Ask HN: How to leave a startup when you own a third of it?
If you had done this the Right Way from the beginning you would have had a vesting agreement that specified what would happen in a case like this.  A typical vesting agreement would have vested your stock over a 48-month period, so after 18 months you would have vested 33% x 18/48 = 12.375%.  So 10% is not unreasonable.However: having 10% of the company owned by a non-particiapting founder is a big red flag for many investors, and would be a significant obstacle to the company's success.  So you might want to consider settling for significantly less than 10% simply because you may end up making more money in the long run.  10% of zero is still zero.  If I were setting up a company today I would have a back-loaded vesting schedule: 10% the first year, 20% the second, 30% the third year and 40% the fourth year.  On this schedule, your share at 18 months would have been 4.95%.  If I were you, I would offer to take that.If you're raising $500k for 25% of the company, that means you think that the company is worth $1.5M now (because the company now plus the $500k would be worth $2M).  So your share on a flat vesting schedule is about $180k, on the back-loaded schedule it would be about $74k.  If you want to cash out you should offer to sell for significantly less than those numbers because you want everyone to think they've gotten a good deal.  You never know when you'll want to do business with someone again in the future.  A reputation for being reasonable is worth a lot more than $180k.You need to decide what you're doing before you approach investors because you need to achieve clarity on what you're selling them: is this a company with three founders, or a company with two founders and one ex-founder (and is the ex-founder willing to sell and if so for how much)?