A water discount is a reduction in the amount of water needed to dissolve sodium hydroxide lye. When you use a water discount, the soap will harden faster because there is less water in its batter (the mixture that eventually becomes soap). You only use a water discount for cold process or hot process soaps that use lye. You don’t need a water discount for melt and pour soap because the oils are already saponified and the lye has been used up before you ever get the melt and pour container!

Advantages of a water discount:

• Your soap will cure faster
• Your soap will be harder (ideal for Castile soap)
• The soap can be taken out of the mold more easily
• The mold will be easier to clean (less residue = less cleaning of the little corners of your molds is required – a constant problem I’ve had with homemade cosmetics, especially my all-natural conditioner bar).
• A water discount helps balance the recipe if you’re adding other ingredients that contain water such as if you are using milk (including breastmilk) or if you’ve mixed mica powder with water rather than alcohol before adding it to your soap.
• If you want to force a strong gel phase for a specific soap design, a water discount is a great addition to the other things you can do such as using heat pads around your soap while it’s curing.

Disadvantages of a water discount:

• Your soap batter will thicken (solidify) faster, making it harder to work with. If you’re doing a color effect such as a swirl, you will want your batter to reach trace (ideal thickness) then to solidify slowly, to give you time to make your desired effect.
• It can also effect your colors by messing with the heat of the soap. The reaction between lye and oils (saponification) is an exothermic reaction — it gives out heat. And if it heats up too much, it will affect what the soap looks like. If you want to avoid gel phase (e.g. when making cold process breastmilk soap, you do NOT want it to get too hot or the milk will spoil before the soap is done), don’t water discount more than you need to for the extra liquid in the milk.

To calculate a water discount, you use a percentage:

The usual amount of water to lye is 70% water to 30% lye. That means you use 70ml of water for every 30g of lye.

Discounting the water by 10%, you would have 63ml of water to 30g of lye.

Discounting the water by 20%, you would have 56ml of water to 30g of lye (this is a heavy water discount).

You also need to factor in whether your recipe requires a superfat (leftover oil for more nourishing soap bars). In this case, you usually wouldn’t discount your water.

Stuck? The very best resource on calculating the amount of oils, water and lye for your recipe is the Brambleberry Lye Calculator (it also calculates fragrance, but beware in the EU some of the fragrance results are higher than permitted under EU law if you’re selling your soaps). This tool is phenomenal!