Quick answer: Use millimetres as the most reliable hook measurement, then compare the common US label printed on the hook or package.
Common Crochet Hook Sizes
2.25 mm is commonly US B-1; 2.75 mm C-2; 3.25 mm D-3; 3.5 mm E-4; 3.75 mm F-5; 4 mm G-6; 5 mm H-8; 5.5 mm I-9; 6 mm J-10; 6.5 mm K-10.5; 8 mm L-11; 9 mm M/N-13; and 10 mm N/P-15. Labels can vary by manufacturer, so verify the millimetres.
Choose a Hook for Your Yarn
Start with the yarn-label recommendation, then make a swatch. A smaller hook normally creates denser fabric; a larger hook normally creates looser fabric and more drape.
Gauge Matters More Than the Label
Two people can use the same hook and yarn but produce different measurements. Change hook size until the stitch and row gauge match the pattern when fit or finished size matters.
Hook Sizes for Amigurumi
Amigurumi often uses a hook smaller than the yarn label suggests to reduce gaps around stuffing. The fabric should remain workable and should not require painful force.
Measure an Unmarked Hook
Use a hook gauge tool or compare the shaft, not the tapered throat. Do not estimate from the hook head alone. Record the millimetre size in project notes for reproducibility.
Sources
Hook millimetre equivalents follow the Craft Yarn Council crochet hook size standards. Manufacturer stampings can still differ, so confirm the millimetre measurement on the hook itself.
Use the instructions and terminology stated by the pattern you are following. Make a swatch when gauge or finished size matters.