Pan size is measured across the opening, and manufacturers list that top diameter; but because most pans have sloped sides, the base is narrower. Two pans with the same 24 cm label can hold different volumes. So the number on the label is a classification, not an input for calculations; measure from the inside if you are comparing pans.
The number on a cake pan’s box is almost always the top diameter, and this is not an arbitrary choice by the manufacturer: the opening is the pan’s widest point and the easiest place to measure. But batter does not sit at the opening; it spreads upward from the base. In a pan whose sides flare outward, the base diameter is noticeably narrower than the top diameter. The label measures the opening; the batter sits on the base, and the entire discrepancy lies between these 2 statements. The label is not wrong; it simply does not answer the question being asked.
Why does the difference matter? Because pan calculations use the base area. Since area is proportional to the square of the radius, a small error in diameter grows in the area; even a 1 cm difference can reach 10% of the area. A calculation based on the top diameter gives a larger area than the actual one, so the batter sits deeper than calculated and takes longer to bake. If a recipe is being transferred between 2 pans, the discrepancy applies twice and the result shifts even more. If the same error is made with the same 2 pans, the discrepancies do not cancel out; they add together.
Keep 3 things in mind when measuring. Run the measuring tape inside the pan, not around the outside, because the metal’s thickness would otherwise be included in the measurement. Stretch the tape from edge to edge through the center; any measurement that misses the center makes the diameter appear smaller than it is. If the sides are sloped, measure both the opening and the base and record them separately. Measure springform pans with the clasp closed, because an open clasp makes the diameter slightly larger. Set the pan on a flat surface while measuring, because holding it can bend it slightly and change the diameter.
You cannot tell by eye how much volume the slope takes away; you have to measure it. When 2 pans are placed side by side, those same 2 pans may have the same top diameter, but the one with straight sides holds noticeably more than the one with sloped sides. The only method that directly shows this difference is to fill the pan with water and check how many milliliters it holds. If the calculation and measurement do not agree, the measurement is right; a calculation is only an estimate used when you do not have a measuring cup on hand. That is why, with sloped pans, it is faster to measure directly than to spend time calculating.
This distinction also helps when buying a cake pan. Of 2 pans with the same labeled size, the one with straight sides holds more and comes closer to the volumes in recipes; the one with flared sides gives a more impressive slice when served. Which one to buy is a matter of preference, but after buying it, measure its actual volume once and write it underneath. This takes all future recipes out of the realm of guesswork and makes them calculable. That number, once written, remains valid until you get rid of the pan.