Why is a 2 inch pipe not 2 inches?

Nominal size is a label that once referred to the bore of a thick-walled pipe. The outside diameter is standardised so fittings fit, and the actual figure for 2 inch nominal is 60.3 mm.

Does schedule 80 flow less than schedule 40?

Yes at the same nominal size. The outside diameter is identical and the wall is thicker, so the bore is smaller.

Conversionstechnical

Pipe Size Converter

Pipe nominal size to actual dimensions. Nominal size is a label, not a measurement: a 2 inch pipe has an outside diameter of 60.3 mm, and the schedule sets the wall thickness and therefore the bore.

Also called: nps to mm, pipe schedule chart.

Outside diameter

60.3

A 2 inch pipe has an outside diameter of 60.3 mm, wall 3.61 mm and bore 53.08 mm. Nominal size is a label rather than a measurement, which is why the outside diameter is not the nominal figure. At 200 litres a minute the velocity is 1.51 m/s.

Inside diameter

53.08

Wall thickness

3.61

DN equivalent

Flow velocity

1.51

Litres per metre

2.21

On nominal size

Nominal size is a label rather than a measurement, which is why the outside diameter is not the nominal figure.

How this is calculated

Method and background

How this is calculated

Nominal pipe size stopped corresponding to any dimension long ago. The outside diameter is fixed for a given nominal size so that fittings interchange, and the schedule number sets the wall thickness, which means schedule 80 has the same outside diameter as schedule 40 and a smaller bore. That is the practical consequence: upgrading to a thicker schedule for pressure reduces flow capacity at the same nominal size.

velocity is flow over the bore area, and the bore depends on the schedule, not just the nominal size

Q: Flow rate
d_i: Inside diameter

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a two inch schedule 40 pipe

Nominal pipe size: 2 inches
Schedule: Schedule 40
Flow rate: 200 litres/min

Outside diameter60.3

ASME B36.10 gives 60.3 mm for 2 inch nominal

Open this example

schedule 80 has the same outside diameter and less bore

Nominal pipe size: 2 inches
Schedule: Schedule 80
Flow rate: 200 litres/min

Outside diameter60.3

boundary: the property that makes fittings interchange

Open this example

Method and limits

What it assumes

ANSI/ASME B36.10 dimensions for carbon steel pipe.

What it deliberately does not model

Copper tube and plastic pipe use different sizing conventions again.
Velocity here ignores fittings and friction losses.

Formula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator

Frequently asked questions

Why is a 2 inch pipe not 2 inches?: Nominal size is a label that once referred to the bore of a thick-walled pipe. The outside diameter is standardised so fittings fit, and the actual figure for 2 inch nominal is 60.3 mm.
Does schedule 80 flow less than schedule 40?: Yes at the same nominal size. The outside diameter is identical and the wall is thicker, so the bore is smaller.