One measurement, four sizes

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Short answer: measure the neck where the collar will sit, add 5-7 cm (2-3 in), and match the result to a size band — S, M, L or XL. The fit is right when two adult fingers slide flat underneath. Everything else — strap width, harness size, leash length — follows from the same tape.

Neck and chest girth mapped to collar sizes, strap widths and harness bands
Gear and size	Body measurement (cm)
Collar S	Neck 25-35 cm
Collar M	Neck 35-50 cm
Collar L	Neck 50-65 cm
Collar XL	Neck 65-80 cm
Harness S	Chest 30-45 cm
Harness M	Chest 45-65 cm
Harness L	Chest 65-90 cm

The vocabulary of the tape

Neck girth: The circumference of the neck measured with a soft tape where the collar will sit. It is the single input for every collar size chart.
Chest girth: The circumference of the ribcage at its widest point, used to size harnesses. Every strap of the fitted harness should pass the two-finger check.
Two-finger rule: The standard fit test: two adult fingers must slide flat between the gear and the dog. Fingers on edge do not count.
Martingale collar: A limited-cinch collar for dogs whose heads are narrower than their necks. It tightens only as far as its control loop allows and never closes fully.
Control loop: The 5-8 cm (2-3 in) webbing loop that sets a martingale's maximum cinch. It is what stops a whippet or greyhound from backing out.
Long line: A 4.5-9 m (15-30 ft) leash used for recall training on open ground, giving the dog range while keeping a physical connection.

Step one: measure the neck where the collar sits

A soft tape, a standing dog and about two minutes produce the only number sizing needs.

Place the tape mid-neck, exactly where the collar will rest — not up under the ears and not down on the shoulders. Keep it snug against the coat without pressing into it, and write the figure down in both centimetres and inches so any chart can be read without conversion errors.

Add 5-7 cm (2-3 in) to that reading: the allowance is what turns a body measurement into a collar's working length. Once buckled, the collar passes the fit test when two adult fingers slide flat underneath — lying flat, not stacked on edge.

Three named failures account for most wrong sizes. The high-neck error: the tape rides up under the ears and reads small, so the finished collar pinches. The fur squeeze: pressing the tape into a thick double coat reads smaller than the real neck, and the collar tightens as the coat settles. The dangling tape: a loose loop reads large, and the collar slips over the dog's head on the first walk. Puppies add a fourth failure — outgrowing — which is why the schedule is re-measurement every 3-4 weeks through the first six months.

Stand the dog and place a soft tape mid-neck, where the collar will sit.
Read the girth snug, not tight; note it in centimetres and inches.
Add 5-7 cm (2-3 in) to get the collar's working length.
Buckle up and run the two-finger check: two adult fingers, flat, underneath.
Re-check after the first walk; re-measure puppies every 3-4 weeks.

A puppy's first six months, on the tape

Neck girth moves fast enough in this window that fitting is a schedule, not a one-off purchase.

Timeline: 4 dated entries

Step two: turn the number into a size label

Most retail charts run S to XL, and the measured neck lands in one of four bands.

The standard bands are S = 25-35 cm (10-14 in) neck, M = 35-50 cm (14-20 in), L = 50-65 cm (20-26 in) and XL = 65-80 cm (26-31 in). The four bands meet edge to edge, so every neck between 25 and 80 cm has a label.

Three numbers — 35, 50 and 65 cm — sit on a boundary and appear in two bands at once. For a growing puppy, take the upper band so the collar has adjustment room for the 2-5 cm a month the neck may add; for an adult dog, check where the working length (neck plus 5-7 cm) falls inside each band's adjustment range.

The full mapping, including the strap width each band usually carries, is laid out in the Dog Collar Size Chart page, and the measuring technique itself is broken down in How to Measure a Dog for a Collar and Harness.

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Step three: match the strap width to the dog

Width is a strength and comfort decision, not a style choice.

Flat collars follow three width steps: 10 mm (3/8 in) for toy and small dogs, 15-16 mm (5/8 in) for medium dogs, and 20-25 mm (3/4-1 in) for large dogs and heavy pullers.

A wider strap spreads the load across more of the neck, which is what a heavy puller needs; the 25 mm (1 in) end of the large band exists for exactly that dog. On a toy breed the logic reverses — a wide, heavy webbing collar is a burden on a small neck, so the 10 mm band keeps weight down.

Width and length are independent choices: a dog can need an L length on the narrower end of the width range, or an M length on 16 mm webbing if it pulls hard for its size.