Boil-Off Gravity Calculator

About this calculator

During a brewing boil, water evaporates while sugars stay behind, so the wort's gravity rises as its volume shrinks — a straightforward mass-balance calculation: pre-boil volume × pre-boil gravity points = post-boil volume × post-boil gravity points, since the total dissolved sugar (in gravity points) doesn't change even though the water it's dissolved in does.

Homebrewers and professional brewers use this to plan a recipe's boil: knowing your kettle's typical boil-off rate (often 1-1.5 gallons per hour, depending on kettle size and boil vigor) lets you predict what gravity you'll land on for a given boil time, or work backward to figure out how long to boil to hit a target original gravity before pitching yeast. It's also useful mid-boil if a gravity reading comes in off-target and you need to decide whether to extend or shorten the boil.

This calculator takes your pre-boil volume and gravity plus the expected post-boil volume and returns the resulting gravity.