Work out boil off rate instantly with clear inputs, formula shown and shareable results.
Boil-off rate is volume evaporated per hour and is a fixed property of your kettle geometry, burner and lid arrangement rather than a percentage of batch size. Since sugar is conserved, gravity concentrates in inverse proportion to volume: post-boil points equal pre-boil points times the volume ratio.
Evaporation rate
Rate (L/h) = (pre-boil - post-boil volume) x 60 / boil minutes
Gravity concentration
Post-boil points = pre-boil points x pre-boil volume / post-boil volume
A fixed volume for a given kettle and vigour, because it depends on surface area and heat input, not on how much wort is underneath. Scaling recipes by percentage boil-off is a common error.
To drive off dimethyl sulphide precursors and to promote hot break protein coagulation. A vigorous uncovered boil of 60-90 minutes is standard for pale malt beers.