One of my co-workers mentioned in passing that he had a digital caliper set that could measure down to 0.00mm and below. I was working on knit project that was to the point where I needed to use several sets of needles at the same time to get around the circumference, and one set in particular wasn't marked as to size. Using the needle gauge, it was pretty close to what I needed. So I asked him if I brought all my hooks and needles to work one night, could he bring it in so I could measure what I had. The next night he surprised me by handing the set to me and saying have fun! Smart move on his part! It took me two hours to go through all my "utensils", marked and unmarked!
So here's what I found out. Very few of the needles and hooks were exactly to stated size. The range varied from a couple hundredths of a millimeter to almost one quarter of a millimeter.
So I thought I would share some pictures with you of my "experiment". All the hooks used in these pictures were labeled as size H/5.00mm.
All measurements were taken right at the bottom slant angle of the throat area to start with. Then measured further down the shank to see if any of them came out to exact stated mm size. The Boye hook was the biggest surprise for me. I didn't get to 5.00 mm until just above the thumbrest area!!. I don't know about you, but I sure don't have my yarn that far down the hook to make my stitches! I'm usually just into the shank portion from the throat.
It just proves to me that the hook size stated in a pattern really is just a suggestion, and that a gauge swatch really should be done to get an accurate finished product! But be aware! Even using the same hook throughout a project doesn't mean your gauge won't change! Mood has a lot to do with it, too!