Many taxpayers must file an estimated quarterly tax with the Internal Revenue Service. This allows for those taxpayers to better assess their tax positions throughout the year instead of having one large lump-sum tax payment at the end of the year for taxes outside of those recorded on their W-2 forms. This is often the case for self-employed individuals. To estimate tax liability, taxpayers should use Form 1040ES. This form estimates the current year's tax liability. Form 1040ES is a simplified version of Form 1040, which taxpayers must file each year.