Converting a 9×5″ loaf recipe to a 10″ Bundt pan
Batter scales with volume, not with width. A 9×5″ loaf pan holds about 8 cups filled to the brim, and a 10″ Bundt pan holds 12 cups, so you need 50% more batter. Multiply every ingredient — including the eggs, which is the awkward one — by 1½×.
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 1.7″ to 1.6″. Batter depth is similar in both pans. Bake time should be close to the original; start testing 5 minutes early. (Bundt and tube depths are approximate: their ring-shaped bake area isn't published, so we estimate it.)
How many pans one batch fills
Going this way, a 9×5″ loaf recipe doesn't fill a single 10″ Bundt — it comes to about 67% of one, so you'd get a noticeably shallow bake. Scale the recipe up by 1½× 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.