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A soft tape and about two minutes: mid-neck for collars, the widest rib point for harnesses, two fingers of clearance for both.
For a collar, stand the dog and wrap a soft tape around the neck where the collar will sit — mid-neck, not up under the ears and not down on the shoulders. Keep the tape snug against the coat without pressing in, and record the figure in centimetres and inches.
Add 5-7 cm (2-3 in) to the neck reading to get the collar's working length, then verify on the dog: two adult fingers must slide flat under the buckled collar. That check is the pass mark, and fingers held on edge do not count.
For a harness, the tape moves to the chest: measure the ribcage at its widest point. The three common bands are 30-45 cm (12-18 in) small, 45-65 cm (18-26 in) medium and 65-90 cm (26-35 in) large, and every strap of the fitted harness — not just the chest piece — should pass the two-finger check.
On narrow screens, swipe or scroll the plate sideways.
Puppies break the one-and-done assumption. Re-measure every 3-4 weeks through the first six months, because neck girth often gains 2-5 cm (1-2 in) a month in that window; each re-measurement is a fresh sizing decision, not a tweak.
Four named failures cause most bad fits. The high-neck error reads small because the tape rides toward the ears. The fur squeeze reads small because the tape compresses a thick coat. The dangling tape reads large and produces a collar that slips over the head. The skipped strap check leaves one harness strap — usually the one behind the elbows — tighter than the rest.
Once the numbers exist, the Dog Collar Size Chart converts them into a size label and strap width, and the Leash Length Guide picks the lead length for the walks planned.
Further reading