This article explains the difference between weight and volume. Specifically, it explains why ounces (oz) and fluid ounces (fl oz) are different.
The main ways that you measure things in the food industry are by weight or by volume. Everybody understands weight. It is how heavy something is. When you get on the bathroom scale in the morning you are measuring your weight. When you put something on the scale in your kitchen you are measuring weight.
The common units of weight are pounds (lbs), ounces (oz), grams (g) and kilograms (kg). (Actually grams and kilograms are measures of mass. Mass and weight are closely related, but we will consider them equivalent).
Two different items that have the same weight will balance exactly on a balance scale.
Volume is a measure of size. It tells you how large something is, or how much space it takes up.
The common units of volume are gallons (gal), quarts (qt), pints (pt), cups (cup), fluid ounces (fl oz), tablespoons (Tbs), teaspoons (tsp), liters (l) and milliliters (ml).
Two different items that have the same volume are NOT necessarily the same weight. For example, a cup of feathers is the same size as a cup of sand, but they definitely do not weigh the same. They will not balance on a balance scale.
Also note that even though one of the volume measures is called a fluid ounce, it doesn't mean this is a liquid measure. You can measure a fluid ounce of flour just as easily as you can measure a fluid ounce of water. Don't be confused by the name. Remember, volume is a measure of size for any substance.
Another way to realize that volume is a measure of size is to think about the relationship between length, area and volume.
For example you can measure the length of a line in inches or feet (or centimeters or meters). Length is a 1-dimensional measurement of size.
If you make a square, you can measure the sides in inches or feet (or cm or m). The area of the square is the number of square inches (a square 1 inch on a side) or square feet in the square. Area is a 2-dimensional measurement of size.
If you make a cube, you can again measure the sides in inches or feet (or cm or m). The volume of the cube is the number of cubic inches (a cube 1 inch on a side) or cubic feet in the cube. Volume is a 3-dimensional measurement of size.
Since both gallons and cubic feet are measures of volume, there is a simple conversion between them. However, you will never see measures of cubic feet used in a formula. (You will see the size of your oven or freezer described this way, though.)
nutraCoster is supplied with the USDA library of nutrition values. Many of the items in the USDA library have volume units already defined for them (e.g., water, milk, flour, etc.).
When you add a new item to the Master Items List, nutraCoster can automatically define the weight units for you, because a pound of flour is the same as a pound of water.
However, because a cup of each item has a different weight, nutraCoster has no way to know how to define the volume measures for a new item until you tell it.
Generally you will weigh a volume of the item, and use the weight to define the volume unit. For example, a gallon of water weighs 8.33 lbs. So you will define a unit as: 1 gal = 8.33 lbs.
All of the volume units are related to each other. Whenever you define any of the volume units, nutraCoster will automatically define all of the others for you. So if you tell nutraCoster that 1 gal = 8.33 lbs, nutraCoster automatically knows the weight of qt, pt, cup, fl oz, Tbs, tsp, l, ml and defines them for you.
When you weigh an item to define the unit, weigh the largest quantity you can. This minimizes the effect of measurement errors and roundoff errors.
Finally, you have a choice of units used on the reports. As nutraCoster scales recipes it can convert the item quantities to any units you choose. Check the report options to choose the units to be used for each of the reports.