225 or 228
The Bordeaux barrique is 225 litres and the Burgundy piece is 228, and both get called a barrel in conversation. The gap is 1.3 percent, which is nothing on one vessel and 120 litres across a forty barrel cellar — half a barrel of wine that either exists or does not depending on which convention the spreadsheet was built on. Hogsheads at 300 and puncheons at 500 are the other two common sizes. If your cooper quotes something else, the last option in the list takes the number straight.
A used barrel is also not necessarily the volume stamped on it. Barrels get shaved, rebuilt and re-headed, and the working capacity drifts. If a topping figure keeps coming out wrong against the cellar record, the capacity is a good place to look before the evaporation rate.
The rate compounds
Four percent a year over eighteen months is not six percent. It is 5.93 percent, because the second year applies to what the first year left. Forty barriques at 225 litres lose 534.6 litres over eighteen months on the compounded arithmetic and 540 on the straight-line one — five and a half litres, or seven bottles, across the whole cellar.
That is a trivial difference and it is worth seeing precisely because it is trivial at eighteen months. Take the same rate out to three years and the two methods part by 1.07 litres a barrel, or 43 across the cellar. Take a 12 percent rate out to three years and they part by 9.33 litres a barrel, which is 373. The straight-line shortcut is safe for one elevage and unsafe as a habit.
The interval sets the void, not the total
This is the part that catches people. Topping every three weeks instead of every six does not save wine — the same volume leaves through the staves either way, and all the topping does is put it back. What the interval changes is how large the headspace gets before somebody closes it. At four percent a year on a 225 litre barrique, three weeks opens 0.53 litres of void and six weeks opens 1.06. The wine consumed over eighteen months is the same 13.36 litres a barrel in both cases.
So the topping schedule is a decision about void, and the total topping wine is a decision about the cellar and the vessel. They are different questions and the page keeps them apart deliberately.
New oak drinks once
A new barrel absorbs wine into the staves the first time it is filled and never gives it back. Five litres is the placeholder in the form and your own coopers figure or your own fill records are better. It is a one-time cost, not an annual one, which is why the page charges it only against the share of the programme that is new — and it is a real reason a first-fill barrel appears to lose more than an old one in its first year even when the evaporation rates are identical.
What has to exist somewhere
The reserve figure is the practical output. Forty barriques, eighteen months, four percent a year, thirty percent new, two rackings: 535 litres of topping wine, 60 litres into new staves, 168 litres left with the lees. That is 763 litres, or 3.4 barrels of wine that has to be held aside and cannot be sold. Cellars usually hold it as whole barrels of the same wine rather than as a measured volume, which is why the page prints the vessel equivalent.
What the topping wine may be is not an arithmetic question. Whether it can come from another lot, another vintage or another variety, and what that does to how the finished wine may be labelled and taxed, is regulated and differs by country. That is a question for your regulator and your enologist and this page takes no position on it.
Questions people ask
How much wine does topping barrels use in a year?
It is your own evaporation rate times the volume under barrel, compounded. Forty barriques of 225 litres at 4 percent a year lose 360 litres in twelve months and 535 over eighteen. The rate itself is a cellar property — humidity, temperature, new against old, and whether the barrel is bung up or on its side all move it — and the easiest way to measure it is to keep count of the topping wine you use over a year rather than to weigh anything.
How much headspace opens up between toppings?
At 4 percent a year on a 225 litre barrique, three weeks opens 0.53 litres and six weeks opens 1.06. That is the whole practical meaning of a topping interval: it sets how big the void gets, not how much wine you use. The total consumed over the ageing period is the same at either interval.
Is a barrel 225 litres or 228?
Both, depending on the convention. The Bordeaux barrique is 225 and the Burgundy piece is 228. Hogsheads are usually 300 and puncheons 500. The page carries both and lets you type in a third figure, because coopers vary and a rebuilt used barrel may not hold what it did. Across forty barrels the 3 litre difference is 120 litres, which is worth getting right once rather than arguing about later.
Why is the compounded loss smaller than the annual rate times the years?
Because each period applies to what is left rather than to the original fill. Four percent a year over three years leaves 0.96 cubed, or 88.5 percent, so 11.5 percent is gone rather than 12. Over eighteen months the difference is a few litres across a whole cellar and safely ignorable; over several years at a higher rate it is not, which is why the page shows both figures side by side.
Does a new barrel lose more wine than an old one?
It takes more wine, which is not quite the same thing. New oak absorbs a volume into the staves at the first fill and does not return it, so the first year of a new barrel shows a larger total draw even at the same evaporation rate. The page charges that absorption once, against the new share of the programme only. Your own fill records are the source for how much it is.