Why pan size changes everything in baking
Changing the pan you bake in is never a neutral swap. A cake batter that rises beautifully in a 20 centimetre round tin can overflow a smaller tin, or turn dry and flat in a much larger one, even when the oven temperature and baking time stay exactly the same. The reason is simple geometry: the pan controls how deep the batter sits, how much surface is exposed to heat, and how far heat must travel to reach the centre. A deeper batter takes longer to set, so the outside can over-brown before the middle is cooked. A shallower batter sets faster and loses moisture more quickly. PanConvert exists to remove that guesswork by scaling your recipe in proportion to the actual baking area, so the batter depth stays close to what the original recipe intended.
How the conversion actually works
The honest way to convert a recipe between pans is to compare their base areas, not their diameters or their names. A round pan's area grows with the square of its diameter, which is why a 23 centimetre tin holds far more than a 20 centimetre tin, even though the diameter has only grown by three centimetres. A rectangular pan's area is its length multiplied by its width. Once you know both areas, the scaling factor is simply the new area divided by the old area. Multiply every ingredient by that factor and the batter depth remains almost identical. This calculator does exactly that arithmetic for you, including the familiar round and rectangular sizes bakers actually own, and it also handles the reverse direction, scaling a large recipe down for a smaller household tin.
Choosing the right pan for the recipe
Before you convert, check that the new pan is a sensible match at all. As a rule, fill any cake pan only about two-thirds full, leaving room for the batter to rise without spilling over the rim. If the scaled recipe would fill the new pan more than three-quarters full, bake the extra batter as cupcakes or in a second small tin rather than risking an overflow. Equally, if the batter barely covers the base of a very large pan, the cake will bake too fast and dry out, and no amount of careful scaling can fix a pan that is simply the wrong shape for the job. Loaf tins, springform tins and square tins all follow the same area rule, but deep loaf tins behave differently from shallow sandwich tins, so keep the batter depth in mind as well as the area.
Step by step: converting a recipe with PanConvert
Start by selecting the pan the recipe was written for, then the pan you actually want to use. Enter one ingredient quantity, for example 200 grams of flour, and the calculator shows the scaled amount for the new pan. Repeat for each ingredient, or apply the same scaling factor to the whole list by hand. If the recipe gives ingredient weights, use weights rather than cups, because scaling by weight is far more accurate. After scaling, check the batter depth: pour the batter in and confirm it sits at roughly the same depth as it would have in the original pan. That single visual check catches most conversion mistakes before the tin ever reaches the oven.
Adjusting baking time and temperature
Scaling the ingredients does not automatically scale the baking time. When the batter depth stays the same, the baking time stays very close to the original, which is the great advantage of area-based conversion. When you deliberately change the depth, for example by baking the same batter in a deeper tin, add time in small steps and test early. Start checking five to ten minutes before the recipe time if the pan is smaller or shallower, and allow extra time if it is deeper. Keep the oven temperature the same in almost all cases; lowering it for a deeper cake often just prolongs the bake and dries the crumb. Use the classic tests for doneness: a skewer inserted in the centre comes out clean or with a few moist crumbs, the top springs back lightly when pressed, and the cake just begins to pull away from the sides of the tin.
Common pan conversion mistakes
The most common mistake is scaling by diameter instead of area, which understates the change badly. A 25 centimetre tin is not 25 percent larger than a 20 centimetre tin; by area it is more than 50 percent larger. Another frequent error is forgetting the pan's depth, especially when converting between a shallow sandwich tin and a deep springform tin of the same diameter. A third mistake is scaling the leavening agents, such as baking powder, in exact proportion for very large changes; for conversions beyond about double or half the original, keep leavening slightly conservative, because too much raising agent can make a large cake rise and then collapse. Finally, remember that eggs do not scale neatly: when a conversion calls for two and a half eggs, use two whole eggs plus one yolk, or round sensibly and adjust the liquid slightly.
Round, square and rectangular pans compared
Round pans are the standard for celebration cakes because heat reaches the centre evenly and the shape releases cleanly. Square pans hold more batter than a round pan of the same nominal size: a 20 centimetre square tin has an area of 400 square centimetres, while a 20 centimetre round tin has about 314 square centimetres, so the square tin needs roughly a quarter more batter. Rectangular pans are the workhorses for traybakes, brownies and sheet cakes, and their area rule is the most transparent of all. Loaf pans deserve special care because their sloping sides mean the base area understates the volume; measure across the top for a practical estimate, and fill loaf pans only about two-thirds full so the loaf can dome without spilling. Bundt pans and novelty shapes cannot be converted by base area alone, so for those, measure capacity by filling the pan with water and comparing volumes.
Tips for reliable results every time
Weigh your ingredients, line the base of the new pan, and grease the sides as the original recipe directs. Preheat the oven fully before the batter goes in, because a converted recipe is more sensitive to a cool start. If you are scaling up for a party, prefer baking two normal cakes rather than one enormous one; large cakes bake unevenly and are harder to handle. Keep a note of conversions that worked, including the pan, the scaling factor and any time adjustment, so next time you can repeat the success without recalculating. Baking is a craft of small, repeatable decisions, and a correct pan conversion is one of the simplest ways to protect a recipe you already trust. With the area matched and the depth preserved, your favourite cake should taste the same in the new tin as it did in the old one.
Working with different batters and fillings
Not every batter behaves the same way when it moves to a new pan. Dense batters, such as fruit cake or mud cake, rise less and are more forgiving of a deeper tin, because they do not depend on a dramatic oven spring. Light sponge and chiffon batters are the opposite: they need room to climb and they collapse easily if the tin is too deep or the oven door is opened too early. When converting a delicate sponge, prefer a pan that keeps the depth almost identical to the original, and avoid filling the tin above two-thirds under any circumstances. Brownies and traybakes are more tolerant, since a slightly thinner or thicker slab still bakes well, but the texture changes: a thinner brownie will be chewier at the edges, while a thicker one stays fudgy in the middle and needs a longer bake. Cheesecakes deserve extra care because they bake in a water bath or at a low temperature; a larger springform tin changes the setting time more than a normal cake, so check the centre for a slight wobble rather than relying on time alone. Whatever the batter, write down the pan, the factor and the result, because a conversion you record once becomes a reliable recipe of its own.