My husband had a question I don’t know the answer to,so here goes:
Can you use calories and fats to calculate carbohydrates. My answer,I don’t know. He asks this as restaurants show calories and fats. But no carbohydrates. Any ideas? Nancy
You can use calories and fat to calculate the total amount of both protein and carbs.
(Calories - (fat grams x 9))/4 = carb grams + protein grams
So if something had 500 cal, 10 grams of fat, you could go 500-(9 x 10) = 410, 410/4 = 102.5, so roughly 103 grams total of both carbs and protein.
You’d have to be able to estimate the amount of protein to estimate the amount of carbs from that. At that point it’s more of a guessing game though. That said, there are some meals where I know I’d feel able to guess at that pretty accurately, since there usually are fewer sources of hidden protein than carbs or fat (which can be snuck in via sugar or butter or oil in hard to guess ways).
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It may be possible but it would not be easy. Carbohydrates do provide calories but they are not the only source. Calories can also come from proteins and fats.
I suppose you could subtract other sources of calories from total calorie count to find how many calories are from carbs and then calculate the total, each gram of carbohydrate equals four calories.
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It’s also worth pointing out that restaurant-provided nutritional info is unlikely to be very precise anyway—even packaged foods can be off by quite a bit, and in restaurants, unless it’s a fast-food joint with entirely pre-made and pre-portioned food, often employees are creating portions themselves, so for example the salad I get from Chipotle one day likely isn’t exactly the same as the one I get another time, even if I order it the same way. (They can make a pretty good low carb salad by the way!) I usually assume restaurants are reporting the minimum possible calories/macros, vs the definite amount I’m getting.
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Thanks,good thing I know what I eat most of the time. This all started when I wanted a donut. Yes,it is a once in a while treat after I swim. Thank you so much. Nancy
For a donut, it would probably work or at least give you a close estimate, since there is almost no protein in it.
My vote: no
There is a pocket book that has a lot of restaurants and other food info called Calorie King, I think. It’s on amazon site. Easy to take to restaurant and eliminate the guesswork. I have 3 copies stashed in different places. It has carbs, calories, fat info for lots of food choices.
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I have a copy at home,I need to use a magnifier on my desk to read it. I am pretty good looking it up online before I eat out. We live in a small town, eating in the city is rare. Thanks! Nancy
I forgot to add there is an app, too. I use it on my phone now and then. Most of what I eat, I have the carb amounts memorized. 
There is a large print version, too.
Thanks,I will check out the App. Nancy
What I do to get me close is I use the Calorie King app, there’s quite a few similar apps out there, and look up the same food in question. It’s just a ball park count, but it gets me close enough. I may have to take a small correction afterward or maybe a half a glucose tab if it’s too much. If I think I’ll eat there again, I’ll record my results and perhaps slightly modify the carb count.
Also, I now look up the menu and figure out what I’m going to eat before I go. I can do my carb research and can bolus more confidently.
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Calorie king
Also if you have a iphone there is a free app to download for restaurant info. Also I bought a digital scale off of amazon that calculates foods. It comes with a thorough book of codes for foods. The side looks like the nutritional label on the back of packages it breaks down everything
I gave up doing these calculations in my head years ago, instead I carry along a handy guide book that tells me the amount of calories in just about any kind of food I might encounter, when I go grocery shopping or eat at restaurants or even when analyzing a meal I make at home.
The book I use does not seem to be available but I noticed one that looks like it and goes for less than 10 bucks at Amazon: You can find it by googling or doing a search at amazon.com for The Everything Calorie Counting Cookbook.
If the restaurant is a chain or franchise and you’re in the US, ask for a nutrition sheet. Or look up the menu online using your phone. Also, most restaurant meals can be found in the MyFitnessPal database. You can look information up by searching google and typing nutrition after the meal name.