Boil-Off & Post-Boil Gravity Calculator

How it works

Points × volume is constant: OG − 1 = (pre-boil SG − 1) × pre-boil volume ÷ post-boil volume; post-boil volume = pre-boil − rate × hours

Boiling removes water and nothing else, so the sugar that was in the kettle at the start is still there at the end, just in less liquid. That makes the arithmetic simple: gravity points multiplied by volume stay constant, and the post-boil gravity is the pre-boil gravity scaled by the volume ratio. Taking a pre-boil reading (cooled or temperature-corrected) is the moment to catch an efficiency shortfall while it can still be fixed — by boiling longer, adding a little dry malt extract (about 384 gravity points per kilogram per litre), or, if the wort is too strong, topping up with water. Vigorous boils in wide kettles lose more; lids trap steam but also DMS, so most brewers boil uncovered and accept the evaporation.

Worked example

26 L at 1.042 boiled for an hour at 3.5 L/h: 22.5 L at 1.049. To reach 1.052 you would need to boil 20 minutes longer or add about 180 g of DME.

Frequently asked questions