A cup of flour is not a weight
A recipe that says "one cup of flour" is not telling you how much flour to use. It is telling you how much space the flour should fill. Those are different things, and the distance between them is why a recipe can work one week and disappoint the next with nothing obviously changed.
Volume and mass measure different properties
A cup is a unit of volume, and in US measures it sits on a chain of exact definitions. One US gallon is exactly 3.785411784 L. A US fluid ounce is a hundred-and-twenty-eighth of that, so exactly 29.5735295625 ml. A cup is eight fluid ounces, so exactly 236.5882365 ml. Nothing anywhere in that chain says a word about what is in the cup.
A gram is a unit of mass. Getting from the first to the second needs a third number that the recipe usually does not give you: how much mass that particular ingredient packs into that much space. Water makes it look easy, because 1 ml of water weighs very nearly 1 g, which puts a cup of water at roughly 237 g. Almost nothing else in a kitchen is that obliging.
Why flour is the awkward case
Flour is a powder with a great deal of air in it, and how much air is in it depends entirely on what happened to it in the last ten seconds. Push the cup down into the bag and you compact it. Spoon it in loosely and level the top and you do not. Sift it first and it is looser still. Tap the cup on the counter and it settles. None of that changes the volume — the cup is the same cup — and all of it changes the mass.
Figures commonly quoted for a US cup of plain wheat flour spread across a range of roughly 120 g to 150 g, and that spread is the honest answer rather than a failure to look it up. It moves with the milling, the variety of wheat, how humid the room is, and how the cup was filled. Treat any single figure as approximate and specific to one ingredient, and do not lean on it for a bake where the ratio genuinely matters.
That range is also large enough to matter. The difference between the bottom and the top of it is about a quarter of the flour in the recipe, which is the sort of change you would notice if somebody made it on purpose.
So the conversion cannot be one number
Every ingredient has its own volume-to-mass relationship, and they are nothing like each other. Granulated sugar is dense and pours consistently, so it behaves far better than flour. Brown sugar is usually packed, which means the recipe is quietly asking you to compress it by an unspecified amount. Cocoa and icing sugar are light and clump. Oats, chopped nuts and anything in irregular pieces depend on how the pieces happen to stack that day.
This is why recipes written to work in more than one kitchen are written in grams. A gram is a gram in every kitchen on the planet, and it does not care how the ingredient got into the bowl.
What the converter does, and what it will not do
The free Cooking Converter moves between milliliters, teaspoons, tablespoons, cups, fluid ounces, pints, quarts and liters. Every one of those is a measure of volume, so every conversion it does is exact arithmetic on defined quantities — no ingredient, no assumption, nothing to be wrong about.
It will not turn cups of flour into grams, and that is not a missing feature. A tool that offered it would have to pick one number for an ingredient that does not have one, print it with the same confidence as 3 teaspoons = 1 tablespoon, and hand you a false precision that reads as authority. The number you would need is a property of your flour, not of the software.
The relationships worth knowing by heart
All of these are exact by definition, not roundings:
3 teaspoons = 1 tablespoon16 tablespoons = 1 cup, which is also48 teaspoons = 1 cup1 cup = 8 US fluid ounces2 cups = 1 US pint, and2 pints = 1 US quart, so4 cups = 1 quart
In metric terms a tablespoon is exactly 14.78676478125 ml and a teaspoon exactly 4.92892159375 ml. Nobody needs those digits, but they are where the converter's answers come from.
What to do instead of converting
If a recipe gives both weights and volumes, follow the weights. If it gives only volumes and it is a bake where the ratio carries the result, a kitchen scale with a tare button removes the whole problem: put the bowl on, zero it, add the ingredient, zero it again for the next one. There is one bowl to wash and no scooping technique to get wrong.
And when you do measure by volume, be consistent with yourself. A recipe you wrote down after scooping straight from the bag is a recipe that expects you to scoop straight from the bag. The measurement is only half the instruction; the habit is the other half.