I'm going to start off assuming that you're relatively well-off and your quality of life wouldn't fall off a cliff if you spent 1.4 years on this and failed. I

I'm going to start off assuming that you're relatively well-off and your quality of life wouldn't fall off a cliff if you spent 1.4 years on this and failed. If that's not true, you should probably pass (because most startups fail). Ultimately, you want to maximize your expected value per year of work. Expected Value = Probability of success * Business Value * Your Equity Stake When estimating the startup's probability of success, you should take into consideration that it's abnormal to have only 1 technical founder in a team of 4 (assuming this is a tech startup). It's also abnormal that two founders are part-time. These abnormalities should decrease your estimation for probability of success. If you don't feel confident choosing a success probability, I would advise using 20% (based on what you've mentioned). To estimate the startups' business value, first make an educated guess at how much profit the company will make per year if successful. Then, multiply this yearly profit by 5-10 to get a business value estimate. (Note: the 5-10 multiple actually depends a lot on your industry, and how sustainable the business is. You can find comparables by looking at P/E ratios for similar businesses being sold in your industry). Once you've figured out what your expected value is, divide it by the number of years it will take you to reach success. Then, compare this Expected Value / Year to your alternatives to determine if it's a good opportunity. A final note: Startups are all about execution. Good ideas are a dime a dozen. Ultimately, the CEO/chairman is planning on bringing ideas and letting you and one other person execute. That means you will likely be pounding pavement while he is nowhere to be found. If this personal dynamic irks you, you should think about negotiating the equity split so you feel well compensated for your sweat equity.