How do I calculate my pre-money valuation?

Pre-money valuation = fully diluted shares × price per share in this offering.
Fully diluted shares typically include all issued and outstanding equity plus any potential shares from unexercised options, warrants, and convertible securities.

For example, ABC Corp currently has 5,000,000 shares of common stock issued.  It wants to raise $1,000,000 at $1 per share.  This means;

5,000,000 issued shares x $1/share = $5,000,000 pre-money valuation.

*Because the fully diluted share count is treated as fixed for purposes of calculating valuation, the price per share and valuation are directly related (increasing one increases the other)