Converting a 9×13″ pan recipe to a tray of 12 cupcakes
Batter scales with volume, not with width. A 9×13″ pan holds about 14 cups filled to the brim, and a tray of 12 cupcakes holds 6 cups, so you need 57% less batter. Multiply every ingredient — including the eggs, which is the awkward one — by 0.43×.
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. Bake cupcakes 18–22 minutes at the recipe temperature; test at 18.
Cupcakes are the one conversion where time changes more than anything else: they're shallow, so they bake in 18–22 minutes rather than the 30–40 a layer takes. Fill each liner about two-thirds and don't be tempted to go higher — the overflow is what gives you those flat, spreading tops.
How many pans one batch fills
One 9×13″ pan recipe comes to roughly 2½ trays of 12 cupcakes, which doesn't divide neatly — you'd need 3 of them, with the last only part full. Two honest options: scale the recipe by 0.43× and bake a single pan, or make the full batch and accept that one comes out shallower than the rest. Batter left standing while an earlier pan bakes loses its lift, so get them all in the oven together where you can.
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.