An accurate answer depends on specifics - e.g., the valuation and other variables. As a very general guideline, the average runs roughly:
10% to 25% for Seed Rounds
25% to 50% for Series A
33% for Series B
As you advance to the next funding round, realistically expect further dilution. Founders start with 100% ownership. Seed rounds - the earliest stage of funding, usually from family and angel investors - typically dilute founders’ ownership by an average of 15%.
By the time you reach the Series A stage, you need to be prepared for further dilution. Since their funding typically exceeds $2 million, their percentage of ownership can be as high as 50%.
If you get to the Series B round, expect a dramatically different mindset from earlier funders. Whereas Series A and seed investors believe in your vision and have bought into the prospects of your company, those in Series B want to see that you’ve successfully progressed and satisfied important milestones. They typically see about 33% ownership, which will dilute all previous ownership percentages.
You can find more information in this recent post, where I discuss the usual percentage of shares that go to seed, Series A and Series B rounds.
Protect Your Equity Stake
You’ll want to have an attorney review all your funding documents so that you understand what you actually own and what you’re sharing. While each round of investment presents a new valuation, you can ensure that there are minimal - if any - restrictions on your equity, such as vesting periods that give you less than what you might think you own.
These guidelines are offered to you to give you some idea of the range of possibilities. It’s not recommended that you rely on any of these ranges as a substitute for sound legal advice. You’re going to have questions that are very specific to your own unique circumstances, requiring competent legal advice. This includes everything from vesting cliffs to intellectual property and other issues beyond the horizon.