SDR and PN Pressure Class of Polyethylene Pipe: Formula and Table

SDR and the PN pressure class are two numbers seen on every PE pressure pipe, and they depend on each other. SDR is the ratio of outside diameter to wall thickness and describes the pipe's geometry; PN is the nominal pressure in bar and says what continuous pressure the pipe is made for at 20 °C. When the market says "110 pipe, six bar" or "ten bar", it is giving the PN.

The smaller the SDR, the thicker the wall and the more pressure the pipe withstands; so a smaller SDR means a higher PN. This holds for both PE100 and PE80, but because PE100 has a higher design stress it gives a higher PN at the same SDR.

In this guide you will find the formula, a full SDR and PN table for the two grades, the wall and inside diameter of 110 mm pipe at each SDR from our catalogue rows, the effect of temperature and water hammer, and a guide to ordering.

• PE pipe size chart in inches and millimetres
• PE100 water pressure pipe

SDR comes from dividing the outside diameter by the wall thickness. A 110 mm pipe with a 10 mm wall has an SDR of 11 (110 ÷ 10 = 11). The same pipe with a 6.5 mm wall is SDR 17 (110 ÷ 6.5 ≈ 17) and with a 14.9 mm wall SDR 7.4. So from diameter and wall you can work out SDR, and from SDR and diameter the wall.

The important result for the buyer is that two pipes of the same diameter but different SDR have a different inside diameter and weight. The inside diameter comes from "outside diameter minus twice the wall"; for 110 mm with a 10 mm wall it is 90 mm. This is what you see in the wall table below.

The markings printed on the pipe body normally show the material grade, outside diameter, SDR and nominal pressure side by side. Reading them before installation helps you confirm that the pipe delivered is the one you ordered and that a pipe of the same diameter but another class has not entered the line.

Nominal pressure is calculated from PN = 20 × design stress ÷ (SDR − 1). The allowable design stress is 8 MPa for PE100 and 6.3 MPa for PE80. With SDR 11 in PE100 we get 20 × 8 ÷ 10 = 16 bar; with SDR 17 we get 20 × 8 ÷ 16 = 10 bar; and with SDR 7.4 we get 20 × 8 ÷ 6.4 = 25 bar.

In PE80 the same SDR 11 gives 20 × 6.3 ÷ 10 = 12.6, which the standard table rounds to 12.5 bar. These two design stresses come from the minimum required strength (MRS), which is 10 MPa for PE100 and 8 MPa for PE80. The pipes are listed to ISO 4427 and EN 12201-2 with these relations.

In practice you do not have to solve the formula each time; the table in the next section has done the calculation for all common SDRs. Still, knowing the formula lets you see the logic of the table: each step up in wall thickness raises the allowable pressure in inverse proportion to SDR minus one, so the first steps have the largest effect.

The table below shows the pressure class of each SDR in the two grades. For the same PN, PE80 needs a smaller SDR, that is a thicker wall: for 16 bar, PE100 at SDR 11 is enough but PE80 must be SDR 9. SDR 41 appears only in PE100, for 4 bar.

If the designer has given only PN, you can find the SDR from this table, and if only the SDR is printed on the pipe, you can find the PN. In an order it is best to write both so there is no mistake.

Another important point is that the table shows only the standard rows, and intermediate values such as 7 bar are not in it. If your design pressure falls between two classes, always take the higher class and never accept the lower one on the argument that it is "only a little less".

The table below puts the wall thickness, inside diameter, weight per metre and PN of each SDR side by side for PE100 pipe with a 110 mm outside diameter (4 inch). You can see that from SDR 41 to SDR 7.4 the wall rises from 2.7 to 14.9 mm, the inside diameter falls from 104.6 to 80.2 mm and the weight per metre rises from 0.943 to 4.49 kg.

That means every step up in pressure has a price: more weight and less flow capacity. So do not take a class above what you need; choose the class from the real line pressure and the considerations in the next sections.

If you use the pipe for drinking water or irrigation, the inside diameter determines flow velocity and changes with SDR. To choose the right diameter use our flow calculator and enter the inside diameter of each SDR in the calculation, so you see the real water velocity and not that of the nominal diameter.

• 110 mm PE pipe
• PE pipe simulator
• Flow and pipe size calculator

The pressure classes in the table are valid only for water at 20 °C. If the fluid temperature rises, the polymer loses strength and a pressure reduction factor from the standard table must be applied in the calculation; for this reason ordinary PE pressure pipe is not suitable for hot fluid. These factors are not given in this article and are taken from the standard and the designer.

When choosing a class, consider the real water temperature too; for example a pipe laid on the ground in the sun, whose water warms up, is not in the same conditions as a buried pipe. If you are not sure, state the temperature and installation conditions in your enquiry and agree them with the designer.

The static pressure of a line changes with elevation: each 10 m of water column creates about 1 bar, so at the lowest point of a transmission line with a large elevation difference the pressure is higher than at the highest point. That is why classes of 16, 20 and 25 bar at SDR 11, SDR 9 and SDR 7.4 are used on steep lines.

Dynamic pressure matters too: water hammer from a valve closing suddenly or a pump stopping, and the hydrostatic test pressure on site, can exceed the working pressure. Close valves slowly, fit check valves and air valves correctly, and set the final class with the design engineer from these pressures.

On large projects it is better to get the pipe data sheet and the class needed for each part of the route from the designer and send it as a table with the enquiry. This avoids the common mistake of ordering the whole line in one class when the low-pressure sections could have been lighter.

• Valves for polyethylene pipe

In your enquiry write the grade (PE100 or PE80), outside diameter, SDR and PN together; for example "PE100, 110 mm, SDR 17, 10 bar". If you have only the PN, find the SDR from the table above, and for PE80 watch for the smaller SDR. Add the supply form (coil or bar) and the length.

We give technical advice on choosing the class before you buy and issue an official invoice, but the final pressure class of each line is for the designer. You can see the approximate daily price of each size on the price page, and you send your list from the contact page.

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• PE100 vs PE80 polyethylene pipe
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