Since 1 cup holds 8 ounces, 2/3 of a cup is 5 1/3 ounces. As most measuring devices are not accurate to 1/3 of an ounce, an alternate method of measure may be easier to use.
Keeping with measuring devices that are found in the average home, 2/3 of a cup also equals 10 tablespoons plus 2 teaspoons. If using a 1.5 ounce jigger, use a scant four measures.
If you have metric measurements listed on a liquid measuring cup, you can measure in milliliters instead. Since 1 cup equals approximately 236 milliliters, 2/3 of a cup measures approximately 157 milliliters.