Understanding Fluid Ounces vs. Net Weight Ounces
When formulating or shopping for ingredients, you will sometimes work with volumetric measurements (i.e. milliliters, fluid ounces and gallons). At other times, you will work with net weight measurements (i.e. pounds, net weight ounces and grams).
Working with ingredients measured in ounces can be confusing because measurements denoted in ounces can apply to either volumetric or net weight measurements.
When an ingredient is specified by volume, it should be listed with the designation fluid ounces or an abbreviation like "fl. oz." When an ingredient is specified by weight, it should be listed with a designation like net weight ounces, "net wt. oz," or "wt oz."
Aren't Liquids Always Measured in Fluid Ounces and Aren't Dry Goods Always Measured By Weight?
Not always. Liquids such as fixed oils and essential oils are sometimes measured by weight. Some dry goods, such as salts, sugars and herbs, are measured by volume such as 1 cup (8 fluid ounces).
One Fluid Ounce of an Ingredient is Not Equivalent to One Net Weight Ounce of the Same Ingredient
To ensure that you replicate a formulation or recipe accurately, it's important not to confuse fluid ounces with ounces denoted by weight (net weight ounces). For example: FNWL sells its Lavender Buds by net weight. 8 wt. ounces of Lavender Buds is much greater in quantity in comparison to the total quantity when using a measuring cup and filling it with 8 fl. ounces of Lavender Buds.
Working With Recipes That Do Not Clearly Designate Fluid or Net Weight Ounces
Sometimes, you may run into a formulation or a recipe that only lists "ounces" without clearly specifying whether the measurement is intended to be in fluid ounces or net weight ounces.
In such instances, look to see if the details that accompany the formulation/recipe states the total amount of product the recipe makes. If the total appears to be designated in volume (i.e. cup, pint, quart or gallon), the ounce measurements are likely intended to be in fluid ounces.
You can sometimes determine whether the formulator's intention is to use fluid ounces or net weight ounces by reviewing the measurements used for other ingredients called for in a recipe. For example, if other ingredients in the recipe call for weight measurements like pounds or grams, the ingredients specified as simply "ounces" likely are intended to be measured as net weight ounces. If in doubt for any recipe, see if you can contact the original formulator for clarification.
I originally wrote the above article for From Nature With Love. A special thank you to Kibby Mitra, CEO of FNWL for warmly granting permission to include the article on AromaWeb. I've slightly modified the article to focus upon calculating essential oils. You can find the original article on FNWL here.