Carry‑On Size vs Linear Dimensions: Understanding the Difference

Airlines love to quote numbers when describing baggage allowances, but not all numbers are created equal. Travelers often confuse the standard “box” dimensions—such as 22 × 14 × 9 inches—with linear dimensions, which add length, width and height together. Misunderstanding these terms can lead to unpleasant surprises at check‑in or the gate. In this post we demystify linear dimensions, explain why some carriers use them and how to measure your bag to meet both types of limit.

The most familiar measurement is the maximum length, width and height of a carry‑on. Many U.S. airlines allow bags up to 22 inches long, 14 inches wide and 9 inches tall (including wheels and handles). International carriers often use similar but slightly different box sizes, and some are stricter on height. These limits are straightforward: if any one side exceeds the published number, your bag may be too large to go in the cabin.

Linear dimensions, on the other hand, sum the three sides into a single allowance. A common linear limit is 45 inches (114 cm). To calculate this, simply add the length + width + height. For example, a bag measuring 22 × 14 × 9 inches has a linear dimension of 45 inches (22 + 14 + 9). Even if your bag exceeds one side of the typical box measurement but stays within the total linear allowance, some airlines will still permit it. Others require you to meet both the individual and the linear maximum.

Why do airlines use linear measurements? It offers flexibility. Because overhead bins and aircraft holds come in different shapes, a bag that is slightly taller but thinner may fit just as well as a standard roller. Linear dimensions allow carriers to accommodate a wider variety of bag shapes without listing every possible combination of length, width and height. This practice is particularly common among smaller, regional or budget airlines, and for checked luggage.

To measure your bag’s linear dimension, use a tape measure and note the longest point of each side. Be sure to include protruding handles, wheels and pockets; airlines measure the full external footprint. Add the three numbers together to get your total. If you’re flying internationally or on a carrier that publishes a linear limit, compare your total to the airline’s rule. Our Understanding Linear Dimensions guide includes a worksheet and examples.

Some carriers publish both a standard size limit and a linear dimension. For instance, an airline might allow 21 × 15 × 7 inches with a linear maximum of 43 inches. In these cases your bag must satisfy both conditions. This prevents travelers from bringing extremely long but narrow items that could disrupt overhead space. Always check the airline’s Conditions of Carriage or baggage policy; sometimes the linear rule applies to personal items, while the box dimensions apply to the carry‑on.

When choosing a bag, aim for versatility. A suitcase with a linear dimension under 45 inches will meet most international linear limits, and if the individual sides are each under the common 22 × 14 × 9 threshold, you’ll be covered for U.S. carriers. Soft‑sided bags with compression zippers can expand or contract slightly to fit different spaces. Measure before you buy and keep your tape measure handy when packing.

Linear rules also appear in checked baggage allowances. Airlines may specify a maximum linear dimension (often 62 inches) for checked luggage, regardless of individual side lengths. Exceeding this can result in oversize fees. If you plan to check a bag on part of your journey, make sure it meets the linear checked limit as well.

Remember that airline policies change and can differ by fare class and route. Always double‑check the specifics for your ticket. Use our Carry‑On Checker tool to see how your bag compares to both box and linear limits for the airline you’re flying.

By understanding the difference between box and linear dimensions and measuring your bag carefully, you’ll avoid unpleasant surprises at the airport. Use the flexibility of linear rules to your advantage by selecting a bag that fits within both standards, and pack confidently knowing that you’ve done your homework.

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