Converting a 9″ round recipe to a tray of 12 cupcakes
Batter scales with volume, not with width. A 9″ round pan holds about 8 cups filled to the brim, and a tray of 12 cupcakes holds 6 cups, so one batch makes about 16 cupcakes — or, if you only want a single tray of 12, multiply every ingredient by ¾×.
The number people miss is depth. Two pans can hold the same volume and still bake completely differently, because what matters is how thick the batter sits: volume divided by bake surface. Here that moves from about 1.2″ to 0.7″. Fill each liner about ⅔ full. Cupcakes bake faster than the cake at the same temperature — King Arthur puts the saving at about 5–10% off a 9″ round layer's time. Start checking early; they're done when a toothpick comes out clean.
Cupcakes are the one conversion where time changes more than anything else. Keep the recipe's temperature — King Arthur's guide puts most cupcakes at 325–375°F — and expect a shorter bake: about 5–10% less than a 9″ round layer, and as much as 40–50% less than a 9×13. Fill each liner about two-thirds; King Arthur fills closer to four-fifths for taller domes, which is why its counts run a little lower than ours (about 12 from a 9″ round pan, against our 16).
How many cupcakes one batch makes
By pan volume, a 9″ round recipe makes about 16 cupcakes — one full tray of 12 and 4 more. That's 8 cups of pan divided by the ½ cup a standard cupcake cup holds, with both filled two-thirds so the fill level cancels out. Recipes and box mixes print their own yields, which vary with how full the liners go — trust yours if it states one.
A caution worth reading
Volume conversions hold well for cakes, brownies and quick breads — anything poured. They hold less well for recipes where structure comes from the pan itself: a delicate chiffon or angel food climbs the ungreased side of a tube pan, and a loaf's shape is part of how it bakes. For those, scale the ingredients by all means, but expect the texture to shift, and use the skewer rather than the recipe's clock.