Baking Pan Size Converter

How to use the baking pan size converter

Choose the shape and size of the pan the recipe names, then the shape and size of the pan you have. The tool computes both areas, divides one by the other, and gives you the ratio that decides whether the swap works. Round pans use pi times the radius squared; square and rectangular pans are plain multiplication.

The default comparison is the swap people search for most: a 9 inch round against an 8 inch square. Their areas are 63.62 and 64 square inches, a difference of 0.6 percent, which is well inside the accuracy of the pans themselves. That is a straight substitution with no change to the recipe and barely any to the time.

Give both pans a wall height and the tool also reports the volume ratio, which is the comparison a deep bake needs, plus each pan brim-full capacity in cups. Set how deep the batter sits in the original pan and it carries that same batter across and tells you the depth it lands at in the new one.

That depth figure is the point of the whole tool. Move a one inch layer from a 9 inch round into a 9 by 13 and it thins to 0.54 inches. Move it the other way, from a 9 by 13 into an 8 inch square, and it deepens to 1.83 inches, which fills a two inch pan to 91 percent before the batter has risen at all.

Where the areas do not match, you have two honest options. Keep the recipe and accept a different depth, adjusting the time and watching closely, or scale the recipe by the area ratio so the depth stays where the recipe intended. The second is the safer route for anything leavened.

Two limits worth stating. The tool measures the pan you describe, not the pan you own, and pan dimensions are nominal. It also says nothing about a precise new bake time, because no honest formula exists for one, only the direction of travel and the advice to check early.

Substitute by area or by volume?

Both ratios are on screen and they answer different questions. Pick the wrong one and the swap that looked fine on paper overflows the pan or bakes into a biscuit. The rule is short: a shallow bake substitutes by area, a deep bake substitutes by volume.

Shallow bakes go by area

Cakes, brownies, blondies, sheet cakes, bar cookies and tray bakes all sit in a thin layer with plenty of pan wall above them. Nothing about the pan height is doing any work, so only the floor area matters, and matching area keeps the depth, and therefore the bake, unchanged.

Deep bakes go by volume

Casseroles, deep-dish bakes, bread pudding, tall layer cakes and anything that comes close to filling its pan are limited by capacity rather than floor space. Here the wall height is load-bearing, and a pan with the same area but shallower walls simply will not hold the food.

The last row is the most useful trick in the list. When no single pan matches, two smaller ones often do, and splitting keeps the depth right rather than compromising it. Two 9 inch rounds stand in for a 9 by 13 with about 9 percent more area between them, which is a small and forgiving error.

Why two 9 inch pans are not the same pan

Pan sizes are nominal, meaning the number on the label is a name rather than a measurement. Bakeware is measured across the top, from inside edge to inside edge, and pans with sloped sides hold noticeably less than that figure implies once you get down to the base.

That is before manufacturing tolerance, wall thickness and rolled rims come into it. Two pans both sold as 9 inch rounds can differ by a good fraction of an inch inside, and since area moves with the square of the width, a small difference in diameter is a larger difference in the surface your batter has to spread across.

• Measure across the top, inside edge to inside edge, not across the outer rim.
• Measure the wall height on the inside too, from the base up.
• Check the base as well when the sides slope, since that is the real bottom area.
• Write the true dimensions on a label under the pan and stop remeasuring.

Metric equivalents carry the same looseness. The inch is exactly 25.4 millimetres under the 1959 International Yard and Pound Agreement, so a 9 inch pan is 22.86 centimetres, but nobody sells a 22.86 centimetre tin. It is sold as 23 centimetres, and a 20 centimetre tin stands in for the 8 inch, which is 20.32.

The formula, worked line by line

Everything here comes from two areas and a division. A round pan is pi times the radius squared, and the radius is half the diameter printed on the label. A square pan is its side squared, and a rectangular pan is its two sides multiplied. Divide the new area by the old and you have the ratio that governs the swap.

Capacity adds one dimension: area times wall height. Batter depth runs the same calculation backwards. The volume of batter is fixed by the recipe, so pouring it into a different pan divides that fixed volume by a different area, and the depth changes in exact inverse proportion.

round area = π × (diameter ÷ 2)²
square area = side²        rectangular area = length × width
area ratio = new area ÷ old area
capacity = area × wall height
new batter depth = old batter depth ÷ area ratio
9 in round: π × 4.5² = 63.62 in²      8 in square: 8² = 64 in²

Run that in reverse and the danger changes sides. A 9 by 13 recipe poured into an 8 inch square has 0.55 times the area, so a 1 inch layer becomes 1.83 inches, filling a 2 inch pan to 91 percent before any rise. Batter that climbs during the bake will go over the rim.

What this tool deliberately will not tell you

Glass and dark metal pull the same result further apart again. Dark surfaces absorb more radiant heat and brown the edges faster, and glass holds heat once it has it, so both tend to run ahead of a shiny light pan even at an identical depth. That is one more reason the doneness test outranks the printed time.

Questions people ask