This question already has an answer here:
This is a general question for company shares:
My partner and I have an equal stake in our company. We want to bring in a designer since both of us suck at UI. We have one in mind.
Since we own the IP and are doing all the backend work for the business, I'm handling sales, marketing, contractors, etc., what percentage of the company would be considered fair for a UI guy? He is strictly design.
Our business is bootstrapped and pre-revenue, but once we launch, we expect to pick up users of our service fairly quickly.
Any and all help in determining the proper percentage would be greatly appreciated.