Buying a needle gauge lets you identify the true diameter of your tools: insert the tip into each hole until you find the size that fits snugly without forcing. If it feels loose or scratches as it passes through, it’s not the right size. With a knitting needle gauge, also check the number engraved on the needle if present, since older or unmarked sets can be misleading. Repeat the check on both needles if you’re working in pairs, to ensure accuracy and avoid tension differences.
With a crochet hook sizer, slide the shaft into the matching slot and confirm that it sits flush along its full length. Avoid measuring only at the tip, as some hooks taper. Before casting on your swatch, note the actual size and yarn you’re using. Keeping that record helps you reproduce results across sessions. If you switch between different brands or materials, measure again: small variations in the shaft can alter your gauge without being obvious to the eye.