Remove stems and crush strawberries thoroughly, one layer at a time. Measure exactly 5 cups prepared fruit into 6- or 8-quart saucepot.
Stir pectin into prepared fruit in saucepot. Add butter to reduce foaming, if desired. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly. Stir in sugar. Return to full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with metal spoon.