In case this is useful to anyone, there are a number of free apps available that do the job quite well. They come with smart kitchen weighing scales and often can be downloaded from the app stores for free and work fine without having to buy the scale (though the scales tend to be quite cheap).
I bought a scale from a company called arboleaf ('arboleaf digital kitchen scales' on amazon.co.uk - currently priced at £22.53) and used it for about six weeks. The app is also called 'arboleaf' on the Google app store - I presume it's the same on Apple. In the app you can search for common foods or add custom foods (where you enter the nutrient and calorie values manually, one-off process per food) and add the foods to 'meal' entries in a food diary. Without the scale element you have to type in the weight for each food into the app manually. Once done you can get a breakdown of carbs and calories on a per-meal basis or for the whole day, and look back in the diary at what you ate on previous days.
If you buy the scale you can weigh each food and it syncs to the app as you're weighing it. Doing this it's a very quick process to find out how much carbs or calories are in a specific portion of fruit or vegetables for example. There's also a 'tare' function that lets you zero the scale each time you add something to the scale....
Let's say you want to make a sandwich for lunch:
1: Put a plate on the scale and tap the tare button in the app to zero it, ignoring the weight of the plate
2: Add the bread to the plate and add that to your 'lunch' meal in the app and tap 'Finish'
3: Tare the scale again to zero it, butter the bread, add butter in the app to your lunch meal and tap 'Finish' again
4: Tare again and add whatever else is going into your sandwich, item by item, zeroing the scale along the way
The process of weighing a sandwich, so long as you have all the foods saved as custom foods or saved as favorites in advance (so you don't have to search through a huge list of foods at each step) takes less than two minutes. The result is you then know how much carbs, protein and calories are in that specific sandwich, right down to the last smear of butter, accurate to around 0.5g, without having to manually type in any weight figures.
One thing the arboleaf app doesn't do is allow you to scan barcodes. That would have been very handy compared to scrolling through my long list of custom foods every time I wanted to weigh something. There is however another scale from a company called Etekcity, also with an app in the app stores called 'Vesync', and I read on another forum that that one does allow barcode scanning and is again free to use without having to buy the scale.
If anyone is reluctant to pay a subscription for a carb counting app I'd recommend checking out one of the apps above before spending any monies.
