How an option pool affects your dilution
An option pool is shares set aside for future hires. Creating one or topping one up adds shares to the total, so it dilutes everybody who already holds equity. What decides who pays for it is when the pool is created relative to the round.
Pre-money pool
Created before the round closes, so it sits inside the pre-money valuation. The existing shareholders absorb all of it and the new investor gets the percentage they paid for, whatever size the pool is. This is what investors will often ask for.
Post-money pool
Created after the round closes, so the new investor holds shares while the pool is created and carries a share of the cost alongside everyone else.
What that looks like
Raise $2M at an $8M pre-money and the investor takes 20%, leaving the existing holders 80%. Add a 15% pool on a pre-money basis and the investor still holds 20%. The existing holders drop to 65% and pay for the whole pool.
Granting from an existing pool does not dilute again
The pool was already counted in your fully diluted total when it was created. Issuing options out of it moves shares from unallocated to allocated. It does not change the total.