This Practice Note provides step-by-step instructions on how to upsize or downsize an initial public offering (IPO), including flow charts showing which SEC filings and communications are required in different situations. This Note includes detailed formulas to make calculations easy. Topics discussed include SEC filing fee increases under Rule 457(a) and Rule 457(o), the 20% safe harbor under Rule 430A, and the 20% cap for short-form registration statements under Rule 462(b). This Note also discusses other liability considerations, particularly when an upsize or downsize is close to pricing.