Every bag or pouch of cat food sold in the UK has to say, somewhere on the pack, whether it's "complete" or "complementary." It's not marketing language. It's a legal labelling requirement, and it tells you the one thing that actually matters before you decide whether something belongs in your cat's daily bowl or just alongside it.
What "complete" actually means
A complete food is formulated to contain everything a cat needs, protein, fat, vitamins, minerals, and amino acids, in the right amounts for daily feeding. You can feed a genuinely complete food as your cat's entire diet and nothing is missing.
To be labelled complete, a food has to meet nutrient levels based on guidelines from FEDIAF, the body that sets nutritional standards for pet food across Europe, and most UK manufacturers formulate against those same guidelines. That's the benchmark sitting behind the word "complete" on the pack, even though the label itself just shows one word.
What "complementary" actually means
A complementary food is missing some of that nutritional completeness on purpose. Legally, it's defined as food that's only sufficient as a daily ration when combined with something else. On its own, over time, it won't cover everything your cat needs.
That doesn't make it a lesser product. Most treats, toppers, and a lot of wet food pouches marketed for variety are complementary by design, they're built to add flavour, moisture, or interest to a meal, not to be the meal. The mistake isn't buying complementary food. It's feeding it as if it were complete.
Why this trips people up
The confusion usually comes from packaging that looks identical to complete food sitting right next to it on the shelf, same brand, same style, similar price. Nothing about the front of the pack tells you which one you're holding. You have to actually find the word "complete" or "complementary" printed on it, usually near the feeding guidelines or the composition panel.
If you can't find either word on a product, that's worth treating as a red flag rather than an oversight. It's a mandatory part of the label, and its absence usually means the packaging wasn't meant for standalone sale as a full meal in the first place.
Where price-per-kg fits into this
Here's where it gets relevant to actually buying food. Two bags can look almost identical and sit at similar prices, but if one is complete and the other is complementary, they're not doing the same job, and comparing their price per kg on its own doesn't tell you which one is the better buy for daily feeding.
On KiloPet, every product's price-per-kg comparison sits alongside whether it's complete, so you're comparing food that's actually doing the same job, not a daily meal against a topper. You can filter to complete foods only here and skip the guesswork entirely.
TL;DR
Complete means it can be the whole meal. Complementary means it can't, not on its own. Check the label, not the branding, and if a food's daily diet is built entirely around something marked complementary, that's worth changing, not something to leave as-is.
Next time you're comparing bags, start with KiloPet's price-per-kg comparison and you'll already know you're looking at food built for the same job.