When you’re making a soap recipe, one thing you might wonder is how to calculate the yield from your soap recipe.
Yield noun – the amount or quantity produced or returned.
There is a really easy way to calculate the yield from any recipe. However, some real world variables will affect the calculation, and in practice, you will find that the yield from any given soap recipe is a bit less than it ought to be.
Why don’t soap calculations produce an exact weight?
A viscous liquid is thick. As a liquid gets more viscous, it pours less readily, until eventually a liquid can be so viscous it doesn’t pour at all (like whipped cream). Soap is a liquid which is viscous, whether it’s melt and pour or a cold process soap that has reached trace. A light trace is less viscous than a heavy trace, but both are more viscous than water.
When you try to pour a viscous liquid, some of it will stick to the sides of the bowl or jug. It will also stick to your stick blender or stirring spoon. This results in a loss of about 10-20g of soap batter per batch. If you are making a small batch such as a single bar of soap, you can lose a significant amount.
The best way to compensate for this loss is to make a bigger batch, because you will not lose much more batter from a larger batch than from a smaller one.
With that in mind, here is how you calculate soap yield:
Add together the weight of all the solid ingredients. Convert water into grams (1ml of water weighs 1g). Other liquids don’t convert 1:1, because a ml is a measure of capacity while a gram is a measure of mass, and the mass of a given capacity is dependent on the density of its molecules. Oils have long chains of molecules, where water has small molecules made of only three atoms, so more water molecules fit into the same space as any oil, so water will always be heavier and more dense than oil for the same capacity.
Confused? Here’s how it works on a practical level. With liquid oils, such as avocado oil, you will need to weigh them separately. Do this before you mix your ingredients together. With electric scales (recommended for soaping), you can do this by turning on your scales and putting an empty container on top, then pressing “tare”. This will set the scales to ignore the mass of the container and just weigh what you put inside it. Next, pour your oil into the container. This will tell you how much it weighs. Add this to the mass of all the other ingredients and this will tell you the total yield of your soap recipe.
Lye dissolves in water, so do I need to weigh it?
Yes. This is because, when you add the lye to the water, even though it dissolves, it is still in the container. The mass of the water increases by the mass of the lye. Any time something soluble (like salt, sugar, or sodium hydroxide lye) is dissolved in water, the mass doesn’t go anywhere.
You can put this to the test if you want to do some at-home chemistry by getting a cup of 100ml water, stir in 50g of salt, and weigh the total mixture. You’ll see the liquid will now weigh 150g and it will have noticeably increased in volume, too. This is because of one of the laws of physics which explain how the universe works.
Melt and pour recipe (taken from my Easy AHA exfoliating melt and pour soap recipe which you can find here). These are the ingredients:
10 ml Cherry kernel oil
90g Melt and pour soap base
1 ml Cherry blossom fragrance
A pinch of sliced up loofah
The cherry kernel oil needs to be weighed. It weighs 8 grams. Add this to the melt and pour soap base and we get 98 grams. It wouldn’t work to try to weigh 1ml of fragrance so we will round it up to 1 gram although it’s more like 0.9g.
In a higher yield recipe (e.g. making a kilo of soap) we would use a lot more fragrance so we would be able to weigh it but here it will not make much difference. So our total is 99g. In a 100g soap mold, this leaves a tiny bit of room for the sliced up loofah to go slightly under the surface of the soap without it spilling over the mold.
90g + 8g + 1g = 99g
So that’s how to calculate the yield for a soap or cosmetics recipe!
PS I’m super excited that my lye just arrived, so I’ll be trying out some cold process soapmaking as soon as my new stick blender gets here (mine died last year before the first lockdown), and I’ll be sure to share my makes (and fails… part of the learning process) as I move into this awesome new world of handmade soapmaking!