Half inch tubing is literally the backbone of many drip irrigation systems. It is by far the most popular size used. The only problem is half inch tubing isn’t half inch. It’s close! Closer than ‘hand grenade’ close. More like ‘electric razor’ close.
Piping has specific dimensions. Steel, iron, copper, pvc all have set standards set by ASTM International. This means that the steel pipe you buy in Maine will fit the fittings you buy in Nebraska and connect to the existing pipe in Alaska.
Plastic tubing? No, no real standards. The size can vary from manufacturer to manufacturer or even within the same manufacturer. The term ½” is known as the nominal size, or the industry trade description of the product. As they say in the diet commercials, your results may vary. A lot, actually.
Irrigation 1/2-inch polyethylene tubing is available in different configurations:
1/2-inch – .570″ ID x .670″ OD 1/2-inch – .580″ ID x .700″ OD
1/2-inch – .600″ ID x .700″ OD 1/2-inch – .620″ ID x .710″ OD
Why do you care? Honestly, the sizes are so close they won’t have much effect on water flow, especially the two biggest. You care because fittings don’t always fit. It’s easy to buy a ½” fitting that won’t fit a ½” tube.
It is important when building a drip system to check the internal diameter of the tubing against the size of the fittings you need. While always buying the same brand of tubing and fittings help it is not a guarantee of fit. The two fittings in the picture are from the same company. They are not interchangeable. If you put the .520” in a .600 ID tube and clamp down tight enough it should hold. You can’t put the .600” in a .520” tube without deforming the tube.
Before you buy your system take a moment and verify dimensions. Look at the barb fittings and you see the specs are given for each piece. All you need to do is match them to your tubing.
Fortunately, ½” tubing seems to be the only product with this problem. The ¼”, ¾” and 1” are all consistent in sizing.