The tablespoon is a unit of volume in the US customary system and uses the symbol Tbsp.
In the kitchen, the tablespoon is the larger of the commonly used spoons served at the table and represents 1/2 US fluid ounce or, expressed in SI / metric format; 14.78676 ml.
This is often expressed as equivalent to 3 teaspoons (3 Tsp) although strictly speaking it is a little under the size of 3 metric teaspoons (5 ml) at 4.92892159375 ml. It was established to assist normal kitchens with recipes without the need for specific measuring equipment or devices. Although many spoons differ in sizes (and very rarely is a spoon filled accurately) it allowed for a decent "catch-all" and gave a "good enough" result most of the time.
The imperial fluid ounce is a unit of volume from the imperial unit system and uses the symbol fl oz.
1 imperial fluid ounce is equal to 28.41306 ml. It is also equivalent to 0.9607599 US fl oz.
The imperial gallon was defined in Britain by Parliament in 1824 and established as ten pounds of water at a normal temperature. This was split into 4 quarts, each quart into 2 pints, each pint into 4 gills and each gill into 5 ounces.
Therefore; 1 imperial fluid ounce is equal to 1/20 imperial pint, 1/40 imperial quart and 1/160 imperial gallon.