It seems simple enough: how do you measure a laptop computer to see if it fits in a specific laptop bag? It doesn’t make sense when you see that 14″ or 17″ size and then neither the length nor the width is equal to that number. Still scratching your head? Here’s how it works.

The measurement that all laptop manufacturers use is the diagonal of the screen. Check out the above image. The diagonal is the measurement that goes from the upper corner to the opposite lower corner. Now that you’ve revisited high school geometry, how does that relate to knowing if your laptop fits in a specific laptop bag?

Here’s the big news: it is a guide, but every laptop in a specific size has different dimensions because of the design of the plastic and screen dimensions. So, if you want to know if your laptop will really fit in that laptop sleeve, you need to measure the length, the width and the depth, just like a box, and then compare those measurements to the size of the bag. Maybe now it makes sense why a 15.4″ Mac fits different bags than a 15.4″ Dell or a 15.4″ Toshiba.