How many calories are in a standard glass of red wine?

How to get close to this number

Go through the alcohol, not the grapes. A standard glass holds a modest volume, of which roughly an eighth is ethanol, and ethanol carries seven calories per gram — the figure in the explanation and the one almost nobody knows. Multiply the alcohol mass by seven and you have nearly the whole answer, because a dry wine contributes very little sugar.

The misconception is that wine calories come from sugar. In a dry red they mostly do not; the sweetness people taste is largely fruit aroma rather than residual sugar. That is why switching to a dry wine saves far less than expected, and why a sweet dessert wine is in a genuinely different category rather than a slightly higher one.

The seven-calories-per-gram figure is worth carrying beyond this question. Alcohol sits between carbohydrate and fat in energy density, closer to fat, and it is metabolised preferentially — the body burns it first and defers everything else. Note too that a "standard glass" is a defined measure that bears little relation to how much most people actually pour, which is the largest source of error in any real-world application of this number.