A 15-Inch Laptop Is Not 15 Inches: Measuring Yours Before You Buy a Bag

To measure a laptop for a bag, ignore the screen size and measure the body: the width along the hinge, the depth from front to back and the thickness at the thickest point, with the lid closed. The "15-inch" in a laptop's name is the diagonal of its screen, measured corner to corner across the glass. The body is a different number in every direction, and two laptops sold in the same size class can differ by almost three quarters of an inch in width. That difference is what decides whether a bag closes.

This guide is about the measuring itself. Which of our bags is built around which screen size is laid out in our laptop and work bags collection; here is how to check that number against your own machine before you trust it.

What "15 inches" actually measures

A laptop's size is the diagonal of its display, not of the machine. Manufacturers measure the visible screen as if it were a plain rectangle, from one corner to the opposite one, and the names are rounded: Apple's 13-inch MacBook Air has a 13.6-inch display, its 15-inch model 15.3 inches, and the 16-inch MacBook Pro 16.2 inches. On many Windows laptops the "15" is a 15.6-inch panel.

None of those numbers describes the outside. The body is the screen plus the bezels around it, the hinge and the keyboard deck, and those vary from model to model. Thinner bezels have pulled bodies in over the last few years, which is why a current 16-inch laptop can be narrower than an older-style 15.6-inch one.

The most common measuring mistake follows from the name. People lay a tape from corner to corner across the closed lid, expecting to find 15 or 16 inches, and get something closer to 16.5 or 17, because the body's diagonal is longer than the screen's. That number is not useful for anything; a bag never measures diagonally.

Same size class, different bodies

Here is what the size classes look like in current models, using the dimensions each manufacturer publishes. Width runs along the hinge, depth from front edge to hinge.

Three things stand out. Across the 15- and 16-inch rows, widths run from 13.40 to 14.11 inches, a spread of 0.71 inch (1.8 cm) between laptops that people shop for as "the big one". The 15.6-inch Dell 15 is wider than both 16-inch machines in the table. And the deepest laptop, the 16-inch MacBook Pro, is 0.42 inch deeper front to back than the 16-inch XPS, which matters more than width in a briefcase with a zip across the top. Figures change between generations and configurations, so look up your exact model rather than borrowing a number from a table, this one included.

How to measure yours

It takes two minutes, and the numbers serve for every bag you buy while you own that laptop:

1. Find the model and look it up. The model name is on the label underneath or in the system settings (About This Mac on a Mac, Settings, then System, then About on Windows). The maker's spec page gives width, depth and height to a hundredth of an inch; trust it over a tape.
2. If you measure by hand, close the laptop and lay it flat on a table.
3. Measure the width along the hinge edge, which is the widest part of most laptops.
4. Measure the depth from the front edge to the back, across the middle, including any lip at the hinge.
5. Measure the thickness at the thickest point, usually the back, including the rubber feet.
6. Measure it in its sleeve or shell if it travels in one, because that is what goes into the bag.
7. Write the three numbers down as width x depth x thickness and keep them on your phone.

Comparing the body with a bag

Compare your laptop's body with the bag's measurements, and expect to need about an inch (2.5 cm) more than the laptop in both width and height. Bag makers, us included, list outside measurements, and the inside loses the walls, seams and lining on every side, as we explained for small bags and big phones. Roughly half of that inch goes to the bag's construction; the rest is the room that lets the laptop slide in and out without forcing. With less, it may still go in, but it is a squeeze every morning.

Which laptop number you compare with which bag number depends on how the bag holds it:

• Briefcases and messenger bags hold the laptop on its long side. Compare the laptop's width with the bag's width, and its depth with the bag's height.
• Backpacks hold it upright. Compare the laptop's width with the backpack's height, and its depth with the backpack's width. Most backpack mismatches come from comparing the wrong pair.

Thickness is rarely the problem on its own; almost every laptop is under an inch. The bag's depth is for everything else you carry, and the charger is part of that: a 16-inch MacBook Pro comes with a 140-watt adapter, a much larger block than the one in a 13-inch Air's box. Our piece on briefcases, messengers and backpacks covers the depth question in full.

Our laptop bags, checked against real bodies

Each bag below lists the laptop size it was built for. The last column applies the one-inch rule to the largest laptop in that class from the table above, using the bag's outside measurements.

Two rows need a word. The Mini Lawbag is our most popular bag and it is not a laptop bag at all; "11 inch" is its own width. And Docbag takes a bare 16-inch MacBook Pro with less than an inch to spare across, so if yours lives in a padded sleeve, measure the sleeve first. If you carry a 15.6-inch Windows laptop or a 16-inch Mac in a sleeve, the 15.6-inch Lawbag is the size with room left over.

For the backpacks, the outside is only half of the check. Where the padded section sits matters too, and our guide to what leather backpacks are good at explains why a sleeve that reaches the floor of the bag is a weak point. The full range is in our backpacks collection.

When the numbers are close

If your laptop comes within an inch of a bag's outside measurements, three things decide whether it will work. A zip across the top is the least forgiving: forcing it closed over a laptop that is a little too tall puts the strain on the stitching at the ends of the zip. A flap with buckles gives more, because it closes over the load rather than around it. And leather does soften and shape itself around what it carries, but over months, not on the first morning; it will not find you the quarter inch you are missing. When in doubt, take the laptop out of its sleeve, or choose the next size up.

Common mistakes

• Shopping by screen size alone, when the body is what has to fit.
• Measuring the closed lid corner to corner, which gives a diagonal no bag uses.
• Borrowing a number from a different model or generation of the same size class.
• Forgetting the sleeve or shell case the laptop actually travels in.
• Comparing a laptop with a bag's outside measurements and no margin, rather than leaving about an inch.
• Reading a backpack the wrong way round, comparing the laptop's width with the pack's width instead of its height.

Quick checklist

• You have your laptop's width, depth and thickness, from the maker's spec page or a tape, with the lid closed.
• The numbers include the sleeve or case it travels in.
• The bag beats the laptop by about an inch in width and in height, measured outside.
• For a backpack, you compared the laptop's width with the pack's height.
• The bag's depth leaves room for the charger and whatever else comes along.
• If it is closer than an inch, you know whether the bag closes with a zip or a flap.

Frequently Asked Questions

Before you buy any bag, look up your laptop's three body numbers and save them. Then check them against the bag's outside measurements with an inch to spare, and start with our laptop and work bags, each listed with the size it was built around.