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What is the Angels' Share? Whisky Evaporation Explained

How the angels' share works in whisky maturation: evaporative loss, humidity, warehouse microclimates, and why older whisky costs more, cask by cask.

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While whisky rests in a wooden cask, a small part of it slowly evaporates through the oak. Distillers call this loss the angels’ share. The name gives a heavenly explanation to a problem that also appears, less poetically, on an inventory sheet.

It is a real cost to the distiller, but it comes with the process that makes whisky mature. The same slow exchange of air and vapour through the staves that lets spirit escape also lets oxygen in, which helps the spirit soften and develop fruity, ester-rich notes over time.

Why does whisky evaporate through wood?

Unlike glass or stainless steel, an oak cask is slightly porous. Coopers build casks from seasoned staves of American white oak (Quercus alba) or European oak (Quercus robur), held together by metal hoops without glue or nails.

Spirit soaks into the staves, and some of it works its way through the wood and evaporates from the outside, while a little air finds its way in. As temperatures change through the seasons, the liquid expands and contracts, which drives more of this exchange.

How much spirit is lost each year?

The rate depends mainly on climate (temperature and humidity) and on the warehouse. The angels, it seems, do not take the same share everywhere:

Angels' share a year, by climate: the share of a cask lost to evaporation each year. In India it is 11% to 12% a year, about six times the Scottish rate. In Taiwan it is up to 15% a year. In Scotland it is about 2% a year.

Does ABV rise or fall during ageing?

Whether a cask’s strength goes up or down depends largely on humidity:

In Scotland, strength slowly falls

Scottish warehouses are cool and humid. With the air around the casks already damp, water evaporates from the cask relatively slowly, and alcohol escapes a little faster than water. Both the volume and the strength fall slowly over the years. Scotch whisky must be bottled at no less than 40% ABV, so a cask that drops below that can no longer be sold as Scotch whisky.

In hot, dry warehouses, strength often rises

Where the air is hot and dry, as on the upper floors of many Kentucky warehouses, water evaporates through the wood faster than alcohol. The volume falls while the strength climbs, sometimes past 65% ABV after several years.

Warehouse styles: dunnage, palletised and racked

How casks are stored changes the conditions around them, and with it the angels’ share:

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Warehouse styleConstructionTemperature and humidityEffect on evaporation
Traditional dunnageLow stone walls, earth floors, slate roofs; casks stacked about three highCool, damp, stable all yearUsually slower, steadier loss; strength falls gently
PalletisedModern buildings, casks stood upright on palletsFairly even conditionsConsistent maturation from cask to cask
Racked / rickhouseTall metal or timber racks over several storeys (common in the USA)Hotter, drier at the top; cooler at the bottomLarge differences between floors; upper floors lose water fastest

Why older whisky costs more

The angels’ share compounds year after year, which is one reason older whiskies cost more. At 1% to 2% a year, this is roughly how much of a Scottish cask is left:

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AgeLeft in the cask at 2% a yearLeft in the cask at 1% a year
12 yearsabout 79%about 89%
18 yearsabout 70%about 83%
25 yearsabout 60%about 78%
30 yearsabout 55%about 74%
How much of a Scottish cask is left, if it loses 1% to 2% of its contents a year to evaporation. After 12 years, about 79% to 89%. After 18 years, about 70% to 83%. After 25 years, about 60% to 78%. After 30 years, about 55% to 74%. These figures are worked out from the yearly rate, compounded, not measurements of real casks.
  1. Less whisky to bottle: After 18 years a cask has lost roughly a sixth to nearly a third of its spirit, and after 30 years between a quarter and almost half.
  2. Years of storage: The distillery has to store, insure and check the cask for decades before it earns anything.
  3. Fewer casks make it: Some casks fall below 40% ABV or become over-oaked, and are never bottled as old single malts.

Our guide why old whisky is expensive covers the other reasons, from storage costs to the leaner production years that today’s oldest whiskies come from.

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About the author

John Fegan

· Editor

Hailing from the land of haggis and deep-fried Mars bars, John was always going to find his way to whisky. He has run Uisge Beatha since 2015 and Whiskipedia.com since 2020.