What percentage to give UI/UX designer co-founder?

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.

That depends on the level of the designer. Is he major league? I assume you can't afford major league ones as contractors? It is really hard to answer without knowing what you build, but I can tell you this: if you are building mobile apps -- even if your functionality is 100% "as expected" and works well. Without a major league design - your application will never be major league. It's almost psychological. People expect great products to have a great UX. Those that don't are automatically cataloged as "inferior", even in functionality and even if they function perfectly. I know this from experience.

In light of this, think hard and well. a UX person is not just a "graphics artist" or "icon painter". He/she has the power to take your app up to the major league -- or make sure it stays in the "Visual Basic for kids" category.

Remember, Apple is where it is today thanks to the genius of designers, both industrial and UX. There were music players before iPod. There are alternatives to OS X, there are alternatives to Macs. But still...