One minute they fit in the palm of your hand, the next they're stretched across the whole sofa. Kittens grow up fast, and at some point you'll wonder whether they're done growing and whether it's time to change their food. Here's the short answer, what the research says, and how to make the switch without any fuss.
When do cats stop growing?
Most cats reach their adult size at around 12 to 18 months old, and most are ready to switch from kitten food to adult food at around 12 months. Some bigger breeds keep growing for longer, and neutering can change how a kitten grows, so it's worth looking at your own cat rather than just the calendar.
Vets usually class a cat as a kitten from birth to one year, and as a young adult from one to six years (Quimby et al., 2021). The PDSA also recommends kitten-specific food for a cat's first year (PDSA, 2024).
So when do kittens become cats? On paper, at their first birthday. In practice, many are still filling out for a few months after that.
A rough timeline for your kitten's first 18 months
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Eight to 12 weeks: most kittens go to their new homes and settle onto kitten food (PDSA, 2024).
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Four to six months: most UK kittens are neutered around now. In the Bristol study, 90% of cats were neutered at six months or younger (Foreman-Worsley et al., 2025).
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Around 12 months: your kitten officially becomes a young adult, and most are ready for adult food.
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12 to 18 months: many cats are still filling out, and some bigger breeds keep going for longer before they're fully grown.
What the growth research shows
Most of the ages you'll read online aren't backed by a source. Here's what the research does tell us.
Researchers have built growth charts for healthy domestic shorthair kittens, similar to the ones in a baby's red book (Salt et al., 2022). A few things stand out:
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The charts run from eight to 78 weeks, which is about 18 months. That fits with many cats still gaining weight after their first birthday.
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Healthy kittens tend to follow their own curve. Kittens whose weight crossed two or more lines on the chart (moving well above or below their usual curve) tended to be the ones that became overweight or underweight by early adulthood.
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The charts are for domestic shorthairs, based mainly on kittens in the USA, so they're a guide rather than a rulebook for every cat.
What this means for you: weigh your kitten regularly in grams, and look for steady growth rather than sudden jumps or stalls. Our guide to the best food for kittens explains how to track this at home.
How neutering changes growth and feeding
Neutering is one of the biggest changes in a young cat's life, and it can affect both how they grow and how much energy they need.
Two studies are worth knowing about:
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Neutering changes growth patterns. In a study of kittens' growth records, bodyweight growth changed after neutering at every age, and the change was greater in female kittens. In a smaller controlled study, female kittens neutered at 19 weeks were heavier at a year old, with more fat and more lean muscle (Salt et al., 2023).
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Neutering age between four and six months made little long-term difference. A University of Bristol study of 584 cats found that cats neutered at four months or younger followed similar weight and body condition patterns to cats neutered at five or six months. Cats neutered between seven and 12 months gained weight more slowly (Foreman-Worsley et al., 2025).
The Bristol researchers recommend weight management advice for all neutered cats, with regular checks of both bodyweight and body condition (Foreman-Worsley et al., 2025). In practice, that means:
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Review portions after neutering, using the feeding guide on the pack and your vet's advice.
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Keep weighing your cat every month or so.
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Check their shape, as well as the number on the scales.
If your young cat starts looking a little round, our guide to overweight cats has practical ways to help.
Do bigger breeds grow for longer?
You'll often read that larger breeds such as Maine Coons, Ragdolls, and Norwegian Forest Cats keep growing until they're two years old or more. That's widely reported, and many breeders say the same, but we couldn't find published research that pins down exact ages for each breed.
Here's how we'd approach it:
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Ask your breeder. They'll know how their lines tend to grow.
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Talk to your vet about when to switch, rather than going by age alone.
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Watch their growth. If your cat is still clearly growing at a year old, they may benefit from staying on kitten food a little longer.
Smaller and more slender cats may finish growing earlier, but the same rule applies: watch the cat, not just the calendar.
Kitten food vs adult cat food: what changes?
Both kitten and adult recipes can be complete foods, but they're designed for different jobs:
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Kitten food fuels growth. It tends to pack more energy into every mouthful, with nutrient levels set for building muscle, bone, and everything in between. The FEDIAF guidelines set separate nutrient levels for growth and for adult maintenance (FEDIAF, 2025).
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Adult food supports everyday life. It's designed to keep a fully grown cat in good shape without the extra energy growth needs.
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Biscuit size can change too. Kitten biscuits are often smaller for little mouths, so your cat may notice the new shape as well as the taste.
Moving to adult food isn't a step down. It's simply the right fuel for the next stage of life.
Signs your kitten is ready for adult food
Most cats are ready to switch at around 12 months. These signs can help you decide:
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They're around a year old, or older for some larger breeds.
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Their growth has slowed, and their weight is levelling off week to week.
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They've been neutered, which can lower their energy needs.
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They're starting to look a little round, with a less defined waist.
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Your vet agrees at their next check-up or booster.
If your kitten is ticking most of these boxes, it's a good time to start the switch.
Can kittens eat adult cat food?
Not as their main food. Growing kittens have different needs from adult cats, and the FEDIAF (2025) nutritional guidelines, which European pet food makers follow, set separate nutrient levels for growth and for adult maintenance. A food made for adults may not give a kitten everything they need to grow.
A stolen mouthful from the adult cat's bowl isn't a problem. If it becomes a habit, try feeding cats in separate spots, or lift bowls once mealtime is over. Until they're ready to switch, keep your kitten on a complete kitten recipe like Smitten Kitten dry kitten food.
Can adult cats eat kitten food?
The occasional nibble is fine. As an everyday food, though, kitten food is designed to fuel growth, so it tends to be richer in energy than an adult cat needs. Over time, that can make it easier for an adult cat to gain weight, especially if they're neutered or mostly indoors.
There are times when your vet might suggest a richer food for an adult cat, for example if they're underweight or need help regaining condition. In that case, follow your vet's advice. If your cat is older and losing weight, our guide to the best senior cat food explains more.
How to switch to adult food
Take it slowly. We always recommend switching over a 2 week period starting with 10% new food, 90% current food, and gradually increasing the new food.
A few tips to make it easier:
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Keep wet food familiar. Our wet pouches, like Paw Lickin' Chicken, are suitable for kittens from eight weeks and adult cats, so you may only need to switch their dry food.
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Weigh food in grams, using kitchen scales rather than cups or scoops, as the density of food differs between recipes.
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Watch their tummy. If you notice loose stools, slow the switch down. If it continues, speak to your vet.
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Feed separately in multi-cat homes, so the kitten food doesn't end up in the adult cat and the other way round.
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Pick a calm moment. Try not to switch food in the same week as neutering, a house move, or other big changes. One new thing at a time is easier on your cat.
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Keep mealtimes the same. Same bowl, same spot, same time of day helps the new food feel familiar.
If your cat turns their nose up at the new food, don't panic. Our guide to why your cat might not be eating has plenty of ideas for picky eaters.
Our adult recipes
Every Meowing Heads recipe is made with natural ingredients from farmers we know and trust, with no artificial colours, flavours, or preservatives. No nasties and no fillers. That's a promise.
Browse our dry cat food and wet cat food collections to find your cat's new favourite.
FAQs
Why does my one-year-old cat still look like a kitten?
Many cats keep filling out after their first birthday, and the kitten growth charts researchers use run to about 18 months. Some cats are also simply small or slender by nature. If they're eating well and their weight is steady, there's usually nothing to worry about, but your vet can check if you're unsure.
Can you tell how big a kitten will get?
Not exactly. The size of their parents, their breed, and whether they're male or female all play a part. Males tend to be heavier than females (Foreman-Worsley et al., 2025). Weighing your kitten regularly and tracking their curve is the best guide you have.
Can I feed my four-month-old kitten adult cat food?
It's best not to. At four months, your kitten is still growing fast and needs a complete kitten recipe made for growth. Keep adult food until they're around a year old, or until your vet suggests switching.
What happens if my adult cat eats kitten food?
A little now and then won't do any harm. If it happens every day, the extra energy can lead to weight gain over time, so it's worth feeding your cats separately.
Can kitten food upset an adult cat's stomach?
It can for some cats, especially if they eat a lot of it or it's a sudden change. Any change of food is gentler when it's made gradually.
At what age do kittens calm down?
There's no set age, and there's little research on it. Many cats mellow a little as they mature, but plenty stay playful for life. Regular play sessions are the best way to give that energy somewhere to go.
