Bag dimensions online are usually the widest point, which flatters the bag and tells you nothing about what fits. These are the numbers that matter and how we list them.

How we measure

• Opening width. Across the top, inside edge to inside edge, with the bag empty and standing.
• Base width. Across the bottom panel. On a tapered bag this is 4 to 8cm narrower than the opening, and it is what a laptop actually has to sit in.
• Base depth. Front to back at the bottom. This is what decides whether a water bottle stands upright.
• Height. Base to top edge, excluding handles.
• Strap drop. Top of the handle to the top of the bag.

Laptops

A bag that meets the opening figure but not the base figure will take the laptop and bow at the corners over time. Check both.

Strap drop

• Under 20cm. Hand carry or forearm only. Will not clear a coat on the shoulder.
• 21 to 25cm. Shoulder carry over a coat. This is the useful range for a tote.
• 26cm and over. Sits at hip, swings when you walk, best on soft bags.
• Crossbody, adjustable. 95 to 140cm covers most adults over and under a coat.

Everyday objects, roughly

• 750ml water bottle upright: 14cm base depth, 26cm internal height.
• A5 notebook: 22cm opening.
• Pair of flat shoes: 30cm of base length.
• Rolled gym kit: about 8 litres, which is a large tote or a small duffle.

If you are between sizes

Go smaller. Almost everyone who writes to us about size regrets going too big, because a large bag gets filled and then it is heavy. The exception is if you genuinely carry a 16 inch laptop daily, in which case there is no smaller option that works.