In viticulture, the yield is a measure of the amount of grapes or wine that is produced per unit area of vineyard, and is therefore a measure of crop yield. Two different types of yield measures are commonly used - the mass of grapes per vineyard area and the volume of wine per vineyard area. The yield can often viewed as a factor for quality, as lower yields are associated with wines with more concentrated flavours, and the maximum allowed yield is regulated for many wine appellations around the world.
Across Europe, yield is measured in hectolitre per hectare - or by the volume of wine. In many of the New World winemaking counties, yield is measured in tons per hectare - or by the mass/weight of grapes, measured in tons or kilograms per hectare.

 

There are a great many variables that affect the yield of a vineyard - vine spacing, row spacing, soil and vintage conditions, the varietal and age of vines all play their part in how many grapes you will harvest from each vineyard.
Due to differing viticultural practices and winemaking procedures for different styles of wine, and diverse composition of different grape varietals, the amount of wine produced from a unit mass of grapes can vary greatly. It is therefore not possible to make an exact conversion between these units. While yield can be seen as an important quality factor in wine production, views differ on the relative importance of low yields to other aspects of vine health and management. There is a consensus that if vines are cropped with a very high number of grape clusters, a poor wine can/will result because of slow and insufficient ripening of the grapes, due to an unfavourable leaf surface to fruit ratio.
As the grape ripens, it draws on reserves from the leaves. The leaves collect sunlight, and use its energy to produce carbohydrates. These are transferred into the grape during ripening, and result in an accumulation of sugars, flavour compounds and tannins. The less fruit the vine has per unit measure of leaf area, the more flavour, colour and tannin it will gather during ripening. Think of the vine leaf canopy as a solar panel, and each grape as a small light bulb that is connected to the panel. One canopy practice involves manipulating the position of the vine shoots to allow better interception of sunlight by the leaves. This is done by trellising the canopy in such a way that it is split it into various layers, by lifting the shoots up off the natural drooping position, into a more upright position using wires attached to the vine trellis. If done well, this can result in higher yields with no obvious reduction in quality.
One school of thought subscribed to in France, claims that great red wine is impossible to produce at yields exceeding 50 hl/ha. Another claims that a yield of 100 hl/ha is possible to combine with high quality, provided that careful irrigation, canopy management is used. In general, white grapes are seen as less sensitive to higher yields, whereas some varietals such as Pinot Noir, Sangiovese and Nebbiolo are particularly sensitive to over cropping.
In France and Italy, the maximum allowed yields are regulated by wine laws, and vary between appellations. Italian wines are labeled with a DOC or DOCG - the yields are controlled and limited by law, so you can not call a wine Chianti or Brunello if the yield is not below a set level. Italy has long recognized that limiting fruit yield is a quality boost to their wines, setting up appropriate regulations & labeling requirements to protect their wine's reputation.
Another factor is how ‘hard’ grapes are pressed, or if they are pressed at all, will impact the final volume of wine yield. A winemaker can choose to use only the free-run juice liberated during crushing and maceration, reducing the yield volume by 30-40%. Yields can vary from commercial quantities of about 35 - 40 tonnes/ha to super low levels of around 1 tonne/ha for example with old, dry grown bush vines. At 'average' volumes - 1 ton of grapes results in a little more than three barrels of wine. Each barrel contains about 225 litres or 300 bottles.
NZ example: Still Wine - on average approximately 1 tonne of grapes = *750 litres of juice.
AOP Limits: Champagne - approximately 1 tonne of grapes = *510 litres of juice.
What is an appropriate yield, depends on a number of factors, but also important is the style of wine being crafted, and the grape varietal grown. If a winemaker wants to make a medium bodied wine, with moderate levels of concentration for early drinking, then it makes no commercial sense to crop at the lower levels. The reason why consumers have such a large choice of good value wines under NZ$15 - is because many wine producers crop their vineyards at the medium to higher end of the yield scale.