Use a cloth or plastic tape measure like the kind for taking your own measurements. If you don't have a helper, go from the center of the chest straight along his body, stop where your arms reach, note the measurement, reposition the tape at your stopping point and continue to measure to the edge or center of the tail. I'd always heard edge of tail but some blankets probably measure differently. Most blankets go by 3" increments. Combine your two measurements and you're good to go.