@amme20 It is helpful to remember that 3.5 ounces is roughly equivalent to 100 grams - knowing this can make it easier to see if there are any glaring errors in your conversions. I still use an online conversion calculator to flip between ounces and grams (I work in grams for measuring).
Getting used to using percentages for your recipes (as everyone has mentioned) will make your life so much easier!
Good luck!
I am also a metric person (well I think most of the world is) and have had endless battles and hair pulling moments through the ounce to gramme issue - then i found soapcalc.net and you can do everything in metric. YAAYYYYYY.
Because of the viscocity (thickness) of oils it is always measured in weight - so if you buy 2 litres of Sunflower oil you can convert it to 2 Kgs - so far I ahve not had any issues with that.
I even weigh the water - i find that if i just weigh everything it all works out fine. But the measuring of ml and then weighing confuses my brain cell. And my brain cell gets confused easily when its faced with digits.
@Donee' - the oil (and other ingredients) are measured in weight, not because of the viscosity, but to take the relative density of each of the ingredients into account.
In detail:
Most oils and fats have a specific gravity (density relative to water) of around 0.9.
What this tells you is that the weight of the oil in any given volume is going to be roughly 0.9 times the weight of the same volume of water (this is why oil floats on water - oil is lighter than water).
The specific gravity of sunflower oil is between approximately 0.91 and 0.93 g/ml at about 20C.
I did a rough calculation, using 0.92 as the specific gravity for your sunflower oil example.
2,000 ml (Sunflower oil) * 0.92 g/ml (approx. Specific Gravity for sunflower oil) = 1,840 grams (anticipated weight of your 2L of sunflower oil)
(Always use actual weight measurements for oils, not calculated (anticipated) weights, however for the sake of this example, lets imagine that the SP was correct and when you weighed it, the 2L of sunflower oil did indeed weight 1,840grams.)
1,840grams of sunflower oil requires 247.96g of NaOH (according to soapcalc) to convert it to soap (at 0% superfat).
2,000gram of sunflower oil requires 269.52g of NaOH to convert it to soap (using 0% superfat).
To get the 1,840gram batch to use the same amount of NaOH as the theoretical 2,000 gram batch, I had to adjust the superfat to -8.7%.
A negative superfat is effectively extra NaOH, so that is
8.7% more NaOH than you need.
In other words, your soap is going to be lye heavy, unless your superfat is above this amount, and/or your sodium hydroxide is a bit clumpy (so isn't as strong as it could be).
In summary:
so if you buy 2 litres of Sunflower oil you can convert it to 2 Kgs
This is not safe to do (the soap could end up being lye heavy).
Reason:
If the specific gravity is ignored and the weight of the oil is assumed to be the same as water, then you are going to calculate the amount of NaOH using an artificially high oil weight, which will result in weighing out more sodium hydroxide than you need for the actual weight of oil you have.
The 2 litres of sunflower oil is going to weigh more like 1,840grams*.
*If you measure oil using volume, you end up being about 8% short of your required oil weight.
Always weigh your ingredients, especially the fats and oils.
... i find that if i just weigh everything it all works out fine.
@Donee' - this is good
