A tasting flight at Garrison Brothers Distillery in Hye, November 2021.
The bottle you opened last month and the one with two fingers left in it are changing at different speeds. The air sitting on top of the whiskey, called headspace, sets the pace.
Why the level in the bottle matters
Arran Distillery says the air goes to work as soon as the seal is broken: “Once you open a bottle of whisky, the air reacts with it, and oxidation occurs.” (Arran)
Marcus Fan, who writes about whisky on his own site, tested how much the fill level matters. He stored the same whisky at several fill levels for a year, in the dark at a steady temperature, then tasted them, and reported that “There was a clear relationship between headspace and the change in taste of a whisky.” (Marcus Fan) It’s one person’s tasting with no lab work behind it.
How long it keeps
Nobody agrees on a number. Margarett Waterbury laid out the range at The Whiskey Wash: “Some say bottles should be finished within one to two years, while others say up to five years is alright, as long as you follow certain rules for keeping your spirit in tip-top condition.” (The Whiskey Wash) What Arran and the Scotch Whisky Association say about an open bottle is in Does whiskey go bad?
What to do about a low bottle
Pour it into something smaller. The Glenlivet says “By decanting it into a smaller bottle, there will be less interaction with the air.” (The Glenlivet)
The closure matters too. Camper English wrote up a talk by Bacardi’s scientists on his site Alcademics, and his notes say “corks allow oxidation; screw caps less.” (Alcademics)
Where the oxidation clock comes from
When you open a bottle in Dram+Vine and log your pours, the app tracks how much is left and sets the oxidation clock from it. At 40 to 50% ABV, a bottle over half full gets 36 months. Under an eighth full, at the same strength, it gets two.
The steps by fill level come from published guidance on headspace, and the top one, three years, falls inside the one-to-five-year range above. I add a proof adjustment on top: more time above 50% ABV, less below 40%. Those are my own numbers. None of the sources here tested proof against how fast an open bottle fades. The clock page also suggests moving a low bottle into a smaller amber one.