Three hundred grams is exactly 2.5 cups of all-purpose flour using the 120 gram cup standard. It is about 1.52 cups of granulated sugar and 1.32 cups of butter, which shows why one weight cannot have a universal cup answer.
For flour, use two level cups and one level half cup only when a scale is unavailable. Spoon and level every cup to avoid compounding a packing error across several measures.
A 300 gram quantity is common in bread dough, larger cakes and family-size bakes. The converter can also calculate cocoa, oats, honey and other ingredients from the same weight.