What is the inner diameter of a 1/4" hex nipple?

Does anyone know the rough inner diameter of a stainless 1/4" hex nipple similar to this? http://www.hoseshop.co.nz/product-detail...

Is it actually 1/4"? Generally I find sizes applied to fittings and pipes are usually not quite the inner diameter.

Pipe sizes reflect inside diameter, tube sizes reflect outside diameter.

The bore through the nipple would be about 10 to 15 thousanth of an inch bigger than quarter inch. The thread size would more more than likely be 1/4" british standard pipe.
1/4 stainless pipe is usually outside diameter and the pipe goes inside the nipple and usually a ferrule is tightened on to make it sealed.

Is that ID or OD Most diameters of pipes refer to ID or interior diameter. The hex part in your description caught me off guard, glad you included the picture. The 1/4 inch could be the thread size as well. I would e-mail the company for clarification. Based on the other pictures (up one page on the web site I would think it is thread size. The thread size most often is also the ID of the pipe. But not always.

It depends on several factors, for example is this American (NPT) or British (BSP) pipe?

1/4" pipe thread can have different inner diameters. The outer diameter is standard, but pipes come in different wall thicknesses, designed to have different maximum pressure limits, which results in different inner diameters.

*** If this is American pipe *** (NPT or NPS), then here are the possible i.d.'s for 1/4":

0.410" for "Schedule 10" pipe
0.364" for "Schedule 40" pipe
0.302" for "Schedule 80" pipe

I am not sure what the inner diameter is if this is British pipe, which unfortunately seems likely as your link points to a New Zealand company's website.

1/4" !