Wash strawberries under cold running water; drain. Remove stems and caps from strawberries. Crush strawberries one layer at a time using a potato masher. Measure 2 quarts crushed strawberries.

Combine crushed strawberries and sugar in a large saucepan. Bring mixture to a boil, stirring until sugar dissolves. Cook rapidly over medium-high heat to gelling point (220°F), stirring to prevent sticking. Remove from heat. Skim off foam if necessary.

Ladle hot jam into a hot jar, leaving ¼-inch headspace. Remove air bubbles. Clean jar rim. Center lid on jar and adjust band to fingertip-tight. Place jar on the rack elevated over simmering water (180°F) in boiling-water canner. Repeat until all jars are filled.

Lower the rack into simmer water. Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner, and bring water to a rolling boil. Process half-pint or pint jars 15 minutes. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12 hours. Check seals. Label and store jars.