Carry-On Size Calculator: Linear Inches, Dimensions, and How to Check Your Bag

Airline carry-on size rules are published in two formats: individual dimension limits (height × width × depth) and linear measurements (the total of all three dimensions added together). Both formats describe the same constraint. Knowing how to convert between them — and how to calculate your own bag's compliance — prevents surprises at the airport.

What Linear Inches Means

Linear inches is a single-number shorthand for carry-on bag size. You calculate it by adding height, width, and depth in inches:

Linear inches = height + width + depth

Most US airlines use 45 linear inches as their carry-on standard. International carriers using the same format often express this as 114 cm total (which is 44.9 inches — effectively the same limit). A few carriers, like Air New Zealand, use 118 cm as their linear limit, which is slightly more generous.

The advantage of the linear measurement system is that it tolerates different bag shapes. A tall narrow bag and a short wide bag can have the same linear measurement even if their individual dimensions differ significantly. This is why US carriers adopted it — overhead bins on different aircraft configurations have different proportions, and a linear limit captures total volume without locking travelers into a specific bag shape.

Checking Your Own Bag

To check whether your bag meets a linear limit:

1. Measure height (top to bottom when upright), width (side to side), and depth (front to back including wheels and handles if they protrude) in the same unit — either all inches or all centimetres.
2. Add the three measurements together.
3. Compare to the airline's stated limit.

Most airlines that publish individual dimension limits (such as 55 × 40 × 23 cm) rather than a linear total are effectively setting a linear limit of the sum of those three numbers. For 55 × 40 × 23 cm: 55 + 40 + 23 = 118 cm linear.

Working Example: 55 × 40 × 23 cm

This is one of the most common carry-on sizes sold in Europe and internationally — it is the approximate size of a typical 40-litre carry-on bag. Here is how to check it:

In centimetres: 55 + 40 + 23 = 118 cm total. This exactly meets Air New Zealand's 118 cm linear limit.

In inches: Divide each dimension by 2.54. 55 ÷ 2.54 = 21.65 inches. 40 ÷ 2.54 = 15.75 inches. 23 ÷ 2.54 = 9.06 inches. Total = 46.46 linear inches.

This is approximately 1.5 inches over the 45 linear inch standard used by many US carriers. A bag this size would technically fail a strict size check on American, United, Delta, or Southwest. In practice, most bags in overhead bins are never measured — but sizer boxes at the gate will reject an oversized hard-shell bag.

Converting Centimetres to Inches

The conversion factor is 2.54 cm per inch. To convert centimetres to inches, divide by 2.54:

• 55 cm ÷ 2.54 = 21.7 inches
• 45 cm ÷ 2.54 = 17.7 inches
• 40 cm ÷ 2.54 = 15.7 inches
• 23 cm ÷ 2.54 = 9.1 inches
• 20 cm ÷ 2.54 = 7.9 inches

For a quick mental estimate, dividing by 2.5 instead of 2.54 is close enough for most purposes. It overstates the inch measurement by about 1.6%, which means your bag appears slightly larger in inches than it actually is — a conservative estimate that works in your favor when checking compliance.

Why Airlines List Dimensions Differently

You will encounter carry-on size rules published in at least three different formats across airlines:

Individual dimensions: 55 × 40 × 23 cm. Each number corresponds to one physical axis of the bag. This format is common among European and Asia-Pacific carriers. It gives you a clear picture of what bag shape fits in their overhead bins.

Linear total: 45 linear inches or 114 cm combined. A single sum. Common among US carriers and some international ones. Allows more bag shape flexibility but requires you to do a calculation.

Linear with per-dimension hints: Some airlines publish both — for example, "no individual dimension over 22 inches, total under 45 linear inches." This is the most specific format but also the least common.

When a size limit is published only as a linear total without individual dimension maximums, a very tall or wide bag might technically meet the linear limit but still not fit in the overhead bin due to bin proportions. In those cases, the gate sizer box is the final arbiter.

Using the CarrySizer Bag Checker Tool

CarrySizer's bag fit checker tool lets you enter your bag's height, width, and depth and check compliance against specific airline policies. The tool handles the conversion math, applies each airline's exact rules (whether they publish linear or individual dimension limits), and flags whether your bag meets, fails, or is borderline for each carrier.

This is useful when comparing airlines for a trip or when you have bought a bag and want to know which carriers will accept it without a fee. Enter your dimensions once and compare across all the airlines you fly.

The Soft Bag Advantage at the Limit

If your bag measures right at the size limit, whether it has a hard shell or a soft shell matters. A hard-shell bag at exactly 55 × 40 × 23 cm cannot compress by even a centimetre. If the gate sizer is slightly smaller than the stated limit — which happens — a hard-shell bag will not fit.

A soft-sided bag in nylon or polyester with some give can compress to fit through a sizer even when its relaxed dimensions are at the limit. Overhead bins also accommodate soft bags better in crowded conditions — the bag can be squeezed to share space with other bags, while a hard shell cannot.

If you regularly travel with bags near the limit, choosing a high-quality soft-sided carry-on rather than a hard shell gives you a real margin that carries no additional cost once the bag is purchased.