The Apple Watch received a Calculator app as of watchOS 6. You can tap its icon on the Apple Watch screen to open it.
To use it, you can say something like: “What’s 73 times 9?” or “What’s 18%of 225?
To perform a quick calculation, tap numbers and operators to get a result.
To split a check or calculate a tip:
° Enter the total amount of the bill, then tap Tip.
° Turn the Digital Crown to choose a tip percentage.
° Tap People, then turn the Digital Crown to enter the number of people sharing the bill.
You see the tip amount, the total amount, and how much each person owes if the bill is split evenly.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)