Inside Diameter (ID) for Flanges

<...what is the parameter that i can calculate the inside diameter...>

The flowrate of the fluid in the pipe, and its temperature and pressure. Then pick the nearest standard nominal size over the diameter value that comes out.

Generally, choose a liquid line size between 1 and 3ms^-1 provided there is no risk of static electricity build-up in a fast-flowing non-conductive liquid, otherwise stay below 1ms^-1. Gases, oh, say 3-10ms^-1.

In the drawing, the inside diameter is B, not A. If the purchaser specifies the inside diameter, you do not need to calculate it--it is already given.

On the other hand, if you are responsible for determining pipe sizes, you will need to use flow/temperature/pressure formulas to calculate the required pipe size, followed by selecting matching flanges.

The inside diameter of the flange is often determined by the inside diameter of what it is attached to - for example a pipe or valve. If something has to go through the pipe ( a cleaning , gauging or inspection tool (PIG), then the bore should be the same throughout. A small difference is allowable but the interface should be tapered (1 in 4 usually) so that the PIG can go through without damage. Even if the line is not piggable, changes in ID affect flow. The flange usually has a smaller ID so that it can be used for thicker wall pipes. The flange spec will state the maximum allowable ID, taking into account pressure and temperature.

For all butt welded fittings (reducers, elbows, tees, weldolets, ... etc.) and flanges (WN and RTJ), the end dimensions (OD, Schedule, and beveled angle) must be equal to the corresponding dimensions of the pipe where the fitting and/or flange shall be connected.

And the main step to find the inner diameter of such a pipe (pipe sizing) is mainly depends on fluid mechanics related to the targeted flowrates, drop of pressures, pipeline length, ... etc. then select the nearest standard nominal size as indicated at post #1 by PWSlack.

When ordering weld neck flanges, because they are butt welded onto the end of pipe the bores of the pipe and flange must be the same. The outside diameter of the weld neck is the same as the outside diameter of the pipe.

It is usually sufficient to order the flange as Nominal Bore, Rating in lbs and pipe schedule.

Some places carry only XXS (the samallest bore) in stock and machine the bore when needed to suit the bore of the pipe.