Converting a 12-cupcake recipe to a 9″ square pan
Batter scales with volume, not with width. A tray of 12 cupcakes holds about 6 cups filled to the brim, and a 9″ square pan holds 10 cups, so you need 67% more batter. Multiply every ingredient — including the eggs, which is the awkward one — by 1⅔×.
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 1.2″. 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 doesn't list a 9″ square, but its 2″ 8″ and 10″ square layers bake 45–50 and 50–55 minutes at 350°F, so estimate roughly 45–55 minutes. Test every 5 minutes once the top looks set.
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 doesn't list a 9″ square, but its 2-inch 8″ and 10″ square layers bake 45–50 and 50–55 minutes at 350°F, so estimate roughly 45–55 minutes for this pan. Test with a toothpick in the centre rather than trusting the cupcake recipe's clock.
How many pans one batch fills
Going this way, a 12-cupcake recipe doesn't fill a single 9″ square — it comes to about 60% of one, so you'd get a noticeably shallow bake. Scale the recipe up by 1⅔× instead and fill the pan properly. A cake baked in a pan that's too big for it dries at the edges before the centre has any height to it.
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.