Converting a 9×5″ loaf recipe to a 9″ square pan
Batter scales with volume, not with width. A 9×5″ loaf pan holds about 8 cups filled to the brim, and a 9″ square pan holds 10 cups, so you need 25% more batter. Multiply every ingredient — including the eggs, which is the awkward one — by 1¼×.
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 1.2″. Batter sits shallower in the new pan. Check 10–15% earlier than the recipe states (an estimate) — shallower batter bakes faster.
How many pans one batch fills
Going this way, a 9×5″ loaf recipe doesn't fill a single 9″ square — it comes to about 80% of one, so you'd get a noticeably shallow bake. Scale the recipe up by 1¼× 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.