SEE HERE The whole idea of posting calories is interesting but making it mandatory is government overreach. As someone who counts calories every day I can tell you that it can be a tricky business and precision is just about impossible because portion sizes vary, the composition of dishes vary. Just go on-line and look at the range of calorie counts for chocolate chip cookies that look otherwise the same. If you want people to be calorie aware than teach some basics about calorie counting.
To start with there are only a few things you need to know to get started. Foods that have calories are sugar, protein, fat, and alcohol. Sugar and protein are 4 calories per gram (BTW a calorie in diet terms is actually a Kilocalorie so eating chilled food doesn't really help much). Fat is 9 calories per gram, and alcohol is 7 calories per gram. There are 28.3 grams in an ounce. Now you can take a shot at guessing the calorie count of stuff.
For example an ounce of sugar would run about 28.3x4=113 calories while an ounce of fat is 28.3x9=255 calories. If you're drinking you can check the proof of the liquor. 200 proof is 100% so if you're having an ounce of 86 proof bourbon the calories due to the alcohol would be 28.3x.43x7=85. Over time you get a sense of what things are: bread tends to run 80 calories per slice plus or minus 20 calories for example. You can develop a standard rule to estimate calories. Cheese most often in about 110 calories per ounce.
But mandating that calorie be published on menus is just a bit on the totalitarian side. It's controlling. It's stupid. And finally, it's really unlikely to be effective if it's supposed to control people's eating which is frankly none of the government's damn business in the first place.