A measuring cup of all-purpose flour can weigh anywhere from 113 g to 150 gdepending on a single decision: whether you scooped the cup directly into the bag, or spooned the flour in. That's a thirty-percent swing on the same ingredient measured the same nominal way. For thirty percent more flour, your cookies stop spreading, your cake gets dry, your pie crust toughens. The recipe is the same; the outcome isn't.
The reason is simple. Flour is a powder full of air. The volume a cup measures is whatever the flour has been compressed (or not compressed) into. Scooping the cup directly into the bag forces flour down through the tip of the cup, packing it as the cup descends; the result is a heavily compressed cup. Spooning flour gently into the cup, then levelling with a knife, leaves a much loftier amount of flour at the same nominal volume.
How King Arthur measures (and what we use)
King Arthur Baking has been the de-facto American standard for ingredient densities for about three decades. Their reference says: 1 cup of all-purpose flour = 120 g, measured by spooning flour into the cup and levelling with a knife. Most American recipe writers either don't name a method (and silently assume scooping) or name spooning and meet King Arthur.
At ScaleRecipe we use 120 g/cup throughout the ingredient converter. The number is right for most American recipes published this century; it's a little light for some older recipes that assumed scooping. When you adapt a recipe and the dough feels persistently wet, the original may have assumed 130–140 g/cup; add a tablespoon at a time until it looks right.
The fix every baker eventually adopts
Buy a digital kitchen scale. Almost any model is fine — they're commodity electronics now, and you can get a perfectly accurate one for the price of a paperback novel. From the moment you start using it for baking, you stop thinking about packed-versus-spooned; you measure 120 g, and the recipe behaves the way the author intended.
Recipes that give weights are the easiest to scale and the most forgiving across kitchens. Recipes that give only volumes are workable but force you to guess what the author meant. The ingredient converter is the bridge: type the cups, get the calibrated grams, never look back.
Why this matters for scaling
When you scale a recipe up, volume errors compound. Two extra grams in 100 is invisible; twenty extra grams in 1,000 is the difference between a moist loaf and a dense one. Working in grams means the scaling math is exact: double a 120 g flour amount and you have 240 g of flour, no matter how you measured.
ScaleRecipe's recipe scalerhandles both — feed it cups and it'll output cups with proper culinary fractions; feed it grams and it'll output grams. But if you're scaling a baking recipe by 2× or more, switching to weight before scaling is the single best change you can make.
One quick test
Next time you bake, weigh the cup of flour the recipe asks for and write the number down. After a few recipes you'll see your kitchen's consistent volume — most home cooks land somewhere between 130 and 145 g/cup, comfortably above the King Arthur reference. Now you know your personal correction factor: when a recipe specifies cups but gives no weight, you can either trust your number or trust the author's 120 g/cup assumption. Either way, you're no longer measuring blind.