Calculating food

Grams is a weight measure, like pounds and ounces (not fluid ounces, weight ounces). Cups and fluid ounces are volume measures. There's no simple generic way to convert between a weight measure and a volume measure. A cup of dry oatmeal (rolled oats), and a cup of vegetable oil, are going to have very different weights.

What to do: If you're not using a food scale to weigh your food, that's how to get a weight measure of the food you're eating. Most people here use grams because food scales typically register individual grams, but only tenths of ounces. Since there are roughly 28 grams in an ounce, gram measurements are a little more precise.

Then, you want to find MFP food database entries that are in weight measures, grams or ounces (usually if one of those is the default, the other one will be in the drop-down list of options). Some MFP database entries even contain both volume and weight measures in the drop-down. (Be careful about confusing ounces (of weight) with fluid ounces, though.)

Most packaged foods (in the US), even packaged whole foods, have both a weight measure and a volume measure on the label. You can use either of those, because they're specific to that food. If the weight measure is in grams, and you know the weight of your portion in grams, you're all set.

If you can't find a database entry in grams (or ounces) for a packaged food, you can use the label information to convert. Let's say the label says half a cup is 50 grams. You find an MFP database entry that has the right macros/calories for half a cup (matches the product label), but you ate 75 grams. In this case, just use arithmetic: 75 divided by 50 is 1.5, so you ate the equivalent of 1.5 servings of half a cup.

There are some other tweaks and wrinkles in doing this in all cases, but the above is the basic idea.