Scale a recipe between different pan sizes by area or volume.
Multiply each ingredient by the scale factor. Adjust bake time by the time factor (its cube root for volume mode, square root for area mode). Watch the pan for doneness — pan colour, depth and oven hot spots matter as much as the math.
Few baking shortcuts are as quietly costly as substituting a different pan from the one a recipe calls for. Cake batters, brownies, sheet bakes, loaves and bar cookies are formulated with a target depth and surface area in mind. When you swap an 8-inch round for the 9-inch round you happen to own, you are not just changing the look of the slice — you are changing how thinly the batter sits, how quickly heat reaches the centre, how much the leavening can lift before the crumb sets, and ultimately whether the final crumb is tender or rubbery. The Baking Pan Converter solves this in a few seconds. Tell it the source pan from the recipe, the target pan you have, and what you want to preserve (batter depth, total volume, or let the tool decide), and it returns a single scale factor for the ingredient quantities and a separate factor for the bake time. Use the scale factor to multiply every ingredient line in the recipe, and use the time factor to estimate how much sooner or later you should start checking for doneness. The math underneath is straightforward geometry, but doing it by hand for a round-to-rectangle conversion at 11 pm with flour on your fingers is exactly the kind of friction that ruins a Sunday cake.
The converter works on two ratios. The first is the area ratio: source pan area divided by target pan area. Round pans use πr², squares use a², rectangles and loaves use a × b. The second is the volume ratio: the area ratio multiplied by the height ratio. The Preserve switch decides which one drives the recipe scale: area mode keeps the batter at the same depth (best for cakes that rise mostly from chemical leavening — sponges, brownies, sheet cakes), volume mode keeps the total batter mass identical (best for loaves, deep batters, and cases where the batter must reach a specific fill line). Auto picks volume when either pan is a loaf and area otherwise — a safe default that matches what most home bakers actually want.
The bake-time factor is not the same number as the scale factor. Heat penetrates from the outside in, so the time needed scales with the linear dimension of the pan, not its area or volume. In area mode the tool returns the square root of the area ratio (since linear ≈ √area). In volume mode it returns the cube root of the volume ratio (since linear ≈ ∛volume). A pan that doubles in area only takes about 1.41 × the time. A pan that doubles in volume only takes about 1.26 × the time. Most home cooks badly overestimate this, end up overbaking, and blame the recipe.
Pick the source shape and dimensions from the recipe — these are usually printed on the first line of the ingredient list, sometimes only as inches (multiply by 2.54 to convert to centimetres). Then pick your target shape and dimensions. Round and square pans only ask for one dimension (diameter or side); rectangles and loaves ask for length and width. Heights are needed in both cases because depth feeds into volume mode and into the bake-time factor. Finally pick what to preserve: leave it on Auto unless you know better. Read the scale factor — for example 1.32× means multiply every ingredient quantity by 1.32. Read the time factor — 1.15× means a 30-minute recipe should be checked at around 34–35 minutes, and probably finished a few minutes after that.
A recipe calls for a 20 cm round cake pan, 5 cm deep. You only own a 23 cm round of the same depth. Source area is π × 10² = 314.2 cm². Target area is π × 11.5² = 415.5 cm². Area ratio is 1.32. With Auto mode (no loaf pans involved) the converter picks area mode, so the scale factor is 1.32. A 200 g flour line becomes 264 g, a 4-egg recipe becomes 5.3 eggs (round to 5 — see the Variations section), and a 30-minute bake becomes about 30 × √1.32 ≈ 34.5 minutes. Start checking at 32 minutes, pull when the centre tests clean.
Cookies do not scale with pan size — they scale with how many you place on the sheet. The pan converter is designed for batters that fill the pan; for cookies, scale by piece count instead. Batters that depend on rising height (chiffon cakes, certain genoise sponges) behave differently when the pan is wider and shallower than the original — they spread out before they rise, which kills the lift. If you are forced to pour a thicker layer than the recipe assumes, drop the oven temperature by 10–15 °C and add 20–30 % to the bake time so the centre finishes before the crust over-darkens. Tube pans and Bundt pans are not solid: subtract the inner cylinder from the area before plugging into the converter, otherwise you will overestimate batter volume by 20–35 %. Springform pans usually have a tighter seal than a solid-bottom pan but identical interior geometry — measure the interior, not the rim. Dark metal pans absorb more heat than light aluminium and bake roughly 10 °C hotter at the same dial setting; glass pans bake about 15 °C hotter and need a small temperature drop; silicone is much slower than metal and may add 20 % to the bake time. Every oven also has hot spots — rotate halfway, especially if your converter says the bake will already run long.
Convection ovens move the air, so heat reaches the surface faster: drop the oven temperature by 15 °C and the bake time by about 20 % from whatever the converter returns. High-altitude baking (above 1000 m) thins the air and weakens leavening; reduce sugar by 5–10 %, add 1–2 tablespoons of liquid per cup of flour, and raise the oven temperature by 10 °C — the converter's scale factor still applies on top. Doubling and halving recipes hits the egg discontinuity: a single egg is about 50 g, so a recipe that wants 1.5 eggs after scaling should round to either 1 (and add a tablespoon of milk) or 2 (and trim a tablespoon of liquid elsewhere) — never beat half an egg into the bowl and call it precision. For cupcakes use the converter on the batter side only: a 20 cm round becomes 12 standard cupcakes, a 23 cm round becomes 16, and the bake time drops to roughly 18–22 minutes regardless of source pan because cupcakes are governed by their own small geometry.