What not to pack in a carry-on

Almost everything confiscated at a US checkpoint falls into five categories: liquids over the allowance, sharp objects, tools, sporting goods that could be swung, and flammable or pressurized items. Learn the categories and you will rarely be surprised — and you will stop packing the three or four things that catch most travelers out.

One rule up front, because it is the one people get backwards: the checked bag is not always the safe option. Several common items are banned from the hold and must travel in the cabin, spare lithium batteries chief among them. Putting a power bank in a checked bag is a more serious mistake than putting a full water bottle in a carry-on.

Rules differ by country and change without much notice. Everything below reflects TSA guidance for US departures; for anything specific, check the TSA's own item search or your carrier before you fly. For the liquids allowance in particular, we cover it separately in our carry-on liquids guide.

1. Liquids over the allowance

The single biggest cause of bag checks. Containers over the standard limit are not permitted through a US checkpoint in carry-on baggage regardless of how full they are — a three-quarters-empty large bottle is still a large container. Medically necessary liquids and baby formula are treated as exceptions and should be declared to the officer. Full detail and the current allowance is in our liquids rules guide.

2. Sharp objects

Knives of essentially any blade length, box cutters, razor blades that are not in a cartridge, scissors above the permitted blade length, and multi-tools with a blade. Disposable and cartridge razors are fine. Safety razor blades are not, though the handle is. Knitting needles are generally permitted; the small scissors in a sewing kit usually are too, within the length limit. The consistent trap is the pocket multi-tool that lives in a bag and is genuinely forgotten about.

3. Tools

Tools over a certain length are prohibited in the cabin — wrenches, pliers, hammers, crowbars and drills among them. Screwdrivers and similar tools under the length limit are typically fine. Anything you would describe as "a proper tool" rather than "a small one" should go in a checked bag.

4. Sporting goods that can be swung

Baseball bats, golf clubs, hockey and lacrosse sticks, ski poles, pool cues and martial arts equipment all have to be checked. Tennis and badminton rackets are usually accepted in the cabin. The line is essentially whether the object works as a club.

5. Flammable, pressurized and explosive items

Fuel, lighter refills, spray paint, most aerosols beyond limited personal-care quantities, fireworks and flares are all prohibited in carry-on baggage — and most are prohibited in checked baggage as well. Camping stove fuel canisters, even empty ones, are a recurring problem. Common disposable lighters and safety matches are handled under separate rules; check current guidance rather than assuming.

The reverse list: what must NOT go in a checked bag

This is the part that catches experienced travelers. Because of fire risk in an unattended cargo hold, the following generally have to travel in the cabin:

• Spare lithium batteries and power banks. Any loose lithium battery, including every portable charger. In a device is usually fine checked; loose is not.
• E-cigarettes and vaping devices. Cabin only, and they may not be charged or used in flight.
• Smart bags with non-removable batteries.If the battery cannot be removed, most carriers will not accept the bag in the hold — the reason this whole category exists is covered in our luggage with USB charging guide.
• Valuables, medication and documents. Not a safety rule but a practical one: airline liability for checked contents is limited, and a delayed bag takes your medication with it.

How to check any specific item in ten seconds

The TSA maintains a searchable "What Can I Bring?" database covering thousands of individual items, with separate carry-on and checked verdicts for each. It is the authoritative answer for US departures and it settles arguments faster than any list can. For international departures, check the local security authority and your airline — the categories are broadly similar but the specifics, particularly around liquids and lighters, differ.

Pack so a bag check is quick

If you do get flagged, the time cost depends entirely on how your bag is packed. Keep liquids in one accessible pouch, keep electronics together near the top, and avoid packing so densely that an officer has to remove everything to reach the middle. Using packing cubes means the officer lifts out two blocks rather than excavating; using a bag with an external pocket for the laptop, as several picks in our business carry-on roundup have, means it never has to be unpacked at all.