Buying is easy to measure because it arrives with receipts. Drinking is not, so a collection tends to grow in one direction. Twelve months of opening history turns the guess into a number you can plan with.
Count what you opened, not what you bought
Purchases are the wrong measure, because they include the case that is still in the hallway and the six bottles you are keeping for later. Openings are the only number that reflects what the collection is actually for. If you record openings, the count takes a minute. If you do not, the opening history guide covers keeping the record without deleting the wine.
Start with twelve months if you have them. A quarter is enough to see a rate, multiplied by four with the caveat that seasons differ: most households drink more in December than in February.
Split the number in two
One yearly total hides the decision you actually make. Everyday bottles are the ones that leave the rack without a reason. Occasion bottles are opened for a meal, a guest or a date. They behave completely differently, and mixing them produces a number that fits neither.
Bottles opened in the last 12 months: Of those, everyday: Of those, occasion or kept for aging: Bottles bought in the same period: Difference, bought minus opened: Bottles in stock now: Years of supply: stock divided by bottles opened:
Two numbers on that sheet do the work. The difference tells you whether the collection is growing, and the years of supply tells you whether that growth is a plan or an accident.
Read the years-of-supply number
Eight years of supply is reasonable for a cellar built around wines that improve with age. It is a problem for a collection of bottles meant to be drunk young, because half of them will be past their best before they reach the top of the list. Compare the number against what you actually own, using the drinking windows you have recorded rather than a general impression.
The other direction is worth noticing too. If you hold three months of supply and buy in a rush every time guests are coming, a slightly larger everyday stock will save money and improve what ends up on the table.
Buy against the rate, not the offer
A case discount is only a saving if you drink the case. At two everyday bottles a week, a twelve-bottle case is six weeks of drinking, which is a fair commitment to one wine. At two a month, it is half a year of the same bottle.
Before ordering, put the yearly rate next to the offer, then check what you already hold with the pre-purchase check. Those two habits together stop the most common way a collection outgrows its shelves.
What the number does not tell you
A year with a wedding, a long trip or a new baby is not the year the rate was built from. Gifts leave the collection without being drunk by you. Neither belongs in the everyday number, so record them as what they were and keep the rate honest.
Holding wine for years is a legitimate choice. The point of the calculation is not to shrink the collection, it is to make the holding deliberate rather than a slow accumulation nobody decided on. Recalculate once a year and after any large purchase.