I make an onion pasta sauce in a Dutch oven the same size. If it's just onions (sliced as like 3 cm x 1 cm slices) I can fill the pot, and it's pretty much half the volume or less when fully cooked. But it is like an all day cook to essentially make an onion pasta.
this post was submitted on 13 Nov 2025
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So this obscure research paper gives the density of onion at 0.86kg/l, while this online calculator has it at 0.55kg/l for cubed (!) onion. The latter is probably the better guide if you slice them. So for 10l, I would probably buy something like 6-8kg, see what fits in the pot and use the rest for something else.
The 0.86kg/l is basically the same as many other veg and fruit, and everyone who had ever placed an onion in water would not be surprised that it floats.
The 0.55kg/l for the cubed onion probably depends on the size of the cubes; especially if they are smaller, there would be a lot of compression with the pot being nearly 40cm high, rapidly approaching the 0.86kg/l mark, at least at the bottom.
I think the 6-8kg is a good and reasonable approach based on that data; I'll need some more onions for two other dishes, so leftovers will not go wasted. Although I need them with vastly different cuts. But the question is more about the amount to buy and peel, the distribution among the dishes can be done rather simply.
Onions don't shrink as much, say mushrooms. This is because of their cellulose structure that holds up fairly wellonheat...for a time.
If you cook 1 cup of onions on a high heat skillet versus a low heat skillet both for 5 minutes, the low heat volume and weight will be more. That's because it takes high heat over time to break down the cellulose walls.
If you're making a gravy, you have some control and variance over how this works. If you hit them with high heat first, THEN add broth to boil them down, you'll have less volume.
I assume you'll be using a flour Roux, you don't want high heat at all because it will burn the flour mixture and give it a horrible taste.
So let's assume you're doing low and slow, and you're thinly slicing the onions and mixing everything in, you'll expect to retain 1/2-3/4 of the volume of the onions depending on the LENGTH OF TIME it is exposed to heat.
The longer you cook, the lesser the volume taken by the onions.
Hope that makes sense.
I know how to cook that gravy, that's not the issue. But usually, I take a "normal" pot, chuck in a few onions, and have gravy for a few servings and for the fridge. Pot space was not the issue.
This time, I want to go big. I know that the onions shrink, so what I need to know is with how much onion I should start to get to the stated 10l limit to have enough headroom to stir the stuff.
Beef shrinks up to 1/2, and onions also shrink up to 1/2 depending on heat.
Should be easy to figure out.
It's not the shrinkage I'm worried about. I know it shrinks. The question is how many kilos would I have to put through the food processor without the pot flowing over.
Okay...so...do the math I just gave.