Vtyjkyuk

I need close to 900 ml of cream 30–35 % for a mousse recipe (which requires part of the cream to be whipped). Is it possible to dilute double cream (48 % fat) with whole milk (3.6 %) to get an average in the range I want?

Using the numbers above, I calculated that mixing 600 ml of double cream with 300 ml of milk should give me 900 ml (if they add up like that) of something with 299 g of fat, which is about 33 %.

Does this make sense or are there any complications I need to be aware of?

Note: The cream/milk packages actually give grams of fat per volume (e.g., 100 ml of double cream has 48 g of fat), but I've taken it to be about the same as percentage by mass since the target fat content is a range rather than an exact figure.

Given two milk or cream liquids with different fat percentage (I will refer to the one with lower fat content as "thin" and the one with higher fat content as "thick"), a liquid quantity Q_target and fat concentration TargetFatConcentration can be achieved by mixing

and

The fraction is a ratio (pure number), but I'm using the word "concentration" instead of "percentage" (which is normally used for fat content) because the products you have (as in my case) might be giving you grams of fat per volume, which is, technically, not a percentage. The point is that units need to be consistent, whatever they are.

Either volume or mass can be used for the "quantities".

It should go without saying that the target concentration can only be between the "thin" and "thick" one; i.e., you cannot dilute a cream by adding a thicker cream to it and vice versa.

This will work just fine. Many people will get the math wrong, but your calculations appear correct. Give it a good stir and everything should come out as expected. Most recipes are sufficiently tolerant that substituting the double cream would likely be fine compared to just using the heavy (if not an improvement).

Technically, double cream has a minimum fat content of 48%, and could be higher, which might throw your numbers off. That’s unlikely with cream from a large commercial supplier, since they can always make more money by diluting it down to 48%, but I buy unpasteurised double cream pretty much straight from the cow from a local dairy farm, and I have to dilute it with milk for recipes that call for double cream, never mind single.

That is the number I get:

600 ml × 0.48 = 288 ml

300 ml × 0.036 = 10.8 ml

(288 ml + 10.8 ml) / 900 ml = 33.2%

I think they would mix fine but I'm not positive on that. Mixed in recipe with dry goods like flour for sure they should mix.

Volume versus mass would be very close to the same here as density is close, if not the same.