I was having coffee over the weekend with a friend that has started to make some “garage” wine.  He is looking to get some Cabernet Sauvignon from a one acre parcel planted on another friend’s property.  “I’ve got one new oak barrel,” he said, “will there be enough grapes to fill it?”

This got me thinking about numbers we use to plan for everything involved in winemaking, from labor to tank space, to barrel purchasing and case production.  These are numbers that winemakers take for granted, but might not be widely known outside the industry.  So, I thought I might take the time to put some conversions down, and highlight the ones that I use every day. You guessed it: it’s time for winemaker math.

So, how does anyone know how many grapes are in a vineyard?  This is a question that could bring a production planner to tears.  We have a pretty good idea, and can even make some “educated” guesses to help out, but in the end, Mother Nature is extremely wily, and she manages to trick us more often than not.

In order to determine the number of grapes in a vineyard we need to know the following information:

1. Weight of a cluster of grapes
2. Number of clusters per vine
3. Number of vines planted per acre

Sometimes these numbers are conflated so that we can estimate the number of tons per acre and then multiply by the acres for an easy answer.

To more accurately figure out that tonnage, we can start with the expected weight of a cluster of grapes.  This can vary by variety from about ¼ – ½ of a pound.  Right there I’ve introduced a factor of 2 on the yield from one acre.  We usually take cluster samples and weigh each cluster to agree on an average weight for the field.

Figuring out the number of clusters per vine and vines per acre planted, we start with pruning. We have to know the style of pruning in the vineyard: cane or cordon pruned, because this will determine how many clusters will be found on each vine.

And finally, we need to know the spacing of the vines, which will give us the number of vines planted per acre.  This is hugely important.  Vineyards in California are planted with a huge range in vine density.  On our vineyard in Knights Valley, for example, we have vinerows that vary from 11 feet between rows and 6 feet between vines (11 x 6 = 66 sq ft per vine) to vinerows that are 6 feet between rows and 4 feet between vines ( 6 x 4 = 24 sq ft per vine).

Here’s a quick rundown on vines/acre depending on our spacing:

11 x 6: 660 vines/acre               8 x 5: 1089 vines/acre               7 x 5: 1245 vines/acre

6 x 4: 1815 vines/acre               4 x 4: 2722 vines/acre               meter x meter:  4046 vines/acre

OK, so let’s say the vineyard block is cordon pruned and 8 x 5 spacing.  Using the numbers above for vines/acre, now the calculation is simple:

Now we’ve got our estimate.  Unfortunately, sometimes despite our best efforts, the estimates are off. It’s not until we pick that we might find out the cluster weight we assumed as our average was off by 20%.  Now, instead of 4.4 tons / acre, we have 5.2 tons / acre and we didn’t buy enough barrels and we have a crunch on fermentation space in the cellar!

So aside from grapes produced per acre in a vineyard, what other winemaker math might you find interesting?

How about: how many gallons in a ton of grapes?  This varies depending on the condition of the grapes and how hard we squeeze them, but normally 140 – 180 gallons per ton is to be expected.  I use 160 gals / ton for most calculations.

The next logical question is: how many cases of wine in a ton?  Using 160 gallons, and the conversion factor we use for a 12 x 750 mL case (2.37753 gals / case), you would get 67 cases per ton.  In reality, we expect more like 60 cases per ton.  This takes into account losses along the way.  It accounts for everything from wine lost when racking, moving and filtering, or from the evaporation that occurs over time while aging in the barrel.  Some people call this the “angels share.”

How many bunches of grapes go into a bottle of wine?  Again, we have to make some assumptions, but using the ¼ lb per bunch we set up above, we can estimate as follows:

Now what about barrels? How many vines would I have to farm to fill a barrel? Using the math derived above, we should expect 32 clusters on our representative Cabernet vine, at 0.25 lbs each that makes 8 lbs of grapes per vine.  Our barrel holds 60 gallons, so 0.375 of a ton, or 750 lbs.  Divided by 8 lbs per vine we get to about 94 vines to a barrel.

So it looks like my friend who is interested in making some garage Cabernet might soon be swimming in wine if he’s not careful. With my math, I’m estimating his acre is going to produce about 700 gallons. That means he’ll need almost 12 barrels. I hope he’s ready.