One thing you can do is give a pretty good guess based on eyeballing the macros. At the end of the day everything is still 4 calories per gram of carbs or protein and 9 calories per gram of fat. Once you’re kind of familiar with the volume and ratios on some common food items, you can make some pretty educated guesses. ‘Hmmm 4oz of chicken breast, a handful of pasta, and we’ll call it two tablespoons of oil’ It won’t be exact, but most of us are not actually losing weight as such slim margins. Being consistent and better most of the time is still progress.
Worst case, what I have done in the past is just mark it as a portion of the same/similar item from like Applebees or some other traditionally calorie heavy chain. Chances are the homemade version isn’t topping those.
I use the app LoseIt which has a pretty big database of foods, including restaurant foods. it also allows you to enter all of the ingredients for a recipe and calculates all the macros depending on number of servings.
It’s not perfect, and like someone else said there is definitely variation in the sizes of eggs or apples or whatever, but it’s close enough.
It’s a pretty big ask, and a pretty difficult thing to do. You’re only going to have best estimates. Even with eggs, the yolk size can vary pretty widely even with large eggs alone and being that the yolk is the fattiest part of the egg, I’ve seen variances large enough to be at least 50-100 calories from the smallest to the largest yolks I’ve seen. Same with steaks from fattier cuts to leaner cuts of the same cut.
I may not be understanding but how are you buying loose food without packaging? Like in bulk? You mention oats, you should be able to find oats that are packaged in the store and use the same nutritional value and use a food scale to measure.