Fortification Calculator

Fortification adds enough high-proof spirit to a wine to raise its overall ABV to a target strength, typically to stop fermentation early (leaving residual sweetness, as in port) or simply to produce a stronger finished wine style.

This solves an alcohol mass balance: the total pure alcohol in the wine plus the total pure alcohol in the added spirit, divided by the final combined volume, must equal your target ABV. The formula rearranges that balance to solve directly for the volume of spirit needed.

The further your fortifying spirit's ABV is above your target, the less volume you need to add. This is why neutral spirit is the efficient choice for port-style production: a small addition goes a long way and adds minimal flavour of its own.

Different fortified wine traditions target different final strengths, largely driven by historical preservation needs and regional style. Approximate targets:

Traditional port-style production fortifies partway through fermentation, while significant residual sugar remains. Adding high-proof spirit at this point raises the ABV enough to kill or stun the yeast, halting fermentation and locking in sweetness alongside the added strength. This is why port and similar styles taste noticeably sweet despite their high ABV.

Fortifying after fermentation is complete, once the wine has fully attenuated to a dry base, produces a dry fortified wine instead, since there's no residual sugar left for the spirit addition to preserve. Both approaches use the same calculation here; the difference is entirely in the timing and the sweetness of the base wine at the moment you fortify.