To judge a bag's size online, compare its width, height and depth with a bag you already own, then check the opening and strap separately. A photograph cannot establish capacity, and three outside measurements do not guarantee that a particular item will fit.
Start with what must go inside. A phone in a thick case, a rigid wallet or a book can be harder to fit than a collection of smaller items with the same total volume.
Read the dimensions in the right order
Look for the seller's stated width × height × depth order; it is not universal. Check whether the height includes handles and whether the width is measured at the top, base or widest point. Curved, tapered and round bags need more context than rectangular ones.
Convert units before comparing. One inch equals 2.54 centimeters. Keep all the measurements in the same unit, and remember that supplier dimensions may be approximate rather than exact.
Make a quick paper template
- Draw the bag's listed width and height on paper or arrange sheets to form the outline.
- Place your must-carry items over it. Check their shape, not only their diagonal measurement.
- Mark the listed depth beside the outline. A wide, shallow bag can still struggle with a bulky case.
- Compare the outline with your existing bag and with where the new bag would sit against your body.
This is a screening tool, not a promise of internal capacity. Lining, seams, a curved base and hardware reduce usable space. Ask for internal or opening measurements if one essential item is a close fit.
Check the opening before the interior
An item has to pass through the opening before it can use the space below. A zipper that stops short of the side seams, a flap or a gathered top can restrict access. Do not assume a bag with a 30 cm outer width can accept a rigid 30 cm object.
Measure the carry position
For shoulder carry, compare strap drop with a bag that feels comfortable over your usual clothing. For crossbody carry, use a tape measure to trace the intended strap path. Confirm whether the strap is adjustable and whether the listing gives full length or drop. Those are different measurements.
For example, Costa Tote currently lists approximate dimensions of 41 × 34 × 14 cm and a strap length of about 62 cm. That strap number should not be read as a 62 cm drop. By contrast, a slim style such as Trama Tote needs a different capacity check even when its front outline looks large.
A useful final question
Would you still choose the bag if it held only the items whose fit you have confirmed? If the answer is no, resolve the missing measurement before ordering. The measurements and care guide explains the basics, and the current product listings provide the starting dimensions for individual styles.