Converting a 12-cupcake recipe to an 8″ round pan
Batter scales with volume, not with width. A tray of 12 cupcakes holds about 6 cups filled to the brim, and an 8″ round pan holds 6 cups — near enough the same that you can bake the recipe as written.
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 0.7″ to 1.1″. Batter sits deeper in a pan than in a cupcake liner, so the center needs longer to set. A full pan bakes much longer than cupcakes — Wilton's chart gives this pan 37–42 minutes at 350°F for a 2″ layer. Start testing a few minutes before 37, then every 5 minutes until a toothpick comes out clean.
Going the other way — cupcakes into a full pan — keep the oven where it is: Wilton bakes 6- to 10-inch rounds and squares and a 9×13 at 350°F, inside the 325–375°F range King Arthur gives for cupcakes. What changes is time. Wilton's chart gives an 8″ round pan 37–42 minutes for a 2-inch layer, far longer than any cupcake. Test with a toothpick in the centre rather than trusting the cupcake recipe's clock.
How many pans one batch fills
A 12-cupcake recipe fills about one 8″ round at the usual two-thirds level — a clean split, which makes this an easy conversion to do without scaling anything.
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.