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 cubic inch is a unit of volume derived from the imperial unit system and uses the symbol in³.
1 cubic inch is defined as equal to the volume of a cube with edges measuring 1 in. It is equivalent in volume to 16.387 ml, 1/231 US gallons and 1/1728 ft³.
Common uses for cubic inches include quantifying the size of electrical boxes and in the automotive industry for measuring engine displacement. The 5th generation Ford Mustang ran on a Boss 302 which was a 302 in³ engine (5.0L).