Beer ABV & IBU

ABV: the standard approximation is ABV (%) = 131.25 × (OG − FG). OG is the original specific gravity before fermentation and FG is the final specific gravity after fermentation. Use hydrometer readings corrected to the instrument’s calibration temperature under comparable conditions. The relation is approximate and becomes less reliable for stronger beers. Equal readings give zero estimated ABV. The calculator rejects FG above OG and estimates outside 0–100%; this validation is not a guarantee of physical accuracy.

Uncorrected refractometer or Brix readings cannot be entered as specific gravity. This tool does not provide wine potential-alcohol tables, sugar-addition calculations, dilution corrections or corrections for additions made during fermentation.

IBU: Tinseth’s empirical model for one boil hop addition uses U = 1.65 × 0.000125^(SG − 1) × (1 − exp(−0.04t)) / 4.15. SG is the average wort gravity during the boil, not the fermented final gravity. Time t is in minutes. Added alpha acids A (mg/L) = AA (%) × hop mass (g) × 10 / finished beer volume (L). IBU = U × A. The displayed utilization is 100U (%). The volume is that of the finished beer, matching Tinseth’s metric definition, not the starting or average boil volume.

Zero alpha-acid content, hop mass or boil time gives zero modeled boil IBU. In particular, zero boil time does not imply that a hop stand contributes no bitterness. There are no pellet, whirlpool, saturation or other process corrections. This is an empirical estimate, not a laboratory measurement or a prediction for every brewing process.

Only the active tab is calculated. Tab changes and edits clear the previous result. Reset restores the standard values in both modes and keeps the selected tab. IBU examples change only the boil inputs. Calculation details contain the gravity drop or utilization and added alpha acids. Valid values are retained across navigation.

Sources: Grainfather, Calculation – IBU and Grainfather, Calculation – ABV.