Why 5 Kilograms of Meat Needs a 12 Litre Pressure Cooker

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Neither, probably. If the meat goes in as one whole joint, 8 litres works and 7 does not. If it goes in as chunks under braising liquid, the same 5 kilograms needs about 12 litres, and the reason is the step everyone skips: a cooker's capacity converts into a quantity of food through three numbers, not one, and the third is a packing fraction that depends on how you cut it.

The counterintuitive result is the one worth carrying away. Cutting the meat into pieces makes it need a bigger pressure cooker, not a smaller one.

Three conversions, and only one of them is a density

Start with mass to volume, which is the easy part. Lean muscle runs around 1,060 kilograms per cubic metre and fat around 900, so a representative boneless cut at 1,050 makes 5 kilograms into 4.762 litres of actual material.

Bone goes the other way from what people expect. Bone is denser than meat, somewhere between 1,800 and 2,000, so a bone-in cut of the same mass takes up less room rather than more. Twenty percent bone by mass gives 1 kilogram at 1,900, which is 0.526 litres, plus 4 kilograms of meat at 1,050, which is 3.810, for a total of 4.336 litres. That is 8.9 percent more compact for the identical weight. The bone costs you on the plate and saves you in the pot.

Second conversion: the fill line. A pressure cooker is filled to two thirds for ordinary food and one half for anything that foams, which means beans, grains, pasta, split peas and oats. That instruction appears in the manual of every cooker ever made and on the listing of none of the seven in this catalogue.

Apply it. Two thirds of 7 litres is 4.667 litres of usable space, and 4.762 litres of meat is 102.0 percent of it. The 7 litre cooker is over the line before a drop of water goes in. Two thirds of 8 litres is 5.333, so the same meat is 89.3 percent and leaves 0.571 litres, which is 2.41 US cups, for liquid. That is enough: our page on why a pressure cooker's minimum liquid is not steam works out exactly what that cup of water is for, and the answer is not making steam, which turns out to need under five grams.

So on two conversions the answer is 8 litres, barely. Then comes the third.

The packing fraction, which reverses the intuition

A pile of irregular solid chunks does not fill the space it occupies. Random loose packing of irregular pieces runs around 0.60, so the same 4.762 litres of meat, cut into stewing pieces, occupies 7.937 litres of apparent bed volume.

And the 40 percent that is not meat, 3.175 litres of gaps between the pieces, does not stay empty in a braise. It fills with liquid. Liquid counts against the same fill line that the meat does, because the line is about total contents and not about which ingredient they are.

Total 7.937 litres, against the 8 litre cooker's 5.333 litre line, is 148.8 percent. Half as much again as the pot allows. To get it under the line you need 7.937 divided by two thirds, which is 11.90 litres.

That is why the answer changes with the knife. A whole joint displaces its own 4.762 litres and sits in a puddle. The same joint in eight pieces creates three litres of void that you then fill with stock, and the cooker has to hold both. The meat did not get bigger. The dish did.

The one mass claim in the catalogue, and it survives

A Ninja HyperHeat 9-in-1 pressure cooker publishes nine functions, a 6.5 quart PFAS-free ceramic nonstick pot, pressure cooking, slow cooking, sous vide, steaming, searing and yogurt, white rice in as little as 8 minutes and quinoa in as little as 5, and one line that no other product in this category offers: "6.5-qt capacity fits a 4-lb chicken or 5-lb roast."

That is a mass claim, which makes it the only capacity statement here that can be checked. Check it.

Six and a half US quarts is 6.151 litres, and two thirds of that is 4.101. The 5 pound roast is 2.268 kilograms, which at 1,050 is 2.160 litres of meat. Add a representative half litre of braising liquid and the total is 2.660 litres, which is 64.9 percent of the line.

The 4 pound chicken is lighter at 1.814 kilograms, but a whole bird is not solid meat. Its body cavity is air, so a trussed chicken roughly 230 by 150 by 120 millimetres is about 2.168 litres of apparent volume, giving it an apparent density of only 837 kilograms per cubic metre. Add the same half litre and the total is 2.668 litres, which is 65.1 percent of the line.

Those two numbers are within three tenths of a percent of each other. A heavier roast and a lighter chicken arrive at the same point on the fill line, because the chicken's lower density exactly offsets its lower mass. That is not a coincidence and it is good evidence somebody calculated the pair rather than estimating them, which is more than can be said for most capacity claims in this aisle. Full record in our review of the Ninja HyperHeat.

Now scale it. If 6.151 litres holds 2.268 kilograms at 65 percent of the line, then 5 kilograms at the same margin needs 6.151 times 5 divided by 2.268, which is 13.56 litres. Running the chicken instead gives 16.95, and the difference between those two is simply that a bird is less dense than a roast.

So three honest routes give three answers, and the spread is the useful information. Eight litres if the joint goes in whole and you use the line right up to it. Eleven point nine if you cut it into chunks and braise. Thirteen and a half if you want the same third of the pot in hand that the one maker who publishes a checkable number keeps. The question offered 7 or 8, and the only version where 8 is right is the whole joint.

A serving is not a volume

An Instant Pot Duo 6 Qt 7-in-1 electric pressure cooker publishes seven functions, 13 smart programs, an 18/8 stainless inner pot with a tri-ply bottom, a pressure release valve and a locking mechanism, and a capacity claim of "6-quart capacity fits up to 6 servings".

An Instant Pot Pro 6 Qt 10-in-1 electric pressure cooker publishes ten functions, 28 smart programs, 1200 watts, a 120 volt 60 hertz supply, UL and ULC listing, and the same claim: "6-quart capacity for up to 6 servings".

Six servings of what? At 150 grams of meat a serving, six servings is 0.9 kilograms, which is 0.857 litres, or 16.1 percent of the two-thirds line. At 250 grams a serving it is 1.5 kilograms, 1.429 litres, or 26.8 percent. So the claim uses somewhere between a sixth and a quarter of the usable volume, and the rest of what fills the pot is liquid, vegetables and whatever else goes in a stew.

Which is to say the serving figure is probably right and it is not a capacity. A serving is a portion decision, not a volume, and nothing on either listing converts between the two. The same trap in the other direction is the subject of our page on why a kitchen scale beats measuring cups, where the problem is a volume standing in for a mass. Here it is a count standing in for a volume, and the fix is the same: weigh the thing.

The 1200 watts on the Pro is the figure that has done real work on this site, because it is what our minimum liquid page uses to turn the problem into a clock, finding that a litre of water and an empty pot is 470.6 kilojoules and 7.69 minutes of preheating, and that each extra cup is 94 seconds. This page takes those as settled rather than recomputing them.

And it is worth setting the Ninja's honest caveat beside them: "Rice and grain times exclude pressure build/release." White rice in 8 minutes is a true statement about the cooking stage and a misleading one about the kitchen, because the preheat is the larger half. That drawback line is the most useful sentence on any of these seven listings.

The third of the pot you are told never to use

Our page on why a pressure cooker is tall and its pot is not works the 6 quart pot out as a cylinder 212.8 millimetres across and 159.6 deep. Put the two-thirds line on that and it sits 106.4 millimetres up a 160 millimetre pot.

The 53 millimetres above it is not wasted space. It is where the steam sits, where foam has room to rise without blocking the vent, and where the lid's internal clearance lives. It is mandatory headroom. But it is also advertised capacity, and on a 6 quart cooker it is 1.893 litres you are instructed not to fill.

Across the seven cookers in this catalogue the published capacities total 39.8 quarts. One third of that is 13.27 quarts, which is 12.56 litres of collective capacity nobody should ever use. That is not a scandal; it is how the appliance works. It is just the reason a 6 quart cooker is a 4 quart cooker, and the reason comparing two cookers on their headline figure compares the wrong number by a factor of 1.5.

An Instant Pot Duo 7-in-1 Mini electric pressure cooker shows what that does at the small end. It publishes 3 quarts, 700 watts, seven functions, 11 smart programs, a stainless inner pot with a tri-ply bottom, UL and ULC listing, a recipe app, and the claim that its "3-quart capacity still holds a small chicken".

Three US quarts is 2.839 litres, so the line is at 1.893. A small chicken of 1.2 kilograms at the apparent 837 kilograms per cubic metre worked out above is 1.434 litres, which is 75.8 percent of the line, leaving 0.459 litres, or 1.9 cups, for liquid. True, then, and used right up to the edge.

Compare that with the Ninja's 65 percent and you have two makers working to two different conventions on the same unstated rule. Neither publishes the fill line, so neither reader can tell that one claim has a third of the pot in hand and the other has a quarter.

Half an inch of hand room, priced in meat

A Cosori Pressure Cooker 6QT publishes 13 cooking modes, a ceramic coated inner pot, a fingerprint resistant stainless housing, double layer anti-scalding, overheat protection, 12 safety features, an angled steam release, a seal and vent button placed away from the valve, sous vide and ferment modes, and one detail that is unusual enough to be worth the algebra: "Widening the pot gap by 0.5 inches" to make the rim easier to clean.

That half inch came from somewhere and the listing does not say where. If it came off the pot, taking the 8.38 inch diameter down to 7.88 at the same depth, the volume falls by one minus the square of 7.88 over 8.38, which is 11.6 percent. On a 6 quart cooker that is 0.70 of a quart, or 0.66 litres, which at 1,050 kilograms per cubic metre is 0.70 kilograms of meat.

If instead it came off the housing, the appliance is half an inch wider on the counter and the pot is untouched. Those are the two ends of a genuine trade, and which one you got decides whether the feature is free. The record is also right to flag what it cannot say at all: its own drawbacks note "No explicit mention of pressure levels or wattage", which means the 13 modes cannot be reasoned about from the listing in any way whatsoever.

Two stovetop cookers separated by 200 grams

A Presto 01365 6-quart stainless steel pressure cooker publishes 6 quarts, stainless construction, a stainless steel rack, a 64 page recipe book, full immersibility, dishwasher safety, a 12 year limited warranty and a claim of cooking 3 to 10 times faster than traditional methods.

A T-Fal Secure stainless steel stovetop pressure cooker publishes 6.3 quarts, a 5 point safety system with an audible lock confirmation, a rotating open and close mechanism, two cooking programs, a steam basket, induction compatibility and a claim of up to 2 times faster.

Those two capacities differ by 0.3 of a quart. Put the fill line on it and the difference in usable space is 0.2 of a quart, which is 0.189 litres, which is 0.199 kilograms of meat. The entire capacity distinction between our two stovetop cookers is about seven ounces of dinner, and it sits on the front of both listings as though it were a reason to choose one.

And the T-Fal's steam basket lives on the same side of the line. A basket that holds food above the liquid occupies volume inside the pot, and nothing publishes how much, so the 6.3 quarts is the capacity with the basket out. The Presto's stainless rack has the same property and the same silence around it. Our roundup of electric pressure cookers covers the category, and the figure to take into it is not the quart number on the box but two thirds of it, minus whatever accessory you intend to leave in.

What none of the seven publishes

Not one of these cookers publishes a maximum fill line. It is the single number that converts a capacity into a quantity of food, it is printed inside the pot on most of them, and it does not appear on any listing. Two publish a serving count, which is a portion rather than a volume. One publishes a mass claim, and it is the only capacity statement in the category that can be verified, which seems unlikely to be a coincidence.

Two publish a wattage and five do not, and one of those five says so in its own drawbacks. None publishes an inner pot diameter or depth, which is why the pot geometry on this page had to come from a sibling page that derived it from the capacity. None publishes the displaced volume of an included rack or basket. And none states the stricter half-fill rule for foaming foods, even on the models that advertise rice, bean and grain programs, where it is the rule that actually applies: on a 6 quart cooker the half line is 2.839 litres, which at a one to three ratio of beans to water is about 0.71 litres of dry beans.

The honest limit, and the three numbers to carry

What is exact here is every conversion once the inputs are granted. Mass divided by density is a volume. A fill fraction times a capacity is a usable volume. A volume divided by a packing fraction is a bed volume, and the voids are one minus the packing fraction. The ratio between the whole joint case and the chunked case is exactly one over 0.60, which is 1.667, and it does not depend on the density at all. The 0.3 percent agreement between the Ninja's two published claims is arithmetic on its own numbers.

What is representative is the physical constants. The 1,050 kilograms per cubic metre for boneless meat sets every litre figure on the page: a fattier cut nearer 1,000 makes 5 kilograms into 5.0 litres and pushes the chunked answer to 12.5, while a very lean cut at 1,060 gives 4.717 and 11.79. The 0.60 packing fraction is the one that moves the answer most, and it is genuinely uncertain: large even chunks pack nearer 0.64 and small ragged ones nearer 0.55, which spans 11.2 to 13.0 litres. The two thirds and one half fill fractions are the standard instruction and not a product specification. The half litre of braising liquid, the 1.2 kilogram small chicken, the trussed bird's dimensions and the 150 to 250 grams a serving are all in the same category.

What this page deliberately does not compute is three things. It does not give a cooking time for 5 kilograms of anything, and that refusal is deliberate and inherited: the temperature coefficient of the reactions that tenderise meat spans a factor of two to four per ten degrees, which makes any such figure a range so wide it is not advice. It does not compute how long the larger load takes to reach pressure, because that is energy divided by power and five of the seven cookers here publish no power; what can be said from the two that do is that the relationship is linear in the mass of contents and that our minimum liquid page has already worked the per litre figure. And it does not say whether a 5 kilogram load cooks evenly, which was the companion question in the vault, because that needs a thermal diffusivity through a bed of meat and liquid and a convection pattern inside a sealed pot, and no listing could carry either. What is safe to say is geometric: the centre of a 12 litre bed is further from the wall than the centre of a 2 litre one, and the distance grows as the cube root of the volume, so going from 2 to 12 litres puts the middle 1.82 times further in.

Three numbers to carry. A capacity is two thirds of what it says, so a 6 quart cooker is a 4 quart cooker and seven of them here hide 12.56 litres between them. Meat is about 1,050 kilograms per cubic metre, so a kilogram is 0.952 litres and bone-in is more compact rather than less. And a bed of chunks packs at about 0.60, so cutting meat up multiplies the pot you need by 1.667, which is the whole reason the honest answer to a 5 kilogram question is nearer 12 litres than 8.