Converting an 8″ square recipe to a tray of 12 cupcakes
Batter scales with volume, not with width. An 8″ square 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 5–10% off a 9″ round layer and up to 40–50% off a 9×13. 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 an 8″ square pan, against our 16).
How many cupcakes one batch makes
By pan volume, an 8″ square 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.