Here’s the maths that I use, I Hope hope it helps. It means you have to weigh your portion befor cooking .
Math 4 carb counting .
I'll use an example of breakfast oats, dry weight they have 60g carbs per 100g, I have 45g so the maths I use is 60 divide by 100 and then multiply by 45 so you take the amount of carbs stated for 100g and divide by that and then multiply by the weight of the portion you are consuming .
Hi Pat, can I suggest an alternative way of getting into this?
This is to get a sheet of paper and write down everything you ate in a week before you started to think about things. It must include everything, including fruit, biscuits, meals, snacks. It does not have not have to be accurate, it just has to give you a good idea.
Then against each item, write down the number of carbs in it.
Some times that is dead easy. You can go to a tin or a packet of the same thing in your cupboard and get it. For example something like crackers. Most packets will tell you what the carbs are in a cracker and you can multiply that be the number you ate to get the total. The packaging standards are a bit variable but use a bit of common sense. You are not looking for accuracy, just a good idea. I sometimes have a tin of soup and usually consume the whole tin. The label on the side might tell me how many carbs there are in 100g of soup and so a 200g tin of soup will have twice that number. Some times it will tell me how many carbs there are in half a tin so in that case, I have to double it.
Sometimes it is a lot harder. What you might have to do is to look up what carbs there are in something and then work out how much of the something was in your portion and do a calculation. Take potatoes for example. Go to a standard list like this one on the diabetes UK website
and look up potatoes. I tells you that 100g of boiled potatoes contains 16g carbs. Next time you have potatoes weigh out your portion. If it is 50g, then that is 8g carbs, if it is 200g then it is 32g carbs. You do not have to be precise, just get a good idea. You only need to do it once, whatever number you get is your number for boiled spuds. Start to do that for everything on your list. You will quickly find that some things can be ignored because the carb content is so low and other things might surprise you because the carb content is much higher than you thought. Might take a few days to complete but when done, add the lot up and divide by seven and you will have (roughly) the number of carbs you used to eat in a day.
Doing this will give you three things.
First off, a lot of practice in carb estimation and the more practice you do the easier it will be.
Second off, you will see how much carb you have been eating. Might be 100g/day, might be 500g/day, depends on your eating habits and what you like to eat.
Third, and the important thing is that you can now set a target to reduce the carbs you are going to eat based on what you have been eating. So if you used to eat 500g/day, you could reduce that to 250/day. If you used to eat 100/day, then maybe you could reduce it to 80/day. You can use your list to see what things you can stop eating (look for the things with a lot more carbs than anything else) or more importantly where you can adjust portion sizes to get your target (small portion of spuds compensated by a bigger proportion of green veg). That way you won't be looking for big changes in what you eat so it might be easier to sort out.
Sounds a bit of a faff but is a lot simpler than it might sound. It is one way into getting to grips with a carb controlled way of eating and might give you something more interesting to do than watching daytime TV in current circumstances!
Well @Docb has summed up what I was going to suggest.
You will also start to get an idea of what 20g carbs looks like in potatoes, ...
Then when we are let out again that will help when you are eating out.
We have a list of our most common foods on our noticeboard, in two ways.
One lists the carbs per 100g for raw ingredients such as all the veg, new potatoes,
old potatoes, rice, .... . When cooking we weigh out stuff and calculate the carbs
Same way as Docb. Divide the carbs by 100, and multiply by the weight of the item we are using
Our other list is how much of each common ingredient we use, to get 25g of carbs.
That is our target for the main carbs bit (potatoes rice pasta) and the extra 5 g for us comes from other stuff. We worked these out, and now don’t need to do it again.
Having said all that the new potatoes bought from different places are labelled with different carbs per 100g. Ho hum. We just stick with our numbers and in the grand scheme of things it is an ish number we are going to get. It still helps us stick to our target of 30g (ish) per meal.
I have a very easy solution if you're not into maths, paper and pencil. Join a website with downloadable app, like NutraCheck (they do a week's free trial before commitment). There are others which do a similar job. It has thousands of foods on it. Just weigh the portion, find the food using the search, enter the weight for that food and it will tell you the carbs. It will keep a running total per meal, per day and per week. For example, my breakfast was 1 24gm Oatibix 15.4gm carbs, 125ml unsweetened coconut milk no carbs, 2gm Truvia 2gm carbs. So total carbs 17.4 gm.
In answer to your original question, if 100gm of food has 20 gm carbs, then 300gm food will have 60 gm carbs.