Measure the same area you count
Choose an area inside your gauge swatch, away from its edges. Measure its width and count the stitches across it; measure its height and count the rows up it. Those are the four numbers to enter. The rectangle does not have to be square.
Use the yarn and stitch pattern called for in your project. Follow the pattern’s directions for preparing or blocking the swatch, and measure without pulling it into shape. If your project is worked in the round, make a swatch that represents that method too.
Compare with your pattern
Gauge is a density: stitches divided by measured width, and rows divided by measured height. Compare your result with the gauge and measurement span written in the pattern. This tool shows both 10 cm and 4 inch spans separately; 4 inches is exactly 10.16 cm.
More stitches across the same span means smaller stitches. Fewer means larger stitches. If your gauge does not match the pattern, try a different needle or hook size and make another swatch. Check row gauge separately, especially where shaping depends on row counts.
A measurement, not a pattern rewrite
The results describe the area you measured, with fractional counts preserved. They do not add edge stitches, round to a pattern repeat, or tell you a cast-on count. Match the pattern’s gauge before using its sizing instructions.