A drink's calories come from two places: the alcohol itself and the carbohydrate, mostly sugar, that is left in it. The alcohol follows directly from the strength printed on the bottle, so the only numbers you need are the volume, the % vol and, for beer, wine and liqueurs, a typical carbohydrate figure.

How it works

The alcohol. Volume × % vol gives the millilitres of pure alcohol, and alcohol weighs about 0.789 g per ml. That is the same conversion behind the US standard drink, where about 0.6 fl oz of alcohol is 14 g (NIAAA).
Its energy. EU food law counts alcohol at 7 kcal per gram (Regulation (EU) No 1169/2011, Annex XIV).
The carbohydrate. Grams per 100 ml × the volume, at 4 kcal per gram. The calculator starts from USDA's figure for the drink you choose: 3.55 g per 100 ml for a regular beer, about 2.6 g for a dry red or white wine, 13.69 g for a sweet dessert wine and none for spirits. USDA gives these per 100 g; beer and wine weigh within a few per cent of their volume, so the figure is used per 100 ml.
A mixer, if you add one, is counted from its own USDA figure.

A worked example

A UK pint (568 ml) of lager at 5%: 568 × 5% = 28.4 ml of alcohol, which weighs 22.41 g and carries 156.9 kcal. Its 3.55 g of carbohydrate per 100 ml adds 20.2 g, or 80.7 kcal. Together that is about 238 kcal.

The same arithmetic for spirits: 100 ml at 40% holds 31.56 g of alcohol, which is about 221 kcal. A 50 ml measure at 40% is about 110 kcal; at 52%, about 144 kcal.

Common drinks, worked out

Common drinks, worked out
Drink	Size	Strength	About
Lager	330 ml bottle	5%	138 kcal
Lager	568 ml UK pint	5%	238 kcal
Dry red wine	150 ml glass	13.5%	128 kcal
Dry white wine	150 ml glass	13%	123 kcal
Vodka, gin or whisky	44 ml US shot	40%	97 kcal
Vodka, gin or whisky	25 ml UK measure	40%	55 kcal
Gin and tonic	50 ml gin, 150 ml tonic	40%	161 kcal

Why drinks rarely carry a nutrition label

In the EU, drinks with more than 1.2% alcohol by volume don't have to show a list of ingredients or a nutrition declaration (Regulation (EU) No 1169/2011, Article 16(4)). Some brands print the energy anyway. When yours does, trust the label; otherwise, the strength is always on the bottle, and it decides most of the answer.

What it can't tell you

Sweetness varies more than strength: a sweet wine or a cream liqueur can carry several times the sugar of the preset, so enter the carbohydrate from the label when it gives one. Home-made spirits often don't state their strength at all; the result is only as good as the % vol you enter. It says nothing about the effects of alcohol on health.

How many calories are in a shot of vodka?

About 97 kcal in a 44 ml (1.5 fl oz) US shot at 40%, and about 55 kcal in a 25 ml UK measure. Vodka, gin, rum and whisky at the same strength are the same, because almost all their energy is alcohol.

Does wine or beer have more calories?

Per 100 ml, dry wine has about twice the energy of beer: roughly 82 to 85 kcal against 42, because it has about two and a half times the alcohol. Per usual serving they are close: a 150 ml glass of dry red at 13.5% is about 128 kcal and a 330 ml bottle of 5% lager about 138 kcal.

Why does the strength matter more than the brand?

Because alcohol carries 7 kcal per gram and most drinks have little else. Each extra percentage point of strength in a 500 ml beer adds about 28 kcal.

Sources

USDA FoodData Central, SR Legacy: beer, regular, beer, light, red wine, white wine, rosé, dessert wine, sweet, distilled spirits, 80 proof, coffee liqueur and tonic water. The other mixers come from the same database, with their record numbers in the tool's data.
Regulation (EU) No 1169/2011: Annex XIV (7 kcal per gram of alcohol) and Article 16(4) (no mandatory nutrition declaration above 1.2% alcohol).
National Institute on Alcohol Abuse and Alcoholism, What is a standard drink?: 14 g of alcohol is about 0.6 fl oz.

For a meal out, add the drinks to the food with the menu calorie calculator. For anything with a label, the nutrition label calculator scales it to your amount.