Converting a 10″ Bundt pan recipe to a 9″ round pan
Batter scales with volume, not with width. A 10″ Bundt pan holds about 12 cups filled to the brim, and a 9″ round pan holds 8 cups, so you need 33% less batter. Multiply every ingredient — including the eggs, which is the awkward one — by ⅔×.
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 1.6″ to 1.2″. Batter sits shallower in the new pan. Check 10–15% earlier than the recipe states — shallower batter bakes faster.
Coming out of a Bundt is easier than going in. A Bundt's central column lets heat through the middle of the batter, so a plain round pan of the same capacity bakes more slowly at the centre. Give it a little longer and test in the thickest part.
How many pans one batch fills
One 10″ Bundt recipe comes to roughly 1½ 9″ round pans, which doesn't divide neatly — you'd need 2 of them, with the last only part full. Two honest options: scale the recipe by ⅔× 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.