US recipes use volume measurements: cups, tablespoons, and teaspoons. The hierarchy: 1 cup = 16 tablespoons = 48 teaspoons = 236.6 ml. 1 tablespoon = 3 teaspoons = 14.79 ml. 1 teaspoon = 4.93 ml. Note that 1 US cup is 236.6 ml, not 250 ml — metric cup sizes vary by country. Australia uses a 250 ml metric cup, which is about 5.7% larger than the US cup. Recipes from different countries using 'cup' as a unit may produce different results without adjustment.
UK recipes often use metric (ml and grams) for precision, plus some imperial measures like fluid ounces (1 UK fl oz = 28.41 ml vs 1 US fl oz = 29.57 ml). This means 'a cup of flour' in an American cookbook and 'a cup of flour' in an Australian cookbook call for different amounts.
Why weight measurements are more accurate
The key problem with volume measures for dry ingredients is that density varies with packing. All-purpose flour: 1 cup can weigh anywhere from 120 g (sifted and spooned) to 160 g (scooped directly from the bag), a 33% difference. For bread baking, a 33% flour error is the difference between a good loaf and a doorstop. Professional bakers and many serious home bakers use a kitchen scale because 120 g of flour is always 120 g regardless of technique.
Common weight reference points for US cups: 1 cup all-purpose flour = 120–128 g; 1 cup granulated sugar = 200 g; 1 cup butter = 227 g; 1 cup whole milk = 240 g (close to ml because density ≈ 1); 1 cup rolled oats = 90 g; 1 cup honey = 340 g. Honey demonstrates the extreme: it's 2.67× denser than the same volume of oats.
Scaling recipes and the limits of proportional adjustment
Most recipe ingredients scale linearly: doubling a recipe means doubling all ingredients. Exceptions: leavening agents (baking powder, baking soda) and salt often should not be fully doubled because excess causes off-flavors or poor texture. Spices may also need less than proportional increase as they become dominant. The rule of thumb: scale leavening by 75% of linear proportion when tripling or more.
Cooking time and temperature do not scale linearly with batch size. A recipe scaled to 2× volume in the same pan depth bakes in roughly the same time as the original — but if you use a larger pan that results in a thicker product, baking time increases. Always test doneness by the recipe's visual and textural cues (golden color, toothpick test, internal temperature) rather than relying solely on scaled time estimates.