Without the Original Gravity (OG) and Final Gravity(FG) it is not an accurate measure of calories because when a beer is done fermenting the attenuation for every beer will be slightly different resulting in more or less unfermented sugar in the beer. The more unfermented sugar left in the beer the more “sweetness” (and calories) a beer has as opposed to a beer with less sugar will taste “drier”(and have less calories then a sweeter beer with the same ABV).

• calories per 12 oz beer = [(6.9 × ABW) + 4.0 × (RE - 0.1)] × FG × 3.55

where

• ABW = Alcohol By Weight
• RE = “Real Extract” (RE, in °P) is a measure of the sugars which are fermented
• FG = Final Gravity

Use a specific final gravity and calculate backwards to determine the ABV (Alcohol by Volume) which you can then convert to ABW by a simple formula:

• ABW = (0.79 × ABV) / FG)

This is the only accurate way to determine calories. Since most breweries do not list these “Beer Secrets” we have to guess based on one data point, ABV, instead of two.

So based on above formula:

• If a beer has a OG (original gravity of 1.048 and a FG of 1.010 would result in a beer of 5% ABV and 163 calories.
• If a beer has a OG of 1.055 and a FG of 1.017 you will get a beer that is also 5% ABV, but has 189 calories.

Source: http://www.simplybeer.com/blog/how-many-calories-are-in-my-beer/

A guestimation formula that works well enough is:

• fl. oz. * ABV = potency / 60 = beers * 150 = Estimated Calories.

With an average beer (indicated as beers below) at 5% in a 12 fl oz size coming out to about 150 calories on average, applying this formula to a Budweiser works like this:

• A 12 fl. oz. Budweiser * 5% ABV = 60 (potency) / 60 = 1 beers * 150 = 150 estimated Calories

Calculating calories per gram of carbohydrates, fat or protein is easy. All you need to remember is that:

• 1 gram of carbohydrate = 4 calories
• 1 gram of protein = 4 calories
• 1 gram of fat = 9 calories
• 1 gram of alcohol = 7 calories

So with this in mind, we can derive a scale with which we can fairly accurately guess at the caloric content of beers according to the ABV rating.

• 12*5% = 60 / 60 = 1.0 * 150 = 150C (e.g. Budweiser American Ale)
• 12*6% = 72 / 60 = 1.2 * 150 = 180C (e.g. half a Rogue Shakespeare Stout)
• 12*7% = 84 / 60 = 1.4 * 150 = 210C (e.g. Sierra Nevada Torpedo)
• 12*8% = 96 / 60 = 1.6 * 150 = 240C (e.g. Russian River Pliny the Elder)
• 12*9% = 108 / 60 = 1.8 * 150 = 270C (e.g. half a Brooklyn’s Local 2)
• 12*10% = 120 / 60 = 2.0 * 150 = 300C (e.g. half a Brooklyn Black Ops)
• 12*11% = 132 / 60 = 2.2 * 150 = 330C (e.g. Rochefort Trappistes 10 Ale)
• 12*12% = 144 / 60 = 2.4 * 150 = 360C (e.g. Dogfish Head Santo Palo Marron)
• 12*13% = 156 / 60 = 2.6 * 150 = 390C
• 12*14% = 168 / 60 = 2.8 * 150 = 410C
• 12*15% = 180 / 60 = 3.0 * 150 = 440C
• 12*16% = 192 / 60 = 3.2 * 150 = 470C
• 12*17% = 210 / 60 = 3.4 * 150 = 500C (e.g. Mikkeller 黑)
• 12*18% = 222 / 60 = 3.6 * 150 = 530C (e.g. half a Dogfish Head World Wide Stout)
• 12*19% = 234/ 60 = 3.8 * 150 = 560C
• 12*20% = 246 / 60 = 4.0 * 150 = 590C
• 12*21% = 258/ 60 = 4.2 * 150 = 620C (e.g. Dogfish Head’s 120 Minute IPA)

Source: http://beercritic.wordpress.com/beer-calorie-cheatsheet/