Ensure the box dimensions are not already in inches. If something is written in inches, it is often represented with a quotation mark ("). Example: 24"x19"x20". However, if it is represented with an apostrophe ('), then the units are probably in feet. Example: 24'x19'x20'.
Dissect the box dimensions. Box dimensions are commonly listed in this order: Length, width, and depth (or height). Therefore, 24-by-19-by-20 would be 24 units in length, 19 units in width and 20 units in depth. You will be working with each number individually. Just keep track of what order the numbers were written in, so you can properly put them back together. This is of inestimable importance. If you assemble your dimensions in non-standard order, the results could wreak havoc and confusion.
Multiply each number by 0.39. There are precisely 0.39 international inches in a centimeter. Standard American inches differ by a very small, almost insignificant margin, but in that case 0.39 is a good approximate, and still usable. Using a calculator is preferable to doing the calculations by hand. If you do choose to do the calculations by hand, be certain to double check your work to ensure accuracy.
Reassemble your dimensions. Remember to retain the order that you converted the numbers from. In this case, you have gone from 24-by-19-by-20 cm to 9.36-by-7.41-by-7.8. When you are describing dimensions in inches, remember to use quotation marks as the symbol for inches.