Largest Personal Item Backpack: How Big Can It Be?

By Best Pick Guide Editorial Team · Updated November 7, 2026

The largest personal item backpack you can bring is one that measures 18 x 14 x 8 inches when packed, on airlines that allow that size. As a perfect box, that's about 33 litres. Real backpacks have rounded corners, padding and pockets, so the biggest bags that honestly fit are usually sold as 28 to 30 litres. Anything bigger is a carry-on, whatever its title says.

The biggest limit is 18 x 14 x 8, and not every airline allows it

We read the airlines' own bag pages. Here is what they publish for the bag that goes under the seat:

So the biggest bag depends on the strictest airline you fly. If your trip includes United, a 30-litre backpack is too big for its personal item rule, even if American would take it. Our personal item size by airline guide covers which airlines check at the gate. Rules change, so look at your airline's page before each trip.

How we got from inches to litres

Multiply the three sides to get cubic inches, then multiply by 0.0164 to get litres.

• 18 x 14 x 8 = 2,016 cubic inches, about 33 litres
• 17 x 13 x 8 = 1,768 cubic inches, about 29 litres
• 17 x 10 x 9 = 1,530 cubic inches, about 25 litres

That's the most a rigid box could hold. A backpack loses some of that to its curved shape, the padded back panel and the laptop sleeve. That's why a bag listed at exactly 18 x 14 x 8 is often sold as 30 litres, not 33.

Read the inches, not the litres

Litres are the number makers like to put in the title, but no airline measures in litres. Two "30L" bags can have very different shapes. Always find the inches in the listing and compare them to your airline's limit.

Watch for these when you read a listing:

• Two different sizes. Some listings give one size in the description and another in the product details. The WONHOX bag in our comparison does this: 16.93 x 11.81 x 7.87 inches in one place and 16.54 x 12.2 x 6.69 in another. Use the larger one.
• Empty versus packed. A soft bag is measured empty. Stuffed, the front bulges. A bag already listed at 8 inches deep has no spare room.
• Expandable bags. An expansion zip can push a personal item past 8 inches deep. Check the expanded size, and keep the zip closed on basic fares.
• "Fits under most seats" with no number. If a listing doesn't give inches, you can't check it. Skip it.

The biggest bags in our comparison

From the listings we compared for the best personal item backpacks for men:

• The MOZETO tactical backpack is sold as 30 litres and listed at 18 x 14 x 8 inches. It's as big as the American and Frontier limit allows, so pack it so the front stays flat.
• The MATEIN with a shoe compartment is listed at 18 x 13.5 x 8 inches.
• The tomtoc Navigator-T66 is 28 litres at 17.72 x 11.81 x 7.87 inches, a little under the limit, so it has some room to bulge.

None of these three meets United's 17 x 10 x 9 limit. For United, look for a bag 10 inches wide or less.

How to fit the most into the biggest legal bag

1. Pack flat things against the back. A laptop and a flat packing cube go nearest the straps; soft clothes go in front.
2. Keep the front pocket light. It is the part that bulges past 8 inches first.
3. Wear your bulkiest layer and shoes. A jacket and boots on your body don't count toward the bag.
4. Use the compression straps if your bag has them, to pull it back into shape before the gate.

The bottom line: 18 x 14 x 8 packed, about 30 litres

The largest personal item backpack is an 18 x 14 x 8 inch bag that still measures 18 x 14 x 8 when it's full. That's about 30 litres in a real backpack. Buy one with its inches stated in the listing, check the strictest airline on your ticket, and don't let the front pocket bulge.