Check Google. Searching on this suggests a strong basis for 6 oz being a customary standard. https://cooking.stackexchange.com/questions/25041/is-a-cup-on-a-cof

Check Google. Searching on this suggests a strong basis for 6 oz being a customary standard. https://cooking.stackexchange.com/questions/25041/is-a-cup-on-a-coffee-maker-always-6-oz-is-this-a-standard-in-the-us https://cooking.stackexchange.com/questions/25041/is-a-cup-o... seems to back this up.