It’s easy to sketch a bag that looks good in outline but doesn’t really work when you put actual things in it. This often happens when you pick the width, height, and depth of your bag without really thinking about what it will be used for.
Before you refine a shape, figure out what will go in the bag and how those things will fit. For instance, a small crossbody bag designed for a phone, wallet, and keys will need different dimensions than a tote that is going to hold a notebook, a water bottle, and other bigger items. By deciding what will be carried first, you’ll have a better idea of what the measurements should be instead of just guessing.
Start with a couple of items you know you want to put in the bag, instead of a vague size like “medium.” Hold the items up together and see how wide, tall, and deep they are. You don’t need to measure them exactly. A ruler and a rough estimate are fine.
If you’re carrying a notebook, the notebook might dictate the height of the main part of the bag. If you’re carrying a wallet and a small case, the thickness of those two items will tell you how deep the bag needs to be. But remember, you’ll also want some room to move things around inside the bag so they aren’t pushed up against the zipper, flap, or lining.
Now that you have some rough measurements, you can sketch the front of the bag, which should be a simple rectangle or shape. Write down the measurements you want for the width and height on the drawing.
Now draw the side of the bag, giving it a reasonable depth. The side view is important because you can miss problems when you’re only looking at the front. For example, a wide bag with almost no depth won’t be able to carry the things you plan on putting in it. On the other hand, a small shoulder bag with too much depth will feel huge. Seeing both the front and side of the bag will help you compare the proportions.
The opening can also affect what dimensions work. A bag might have enough room inside but have an opening that is difficult to get things in and out of. Think about what the biggest thing in the bag will be and whether it would fit through the opening. If you have to twist it or push it, maybe the opening is too small or it needs to be in a different place.
The pockets can take up room too. A big pocket, organizer, or stiff panel in the middle of the bag will take up some of the space that you were planning to use for the things in the bag, so make sure you’ve drawn those before you decide what the final measurements are.
Try drawing the same bag three times around the same things you want to put in it. Change the proportions of the bag. Make one wide and low, one tall and thin, and one in between. Give each one a side view with the correct amount of depth. This way, you can see how the same capacity can create very different shapes. You can also see where the handles will go, where the straps will attach, how big the pockets will be, and what size the hardware (like D-rings or buckles) will be.
If you’re still unsure, make a mock-up out of paper. Take a piece of paper or card stock and fold or tape it into the right shape and size of the bag. Don’t worry about getting the seams or lining right; you just need to get an idea of the volume.
Put some of the things you want to put in the bag inside or next to the mock-up, and look at the bag from all sides. You’ll probably find that the bottom panel is too small, the flap is too short, or that the overall size is too big.
There isn’t a perfect set of measurements for every bag. Practical dimensions are just a way of figuring out what the bag will actually hold and making sure the rest of the design makes sense for what you want to put in the bag.
When you have a better understanding of the size, width, depth, opening, and capacity of the bag, it will be easier to decide what materials you want to use and how you want to build the bag. Before you add fancy seams, colors, or hardware, ask yourself whether the basic design will allow the bag to be used the way you want it to.