Why a Cup of Flour and a Cup of Sugar Don't Weigh the Same
A cup measures volume, not weight, and different ingredients pack differently into the same volume based on their density and how compactly they settle — a cup of granulated sugar weighs meaningfully more than a cup of all-purpose flour, which is why a single universal "cups to grams" conversion doesn't exist across ingredients.
This is one of the most common points of confusion in baking, and it's not a flaw in measuring — it's simply how volume and weight relate differently for different substances.
Why volume and weight aren't interchangeable
A cup is a fixed volume regardless of what fills it, but different ingredients have different densities — how much mass fits into that same volume. Granulated sugar's small, dense crystals pack more tightly into a cup than flour's lighter, more irregular particles, so the two ingredients weigh differently at the identical one-cup volume.
Why the same ingredient can even vary cup to cup
Beyond differing between ingredients, how an ingredient is measured also matters — a scooped, packed cup of flour weighs more than a spooned-and-leveled cup, because scooping compacts the flour into the cup more than gently spooning it in. This is exactly why baking recipes that care about precision specify weight in grams rather than relying on cup measurements alone.
What this means practically
For any recipe where precision genuinely matters — most baking — using a kitchen scale and weight measurements, rather than volume, removes this source of variation entirely. Cup-based conversions are still useful for less precision-sensitive cooking, but treating a single "cups to grams" factor as universal across every ingredient is the root of many baking inconsistencies.