Ounce

Ounce and Fluid Ounce

• Ounce (oz) is for weight
• Fluid Ounce (fl oz) is for volume

Both of them are approximately (but not exactly) equal for water. They are based on imperial definition of pound and gallon.

• 1 oz ~= 28.35 gm

• 16 oz = 1 lb (pound)

  (A pound was defined based on differents reference weights at different points in time. Right now it is tied to metric system as 1 lb = 0.4536… kg)

And,

• 1 fl oz ~= 29.57 ml

• 128 fl oz = 1 gallon

  (A gallon was originally and also currently defined as 231 cu. inches. It is known as wine gallon or Queen Anne wine gallon)

But for food labeling US FDA has set 1 fl oz to equal exactly 30 ml for convenience and consistency. Other household measures are also rounded:

• 1 teaspoon = 5ml
• 1 tablespoon = 10 ml
• 1 cup = 240 ml
• 1 ounce = 28 gm