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Baking Pan Size Converter

Converters · Added

A recipe written for an 8 inch round tin does not fit a 9 inch one, and the numbers on the pans hide how far apart they are: area grows with the square of the diameter, so that one inch is 27% more pan. This compares the two by volume, gives the single factor to multiply every ingredient by, and shows how deep the batter ends up if you pour it in unchanged.

The pan the recipe calls for

Cake tins and springforms. Measure the diameter across the top, inside the rim.

in
in

The pan you have

Cake tins and springforms. Measure the diameter across the top, inside the rim.

in
in

How to use the baking pan size converter

  1. 1Set the pan the recipe calls for — pick a preset or enter the shape and measurements yourself.
  2. 2Set the pan you actually have, the same way.
  3. 3Measure across the inside of the rim, and the depth to the inside of the base.
  4. 4Read the scale factor, then check the batter depth and fill level before deciding whether to scale or just accept a thinner cake.

Examples

The classic 8 to 9 inch swap

Input
8 inch round, 2 inch deep, into a 9 inch round
Result
× 1.266 — the 9 inch holds 27% more

Poured in unscaled, the batter sits about a fifth shallower and bakes faster than the recipe says.

A 9x13 into two rounds

Input
9 × 13 rectangle into a 9 inch round
Result
× 0.54 — one round holds just over half the rectangle

Which is why the standard advice is that a 9x13 makes two 9 inch layers, with a little to spare.

Loaf tin to a square

Input
9 × 5 loaf into an 8 inch square, 2 inch deep
Result
× 1.03 — near enough to swap with no change

Within a few percent, which is inside the variation between nominally identical tins from different makers.

About the baking pan size converter

Volume is the comparison that works

The mistake people make swapping pans is comparing the numbers stamped on them, which are lengths. What a recipe cares about is how much batter the pan holds, which is area times depth, and area grows quadratically. That is why an inch of diameter matters so much on a small tin and so little on a large one, and why a 9x13 is not roughly two 8 inch rounds but slightly less than two 9 inch ones.

Once both pans are expressed as volumes, the swap becomes one number: the ratio between them. Multiply every ingredient by it — including the eggs, which is where fractional results get awkward and rounding to the nearest sensible quantity is entirely acceptable — and the batter arrives at the same depth as the original, which means the recipe's baking time still applies.

Depth is why two equal volumes bake differently

Two pans can hold the same amount and produce different cakes, because heat arrives from the outside and has to reach the centre. Batter 25 mm deep in a wide tin sets well before the same batter 50 mm deep in a narrow one, and the deep one browns at the surface for longer while it waits. That is the reason this tool reports the resulting depth as well as the ratio, and why its timing guidance is driven by the change in depth rather than the change in volume.

It is also why the same recipe behaves differently in a dark metal tin, a shiny one and a glass dish. Dark metal absorbs more radiant heat and browns the edges faster; glass conducts poorly but holds heat, so it keeps cooking after it leaves the oven. None of that is geometry, and none of it can be calculated — it is learned from the tin.

Measuring the pan properly

Measure across the inside of the rim, not the outside, and take the depth to the inside of the base. The size stamped on the underside of a tin is often an outside dimension, which overstates capacity by enough to matter on anything small. A tin with sloping sides is somewhere between its top and bottom dimensions, and the honest way to handle it is the water test: fill it from a measuring jug and use that figure as the volume.

Volumes in cups are given here alongside the geometry for exactly that reason. If you have measured a pan with water, you can compare it directly with the figure shown and use whichever you trust more — for a straight-sided tin they will agree closely, and where they do not, the water is right.

Frequently asked questions

Why is a 9 inch round so much bigger than an 8 inch one?
Because area depends on the square of the radius, not on the diameter directly. Going from 8 to 9 inches multiplies the radius by 1.125, and squaring that gives 1.266 — so the pan holds a quarter more than it looks like it should. The same catch works in reverse and bites harder on small tins: a 6 inch round holds only 56% of an 8 inch one, so halving a recipe for it is not enough.
Do I have to scale the recipe, or can I just use the other pan?
It depends on how far apart they are and what you are baking. Inside about 5% by volume, use the other pan and change nothing — that gap is smaller than the difference between two tins sold as the same size. Beyond that you have a choice: scale the ingredients to keep the same batter depth and the same baking time, or pour it in unscaled and accept a thinner or deeper cake with a different time. Thin sponges dry out and deep ones need a lower oven, so scaling is the safer path once the difference passes about a fifth.
How do I know the batter will not overflow?
The tool reports what fraction of the new pan the batter fills before it rises. Cake recipes assume around two thirds, which leaves room for the rise; anything past about three quarters is where it climbs over the rim, and the result is a burnt smell and a cake with a mushroom top. If the fill comes out high, scale down and bake the extra as cupcakes rather than trying to fit it all in — a pan that overflows loses the batter and the shape at once.
Does the baking time change?
It follows the depth of the batter rather than the volume, because heat has to travel in from the surfaces. The same mixture spread thinner sets sooner; the same mixture piled deeper takes longer and can brown on top before the middle is done, which is the case for dropping the oven ten degrees or so and covering the top loosely once it has colour. All of that is a starting point rather than a rule — the batter, the pan's material and colour, and the accuracy of the oven all move it, and a skewer settles it.
Why is a bundt tin treated differently?
Because the geometry lies. A fluted bundt has a chimney through the middle and a profile that is anything but straight-sided, so its outer dimensions overstate what it holds — often by a noticeable margin. The tool models it as a ring and says so, and the reliable answer is to fill the tin with water from a measuring jug once and write the number on a label underneath it. That single measurement beats any calculation, for that tin, forever.

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