The manufacturer should list the amount. Generally it falls between .5 oz and 1 oz PPO (per pound oil). I'll admit it - I go to the fragrance calculator at Majestic Mountain Sage or Brambleberry, pick something similar, and use that number. And I also measure by volume, not weight. (I'm a baaaaad girl.
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ETA: Brambleberry's calculator is for oils + water. Which I think might be the only one that does. So if you are making a batch with 20 oz of oils and 10 ounces of water, if you go to the calculator at Majestic Mountain Sage, put in 20. At BB, put in 30.
How big are the molds? if the manufacture's page doesn't say, fill one with water.
1 ounce of water by volume = 1 ounce of water by weight. Oil is close enough to use this for determining how much soap. So, if your molds hold 4 ounces of water, and you want to make 8 bars, you will need 32 ounces of soap.
Soap batter is approximately 1/3 water and 2/3 oil. So, in this case, you want a batch that is 20 or 21 ounces of oil.
Notice that there is a lot of rounding up or down in my method. There are more precise methods, but this one works for me.