One inch equals 2.54 centimeters. The two pan sizes most commonly used in recipes, 8 and 9 inches, correspond to 20.3 and 22.9 centimeters. The real issue is not the conversion but how close the two sizes appear: the difference in diameter is only 2.6 centimeters, yet their base areas differ by roughly a factor of 1.27. So these 2 similar-looking pans are not interchangeable.
International recipes give pan sizes in inches rather than metric measurements, and the conversion comes down to one fixed value: one inch equals 2.54 centimeters. Multiply the number of inches by this value to get the equivalent measurement. Two sizes appear much more often than the others in recipes. An 8-inch round pan is 20.3 centimeters, and a 9-inch pan is 22.9 centimeters. The same two sizes are used for square pans, where the measurement refers to the side length, not the diameter. Knowing these two equivalents means you rarely need to calculate them again.
The conversion is easy, so that is not the real problem. The problem is that 8- and 9-inch pans look very similar. The difference in diameter is only 2.6 centimeters, and when the two pans are placed side by side, the eye may dismiss it as insignificant. But the amount of batter a pan holds depends on its base area, not its diameter, and area grows with the square of the diameter. So a difference that looks small can translate into a much larger difference in how much batter the pan holds, and using the recipe unchanged can affect the result.
The calculation makes the difference clear. An 8-inch pan has a diameter of 20.3 centimeters and a base area of roughly 324 cm². A 9-inch pan has a diameter of 22.9 centimeters and an area of 412 cm². The ratio between the two is about 1.27 times. In other words, a 9-inch pan needs about one-quarter more batter than an 8-inch pan. This ratio is worth remembering because in international recipes, a switch between two pan sizes almost always means switching between these same two sizes, and the factor is the same each time. For square pans, do the same calculation using the square of the side length, and the result is a similar ratio.
The adjustment works in two directions. If an 8-inch-pan recipe is being adapted to a 9-inch pan, multiply all the ingredients by 1.27; in the reverse direction, divide by the same number. Apply the factor to the entire ingredient list, not just part of it. If you pour the batter into the larger pan without multiplying the recipe, it spreads more thinly, bakes faster, and does not rise to the expected height. In the smaller pan, the batter sits deeper, takes longer to cook in the center, and the edges dry out. In the two cases, check doneness by looking at the center rather than relying on the recipe’s timing.
Because pans sold in Türkiye are made to centimeter sizes, an exact match is often unavailable. In practice, a 20-centimeter pan can stand in for an 8-inch pan, and a 22- or 24-centimeter pan for a 9-inch pan. These are approximate matches, and the difference still needs to be addressed by comparing area. The most reliable approach is to measure the volume of the pan you have with water once, write it down, and compare that value with the pan size the recipe calls for.