Why a Cup of Kibble Weighs 71 to 106 Grams

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If you buy one, buy it for the kibble rather than for the puppy, because both of the things that go wrong with an automatic feeder are decided by the food and not by the animal. There are two pieces of arithmetic here and they are both simple enough to carry around. An auger meters a volume and a feeding guide is written in grams, and the converter between them is the bulk density of extruded kibble, which runs wide enough that one cup is anywhere from 71 to 106 grams. And a hopper outlet jams when it is less than about five kibble across, which means the opening you need scales with the food, and a feeder bought for small puppy kibble loses roughly half its margin when the dog moves to adult food.

Neither of those is a reason not to buy one. They are the two numbers to check, and the awkward part is that one of them is on the food bag and the other is on no listing in this category at all.

The conversion the listing cannot make for you

Start with the hopper, because one record here publishes a pair that actually closes. A TYM automatic cat feeder with a 7 litre programmable hopper publishes 7 litres and "around 29 cups", plus simple button programming with no WiFi or app, a built-in desiccant compartment, mains power with three C-size batteries as backup, and a chew-proof cable.

Check the conversion first, because it is the kind that usually fails. Twenty nine US cups at 236.6 millilitres each is 6,861 millilitres against a stated 7,000. Two percent apart, so the record's own arithmetic is sound. That is worth saying before anything critical, because it is the exception in this catalogue rather than the rule.

What no record can supply is the step from volume to mass. Extruded dry pet food has a bulk density of roughly 300 to 450 kilograms a cubic metre, depending on how far it is puffed in the extruder and how much fat is sprayed on afterwards. So:

One US cup is 236.6 millilitres, which is 71.0 grams of light kibble or 106.5 grams of dense kibble. A spread of 1.50 on the same measure.

The 7 litre hopper is therefore 2.10 kilograms or 3.15 kilograms. For a four and a half kilogram cat eating sixty grams a day that is 35 days or 52.5 days. The same hopper is five weeks or seven and a half, and the difference is entirely a property of the sack you poured in.

Which puts every portion accuracy claim in the category in its proper place. An auger is genuinely accurate, in millilitres, and 1.5-fold uncertain in grams, and grams are the unit the feeding chart on the food uses. The record itself names the mechanism without drawing the conclusion: "portions are limited to what a kibble auger can measure." More in our review of the TYM 7L feeder, and the general case is in our guide to what automatic pet feeders can and cannot do, which asks for "a feeder that sets precise portion sizes" and is right to, with the caveat that precise means precise in volume.

The clean way to handle it costs nothing and needs no equipment: fill one of the feeder's own portion settings into a kitchen scale once, write the number on the lid, and the volume problem is solved for as long as you buy that food. Our page on why a kitchen scale beats measuring cups makes the same argument about flour, where the spread is 40 percent rather than 50.

The jam is a ratio, and it belongs to the food

Now the failure everybody has read about. Granular material flowing through an opening does not clog gradually, it clogs when a few grains happen to arch across the gap, and whether that can happen is governed by one dimensionless number: the opening diameter divided by the grain diameter. Laboratory work on roughly spherical grains puts reliable flow above a ratio of about five, with clogging essentially absent by six or seven.

Kibble sizes, representatively: cat and small breed food runs 6 to 9 millimetres, adult dog food 10 to 15, large breed 15 to 20. So the opening that will not jam is:

About 40 millimetres for 8 millimetre cat kibble. About 65 for 13 millimetre adult dog food. About 90 for 18 millimetre large breed.

The requirement scales linearly with the kibble, which is 2.25 times the diameter and 5.06 times the opening area across that range. Nothing about the animal enters the calculation. A twenty kilogram dog eating small kibble is an easier problem than a four kilogram cat eating large biscuit, and the feeder sees only the biscuit.

Which answers the puppy question in the form it was asked. A puppy eats small kibble, so a feeder bought now is being bought in its easiest condition. The dog then moves to adult food, roughly doubling the kibble, and the ratio that decides whether the hopper clogs roughly halves, with no change to the machine. So the thing to check is not whether the feeder works for your puppy. It is whether it works for the food your puppy will be eating in a year.

And here is the gap. Not one feeder in this catalogue publishes an outlet diameter, an auger pitch, or a kibble size range it is built for. The 7 litres and the 29 cups are the only dimensions on the record. So the ratio has an unpublished numerator, and the honest version of every recommendation in the category reduces to what our own roundup already says: "Prioritise reliability and check reviews for jamming", from best automatic cat feeders. That is sound advice precisely because the specification that would replace it does not exist. The grains in question are also neither spherical nor uniform, which makes the real threshold worse than five rather than better.

The corner that stops moving, and the exact fill where it starts

A hopper empties in one of two patterns and the difference matters for a feature this record publishes.

Mass flow is when everything moves together and food leaves in the order it arrived. It needs hopper walls steeper than the material will stand on, which a shallow-floored box does not have. Funnel flow is the alternative: a channel empties straight above the outlet and the material at the walls waits. First in, last out.

The flow channel spreads upward from the outlet at roughly thirty degrees from the vertical, so it reaches the walls only up to a height of half the clearance divided by the tangent of that angle. Take the published 7 litres as a box 200 millimetres square and 175 deep, with a 50 millimetre outlet:

Half the clearance is 75 millimetres, and 75 divided by 0.5774 is 130 millimetres. That is 5.2 litres, which is 74 percent of the hopper.

Fill it past three quarters and the corners stop moving. Below that, everything in the box is inside the flow channel and the hopper behaves itself. Which turns the built-in desiccant, published as a freshness feature, slightly against itself: above 74 percent it is keeping dry the food that the auger will reach last, after everything poured in on top of it has already gone out. On a hopper that lasts five to seven weeks, a wall pocket can sit for the whole of it.

The footprint and the outlet are ours rather than published, and the threshold moves with them: a narrower, taller 7 litre hopper drops the critical height and makes the stagnant region start sooner, while a wider, shallower one removes the problem entirely. The direction does not move. The practical version is the same either way, and it is free: run the hopper down before topping it up rather than topping it up when it looks low, which keeps everything in the flow channel and makes the order of departure match the order of arrival.

The bowl has the same missing dimension

A slow feeder is the inverse problem, and one record here publishes a pair worth using. A YardEase two-pack of stainless steel slow feeder dog bowls publishes food-grade 304 stainless, a square shape holding 3 cups up to the ridges and 5 cups filled to the brim, which the maker notes is around 20 percent more than typical round bowls, tall closely spaced ridges, a non-slip rubber base and a reversible smooth underside.

Two published volumes give the ridges their own number. Three cups usable out of five cups of bowl means the ridges occupy two cups, which is 473 millilitres of 1,183, so 40 percent of that bowl is ridge. And the 20 percent claim checks out: three usable cups against a typical two and a half cup round bowl is 20 percent.

Now the slowing, which runs on exactly the variable the jam runs on. A channel of width s holds as many kibble abreast as s divided by the kibble diameter, rounded down. At a 12 millimetre channel, 10 millimetre kibble goes single file, one piece at a time. Six millimetre kibble goes two abreast. So the same bowl halves its own slowing effect when the food gets smaller, and the bowl has no way to know.

Which is the honest limit on every slow feeder claim, and it has the same cause as the honest limit on every feeder claim: neither the ridge spacing nor the ridge height is published on any slow feeder in this catalogue. Two products, one missing dimension, and it is the same dimension in both: a length that only means something compared against a kibble. The behavioural side of the question, which is a genuinely different argument, is worked through in our page on whether slow feeder dog bowls actually work, and the record is right that anything about a particular animal's eating is a question for a vet rather than for a bowl. Full detail in our review of the YardEase slow feeder bowls.

One listing that names its own gap, so we can fill it

A Gdsun silicone suction cup slow feeder for dogs and cats publishes 3.54 inches square, under an inch deep, food-grade silicone with textured grooves and raised edges, suction cups beneath, and dishwasher and freezer safety. Its own drawbacks are unusually straight about the problem with its own name: "Called a slow feeder bowl but measures only 3.54 inches square", and "No capacity published in any unit".

The published dimension answers the unpublished one. 3.54 inches is 89.9 millimetres, so the surface is 80.8 square centimetres. A spread laid 2 millimetres thick over that is 16.2 millilitres, which is about one tablespoon. That is the capacity, derived rather than stated, and it settles what the product is for: a tablespoon of something soft, frozen if you want it to last, stuck at nose height while you clip four paws. Not a meal. Our review of the Gdsun pad goes further on the published and unpublished figures, and it is the right shape of record: the gaps are named rather than papered over.

A record of ours that does not close, and which figure is wrong

Worth flagging, because it is in one sentence and it is out by a factor. A gravity pet water dispenser with a stainless steel bowl is described on our own page as follows: "This 8-liter gravity water dispenser holds about 1.1 gallons." Eight litres is 2.113 US gallons and 1.760 imperial gallons. Neither convention reaches 1.1, so the record is out by 1.92 on the US reading and 1.60 on the imperial, and there is no way to read it that makes both halves of the sentence true.

The record's own duration claim settles which half to keep. It states 8 to 12 days for small and medium pets and 5 to 7 days for large pets. At a representative 50 millilitres per kilogram per day:

From 8 litres, 5 to 7 days means 1,143 to 1,600 millilitres a day, which is a 23 to 32 kilogram dog. That is a large dog.

From 1.1 gallons, which is 4.164 litres, the same 5 to 7 days means 595 to 833 millilitres a day, which is a 12 to 17 kilogram dog. That is a medium one, and the record has already used its medium bracket for a longer interval.

So the duration claim only works on the litre reading, and the 8 litres is the real capacity. The gallon figure is the error. We are flagging it here rather than quietly fixing the sentence, because the product name itself carries the wrong figure and that name is the listing's own title. The rest of that record holds up: the bowl height adjusts from 3.15 to 6.3 inches, which is exactly a factor of two and a plausible published range, and the stated limitations are honest, including that the ice pack "must be pre-frozen and cools by room temperature". Our review of the gravity water dispenser and our guide to gravity, plumbed and fountain water dispensers are the pages that need the correction.

The travel version of the same problem publishes its two volumes separately and cleanly. An Aurapaws 2 in 1 portable dog water bottle and food feeder publishes 33.5 ounces of water and 13.5 ounces of food storage in one unit around 5 by 11.5 inches, with reinforced silicone sealing and two detachable bowls. Thirteen and a half fluid ounces is 399 millilitres, which on the kibble densities above is 120 to 180 grams: one adult cat day, or one meal for a medium dog. The water side is 991 millilitres, which at 50 millilitres a kilogram is a day for a twenty kilogram dog. The two compartments are sized for roughly the same animal and roughly the same day, which is a genuinely good piece of design and is nowhere stated as the reason for the ratio. See our review of the Aurapaws bottle and feeder.

The honest limit, and what it leaves alone

What is exact here is short and load bearing. The 29 cups against 7 litres closes to 2 percent. The 3 and 5 cup pair on the slow feeder gives the ridges 40 percent of the bowl and confirms the maker's own 20 percent claim. The 1.50 spread on a cup follows from the density range whatever values you pick inside it. The opening requirement scales linearly with kibble diameter, so the 2.25 and the 5.06 are exact. The 130 millimetre stagnation height follows from the stated geometry and the stated angle. The 80.8 square centimetres on the lick pad follows from one published dimension. And the 8 litre against 1.1 gallon discrepancy is arithmetic, not interpretation.

What is representative rather than published is the rest. Kibble bulk density of 300 to 450 kilograms a cubic metre and kibble diameters of 6 to 20 millimetres are typical ranges from the category rather than measurements of any particular food. The opening to grain threshold of about five is a laboratory result for near spherical uniform grains, and kibble is neither, which moves the real threshold the wrong way. The hopper footprint, the 50 millimetre outlet and the thirty degree flow channel are all ours, and the 74 percent moves with them. The 2 millimetre spread thickness and the 50 millilitres per kilogram per day drinking rate are both conventions.

Three things this page deliberately does not compute. It does not give a jam probability for any specific feeder, because that needs an outlet diameter and no listing in the category publishes one, and guessing at it would turn a real gap into a fake number. It does not give a portion accuracy in grams for any feeder, because that is a property of the sack of food rather than of the machine, and the whole point of the first section is that the machine cannot know it. And it does not say anything at all about what or how much any animal should eat, which is a veterinary question; the slow feeder record says so itself and is right to.

Three numbers to carry. One cup of kibble is 71 to 106 grams, so weigh one portion once and write it on the lid. The opening that will not clog is about five kibble across, which means the food decides it and the animal does not. And keep the hopper under about three quarters full, so the food leaves in the order it went in rather than waiting in a corner for the whole of a seven week fill.