BAKING CALCULATION GUIDES / PAN SHAPES

From a round pan
to a rectangle.

To keep the same batter depth, multiply the recipe by the new pan’s inside area divided by the original pan’s inside area. Comparing the widths alone misses most of the change.

This method models straight-sided pans with a flat base. It gives a quantity ratio, not a tested recipe adaptation. Before calculating, check whether your recipe’s author provides an alternative pan and instructions for it.

1. Measure the usable inside space

Measure a round pan’s inside diameter across its center. For a rectangular pan, measure the inside length and width. Exclude handles, rolled rims, and the thickness of the walls. A square is simply a rectangle whose length and width match.

Check that the recipe refers to one pan. If its ingredient list fills two identical pans, the original area is the combined area of both. Treating that full batch as the contents of one pan would double the amount you intend to make.

The simple model does not describe tapered walls, heavily rounded corners, a central tube, or decorative cavities. A nominal size printed on a pan is also not a substitute for checking its internal dimensions.

2. Compare areas, not diameters

• Round area = π × (diameter ÷ 2)².
• Rectangle area = length × width.
• Equal-depth scale factor = target area ÷ original area.

Both areas must use the same unit. The area units cancel in the division, leaving a multiplier with no unit. A factor above 1 increases the batch; a factor below 1 reduces it.

Worked example: 8-inch round to 9-inch square

Assume both pans have straight sides and the batter will be the same depth. The round pan’s radius is 4 inches, so its area is π × 4², about 50.265 square inches. The square pan’s area is 9 × 9 = 81 square inches.

Factor = 81 ÷ (π × 16) ≈ 1.611443799.

The square needs about 61.1% more mixture under these assumptions. An ingredient listed as 200 g becomes 200 × 1.611443799 ≈ 322.289 g. Keep the unrounded multiplier during the calculation, then choose a measurable final amount with your recipe and scale in mind.

These are arithmetic examples, not ingredients for a complete bake. In the workbench, choose an 8-inch round original pan and a 9-by-9-inch rectangular target, and leave “Keep the same batter depth” selected. Replace the example ingredients with your recipe.

3. Handle mixed units before dividing

One inch is exactly 2.54 centimeters, as documented by NIST’s length reference. An 8-inch diameter is therefore 20.32 cm, and a 9-inch side is 22.86 cm. Calculating both areas in centimeters gives the same factor.

Do not divide square centimeters by square inches as though they were identical. When calculating by hand, convert the lengths first, then calculate the areas. The workbench lets you choose a unit for each pan and normalizes the measurements before comparing them.

What the ratio cannot promise

Equal depth is an explicit assumption. A taller wall does not mean the batter should be deeper. If the intended batter depths differ, use the volume-based depth calculation instead.

Pan substitutions can change baking behavior. King Arthur’s pan-size guide explains the role of surface area and batter depth. This calculator does not predict time, temperature, or doneness, and it does not determine the headroom your recipe needs.

Check fractional ingredients before rounding eggs or tiny quantities. Keep your original recipe’s instructions available, record the change you make, and report a calculation problem if a result seems inconsistent.