Basically, measuring by volume is okay for things that don't have pieces that can change shape. Liquids like water or milk or oil and powders like sugar or flour fill up the whole cup, with no spaces in between. But if it's something like strawberries, in order to be accurate you need to use weight. If you have huge strawberries, only two or three will fit in a cup and there will be big empty spaces between them. If you slice the strawberries, the pieces will fit together and fill up more of the space, so you get more strawberries in the cup. Or think about cereal. You can measure out a cup of cereal that weighs an ounce, but then if you take your finger and smoosh it, you can turn that cup into half a cup (but it still weighs an ounce.) A cup of that smooshed cereal will weigh twice as much and have twice as many calories as a cup of nice, pretty, perfect cereal from the top of the box. This is also why sometimes you open a brand-new box of something like crackers and find it half empty. It was full when it left the factory, but as it bounced around on trucks and got tossed onto loading docks and thrown back and forth by stockboys, some of the pieces broke and fit together more tightly. It still weighs the same (and has the same calories.)
If a recipe calls for a cup of liquid or a cup of flour, you're usually safe just measuring a cup and finding the listing in the database for a cup. But if you're serving yourself a bowl of cereal, it's better to put your bowl on the scales, set it back to zero, and then weigh the cereal. I also weigh things like peanut butter or salad dressing; even though they fill a measuring spoon like a liquid, it's messier to measure them than to put your salad plate or bread on the scale, zero it, and weigh. (If there's anything you should NOT guess at, it's peanut butter, because it's easy to think you're using a tablespoon when you're actually using 1/4 cup, and that's a 300-calorie error.)
When it comes to fruit and vegetables, it's up to you. Most of them aren't very calorie dense, so if you think you're eating 1/2c of broccoli but it's really a cup, that's only about 25 calories. Personally, I weigh fruit and starchy veggies like sweet potato, but I don't worry about other veggies.
The other thing that sometimes confuses people is that there are two different things that are both called "ounce." One measures weight and the other measures how much room something takes up. The second one is sometimes called "fluid ounce." It just so happens that one fluid ounce of water also weighs one ounce-- but that only happens with water. A cup of water is 8 fluid ounces, and it also weighs 8 ounces (half a pound.) A cup of cereal doesn't weigh anything close to half a pound-- it will usually be one sixteenth of a pound (one ounce) to two sixteenths (two ounces.) Usually only liquids and very squishy things like yogurt are sold/measured in fluid ounces.
| current weight: 132.0