here, MAWP is the highest internal gauge pressure the pipe wall can carry. The calculator takes the nominal wall, removes the mill under-tolerance and the allowances, then solves the same equation for pressure. A line as a whole can be limited by a weaker part, such as a flange.

Why include a mill under-tolerance?

Pipe is supplied with a permitted tolerance on wall thickness, so the wall you receive can be thinner than nominal. For seamless pipe 12.5 % under is the usual figure. For plate it comes from the plate specification and is not necessarily zero.

Can I use the answer as the maximum pressure for my whole line?

No. The answer is for the straight pipe wall only. Flanges, valves and fittings can limit the line, so confirm against your design basis and the code edition in force.

Unit conversions

Reference values for the units this tool works in. Rounded for reading; the calculator itself works to full precision.

Pressure and stress

MPa	bar	psi	ksi
0.1	1	14.5	0.0145
0.5	5	72.52	0.0725
1	10	145	0.145
2	20	290.1	0.2901
5	50	725.2	0.7252
10	100	1,450	1.45
20	200	2,901	2.901
50	500	7,252	7.252
100	1,000	14,504	14.5
138	1,380	20,015	20.02
200	2,000	29,008	29.01
250	2,500	36,259	36.26

1 MPa = 10 bar = 145.04 psi. 1 ksi = 6.895 MPa.

Length

mm	in
1.5	0.0591
3	0.1181
5	0.1969
6	0.2362
6.35	0.25
8	0.315
10	0.3937
12.7	0.5
16	0.6299
20	0.7874
25.4	1
50	1.969
100	3.937

1 in = 25.4 mm.

Temperature

°C	°F
20	68
100	212
150	302
200	392
250	482
300	572
400	752
500	932
600	1,112

°F = °C x 9/5 + 32.

You choose the inputs

The tool holds no material tables: the stress, quality factor and coefficients are the ones you type, so they are always the ones in your design basis. Choose a pipe size and schedule (ASME B36.10M and B36.19M) to fill the outside diameter and nominal wall, then change either if your pipe differs.

Allowances stay visible

Mill under-tolerance, corrosion, erosion and mechanical allowances are always shown, never hidden, and added in the working.

Straight pipe only

This is a thin-wall check for straight pipe. Bends, branches, flanges and external pressure need the code and your engineer.