Sizing your puppy through the first year: collar, harness and crate

Short answer

Size your puppy's collar and harness for the puppy it is today, not the dog it will become: the Royal Kennel Club says to buy a collar "that fits them now but is also easy to adjust as puppies grow very quickly"¹, and Bailey & Coco says harnesses need regular checks and adjustments as your dog grows². The crate is the exception, where the RSPCA and PDSA suggest planning for your puppy's adult size^3,4.

In this guide

When can a puppy wear a harness?No UK source we've found gives a minimum age for a puppy harness, and Battersea says you can start getting your dog used to its collar, lead and harness "as soon as your puppy comes home", going at your dog's pace.
Puppy collar size and fitBuy your puppy a collar that fits now and is easy to adjust: the Royal Kennel Club says "Don't get one you think they'll grow into eventually".
How often should I re-measure my puppy?Check your puppy's collar almost every day: the Royal Kennel Club says puppy collars "should be checked almost daily for condition and fit".
Your puppy's first kit: what size to buyBuy your puppy's collar and harness to fit its measurements today and adjust or replace them as it grows, but buy its crate with its adult size in mind, as the RSPCA and PDSA suggest.

Buy for now, or buy for later?

The biggest sizing question with a puppy is whether to buy for its size today or the size it will be as an adult. The answer is different for each item.

Which puppy items to size for now and which for the adult dog
Item	Size for	Why
Collar	Now	The Royal Kennel Club says to buy one that fits now and is easy to adjust, not one to grow into¹.
Harness	Now	Bailey & Coco says regular checks and adjustments are needed as your dog grows². Ruffwear says a growing pup that is truly between two sizes should go in the larger one⁵.
Crate	Adult size	The RSPCA says to get a crate "big enough for your pup to grow into"³. Many owners use a divider while the puppy is small.
Travel crate	Now	The RSPCA says not to travel with your puppy in its adult-sized crate, because it will be better protected in a smaller space³.
ID tag	Any time	Required on a collar in public, with some exceptions⁶. Move it to each new collar.

How fast puppies grow

Fast. Waggel's Golden Retriever chart shows how much a puppy of a larger breed changes in its first year⁷:

Golden Retriever puppy weights by age, from Waggel
Age	Male	Female
8 weeks	5–8 kg	4–7 kg
3 months	9–12 kg	7–11 kg
6 months	21–26 kg	18–23 kg
9 months	27–32 kg	23–27 kg
12 months	29–34 kg	25–29 kg

What this means for sizing

In Waggel's chart, a male Golden more than doubles in weight between 3 and 6 months⁷. A collar or harness that fits at 3 months is very unlikely to fit at 6. Other breeds grow on different timetables: Canine Culture says German Shepherds are fully grown at around 18–24 months⁸. Our breed puppy guides, listed below, cover each breed.

The first days at home

Battersea says: "You can start this training as soon as your puppy comes home, or when an older dog joins your family. Go at your dog's pace."⁹

For a collar, the Royal Kennel Club suggests you put it on and "immediately after give them a treat"¹, and "Keep an eye on them in case the collar gets caught on anything such as furniture"¹. We suggest the same short, positive approach for a harness: a few minutes at a time, with treats, before you ever clip a lead on.

Collar

The Royal Kennel Club says to buy a collar "that fits them now but is also easy to adjust as puppies grow very quickly"¹. It adds: "Don't get one you think they'll grow into eventually, because that's just going to be too big, heavy and intimidating."¹

For fit, the Royal Kennel Club says a puppy's collar should not be so loose it can slip over the head, nor so tight you can't slip two fingers underneath, and should be checked "almost daily"¹⁰. Battersea says a flat collar "should fit snugly - two fingers between the collar and your dog's neck"⁹.

Full details are in puppy collar size and fit.

Harness

No UK source we've found gives a minimum age for a harness. Battersea's advice on collars, leads and harnesses is: "You can start this training as soon as your puppy comes home, or when an older dog joins your family. Go at your dog's pace."⁹

Size the harness by your puppy's chest today: Ruffwear describes the girth as "the widest part of your dog's chest just behind the front legs"⁵. Check where each brand's size chart starts, because a small puppy may be below some brands' smallest size.

See when can a puppy wear a harness? for more, and our harness guides for how tight a harness should be and when to re-measure and adjust it.

Crate

The RSPCA says: "For a puppy, get a crate that's big enough for your pup to grow into."³ The PDSA says: "For puppies - choose a crate based on their adult size or increase crate size as they grow."⁴ Many owners buy the adult size and use a divider: see how crate dividers work for a growing puppy.

Two Royal Kennel Club points to remember: "Always remove the puppy's collar when in the crate in case it gets caught up on it," and "do not leave your puppy in the crate for more than a couple of hours during the daytime"¹⁰.

How often to check and re-measure

Check every day; measure every couple of weeks. The Royal Kennel Club says collars should be checked "almost daily for condition and fit"¹⁰. We suggest the same quick look for a harness, plus a full tape-measure re-measure of the chest and neck every two weeks while your puppy is growing fast, then monthly until the numbers stop changing.

More in how often should I re-measure my puppy?

Stage by stage: our suggested checkpoints

These are our suggestions, not a growth chart. Your puppy's own measurements always win.

8 weeks (or the day you bring your puppy home): measure the neck and chest. Fit a collar that fits now¹ and, if you're using one, a harness that fits now. Set up the adult-size crate with a divider. Start short, positive sessions at your puppy's pace⁹.
3 months: check the collar almost daily¹⁰. Re-measure the chest and neck every two weeks and loosen straps as needed.
4 months: keep re-measuring every two weeks. When the collar or harness is on its loosest setting and still snug, move up a size.
6 months: many puppies have changed size more than once by now. Check the crate divider still leaves room to stand, turn, lie down and stretch³.
9 months: if measurements have barely changed over a few checks, we suggest moving to a monthly re-measure. Keep the daily collar check.
12 months: measure again. Larger breeds can still be growing: Canine Culture says German Shepherds are fully grown at around 18–24 months⁸. Keep checking until the numbers stop changing.

Different breeds, different timetables

There is no single age when every puppy stops growing. Waggel says a Cavapoo typically reaches its full-grown size within the first 12 months¹¹, while Canine Culture says German Shepherds are fully grown at around 18–24 months⁸. A small breed may be in its adult collar and harness size well before a large breed of the same age.

That's why we suggest following your puppy's own measurements rather than its age. When the tape measure gives the same numbers for a couple of months, your puppy has probably settled into its adult size. Our breed puppy guides, below, cover how each breed grows. If you have questions about your puppy's growth itself, ask your vet.

Puppy coats and grooming

A puppy's coat changes in the first year, and it changes the fit. Measure through the coat to the body: Bailey & Coco says "Always measure to the skin, not the outside of the fur"¹². Bailey & Coco also says a Husky's chest measurement can change by 2–5 cm between winter and summer as the undercoat comes and goes¹³, which matters for double-coated breeds from their first winter.

For curly and long-coated puppies such as Cockapoos and Cavapoos, the first grooms can change the fit of a collar or harness in an afternoon. We suggest checking the fit the first time you put it on after every groom.

Car journeys with a puppy

Travel kit is sized for now. The RSPCA says: "don't travel with your puppy in their adult-sized crate - they'll be better protected in a smaller space."³

Rule 57 of the Highway Code says: "When in a vehicle make sure dogs or other animals are suitably restrained so they cannot distract you while you are driving or injure you, or themselves, if you stop quickly. A seat belt harness, pet carrier, dog cage or dog guard are ways of restraining animals in cars."¹⁴ If you use a car harness, it needs to fit your puppy now, like any harness. See wire or fabric crate, and which for the car?

ID tag and the law

The Control of Dogs Order 1992 says every dog "while in a highway or in a place of public resort shall wear a collar with the name and address of the owner inscribed on the collar or on a plate or badge attached to it"⁶. The Order lists some exceptions, for example for certain working dogs, so check the legislation if you think one applies.

A harness doesn't replace the collar and tag. Many owners clip the lead to the harness and use the collar for ID. Ask your vet when your puppy can safely start going out in public.

Your puppy's first kit

Collar and ID tag, harness, lead, crate and bed: each has its own sizing rule. Our first kit guide goes through them one by one.

Puppy guides by breed

Our breed puppy guides cover how each breed grows, the adult sizes to expect, and what to do at 8 weeks, 3, 4, 6, 9 and 12 months.

Labrador Retriever: Labrador Retriever harness · Labrador Retriever crate
Golden Retriever: Golden Retriever harness · Golden Retriever crate
German Shepherd Dog: German Shepherd Dog harness · German Shepherd Dog crate
Cockapoo: Cockapoo harness · Cockapoo crate
French Bulldog: French Bulldog harness · French Bulldog crate
Cavapoo: Cavapoo harness · Cavapoo crate
Border Collie: Border Collie harness · Border Collie crate
English Springer Spaniel: English Springer Spaniel harness · English Springer Spaniel crate

Common mistakes

Buying a collar to grow into. The Royal Kennel Club says it will be "too big, heavy and intimidating"¹.
Buying an adult-size harness for a puppy. A harness that is too large can chafe or slip⁵.
Fitting the collar once and forgetting it. Check it almost daily¹⁰.
Leaving the collar on in the crate. The Royal Kennel Club says always remove it¹⁰.
Travelling with a puppy in its adult-sized crate³.
Forgetting the ID tag when moving to a bigger collar⁶.

Breed puppy harness guides

Our puppy guides take each breed from 8 weeks to 12 months: how it grows, when to re-measure, and when to move up a size.

Find your puppy's harness size All breeds

Questions

Should I buy puppy kit to grow into?

Not the collar or harness. The Royal Kennel Club says to buy a collar that fits now and is easy to adjust¹. The crate is different: the RSPCA says to get one big enough for your pup to grow into³.

How often should I check my puppy's collar?

The Royal Kennel Club says "almost daily for condition and fit"¹⁰.

How tight should a puppy collar be?

Not so loose that it slips over the head, nor so tight you can't slip two fingers underneath¹⁰.

When can my puppy start wearing a harness?

No UK source we've found gives an age. Battersea says you can start getting your dog used to a collar, lead and harness as soon as your puppy comes home, at its own pace⁹.

Should my puppy wear its collar in the crate?

No. The Royal Kennel Club says "Always remove the puppy's collar when in the crate in case it gets caught up on it."¹⁰

Does my puppy need an ID tag?

In a public place, yes, with some exceptions: the Control of Dogs Order 1992 requires a collar with the owner's name and address on it or on an attached plate or badge⁶.