Pan Size Conversion
A recipe written for one pan can be moved to another by area, because at the same batter depth the volume you need scales with the floor of the pan. This works out the factor, then tells you what it does to your batter depth, which is the part that actually changes the bake.
The pan the recipe was written for
The pan you actually have
What the factor does and does not tell you
Scaling by area holds the batter depth constant, which is the thing that keeps the bake recognisable. An 8 inch round holds about 50 square inches of floor and a 9 inch round about 64, so moving up costs you roughly a quarter more batter. Get that wrong in the other direction and a 9 inch recipe poured into an 8 inch tin sits about 27 percent deeper, sets at the rim first, and domes or tunnels in the middle.
Bake time is where the arithmetic stops. It does not scale with the factor, because heat has to travel through the depth of the batter rather than across its width. Deep batter needs longer at a lower temperature; shallow batter overbakes before you expect it. Judge doneness by internal temperature instead of the clock:
- Butter cakes and sponges: about 205 to 210 °F (96 to 99 °C) at the centre.
- Custard cheesecake: about 150 °F (65 °C), deliberately just set, with carryover finishing it.
- Lean bread: 205 to 210 °F (96 to 99 °C). Enriched or soft bread: 185 to 190 °F (85 to 88 °C).
One more variable the calculator cannot see: the pan itself. Dark and nonstick pans absorb more radiant heat and brown faster, and the standard correction is to drop the oven about 25 °F from a recipe written for light metal. Glass and ceramic conduct poorly and hold heat long, so they bake slowly and keep going after you pull them.
Pan materials in full: Tools & Equipment · Cake structure and doneness: Cakes & Sponges · Scaling a formula by weight: Baker's Percentage Calculator