The rapid growth of low-cost carriers operating transatlantic routes has forced more established airlines to compete on price by offering cheaper hand baggage only (HBO) fares. This trend presents HBO passengers with quandaries: how much, exactly, can I carry on board? And how does this differ between airlines?

Of the carriers offering nonstop flights across the Atlantic, British Airways offers the most generous hand-baggage size allowance. The rest of the airlines’ size restrictions are anywhere from a half-inch to 4 inches less than BA, often only differing in one dimension (length, width or height). This can be a problem if you purchase a carry-on bag specifically designed to fit BA’s overhead bin, but then end up on a flight with Delta or United, which both have the same length limit as BA (22 inches), but only allow widths of up to 14 inches (compared to BA’s generous 18 inches).

These small differences in size restrictions are often enforced, further complicating which is the best carry-on suitcase to buy. A bag that meets the smallest requirements can be used everywhere, but you could be missing out on up to 4 inches of space when you fly certain airlines. If you want to travel with only a carry-on, extra inches makes a big difference.

The weight limit of a carry-on also varies, though this is easier to plan for. The most common cut-off is 22lbs. The largest allowance is a generous 51lbs., again with British Airways. None of the three major American airlines (American, Delta, United) gives an official weight restriction for carry-on baggage, although they do clarify that anything too heavy for you to lift is too heavy to take on board.

Primera Air and the trailblazer of the low-cost transatlantic fare, Norwegian, both give a maximum weight limit for the carry-on bag and personal item combined (22lbs. or 33lbs. depending on ticket).

Most airlines also allow you to carry a “personal item” (handbag, satchel, etc), but its size threshold also varies by airline. Delta and Virgin both shy away from specifying how big is too big, instead suggesting the item be a “purse, briefcase, camera bag, diaper bag, laptop computer or an item of a similar or smaller size.”

Other airlines are clearer. The generous allowance on American Airlines (18 x 14 x 8 inches) can easily accommodate a backpack, while the smaller limits on Aer Lingus, Norwegian and Primera leave far less wiggle room. Currently, none of the airlines count a jacket, book, umbrella, or other similarly small item you may happen to have in your hand while boarding as your personal item.

The easy solution to this confusion would be for the airlines to agree on a universal baggage size and weight limit, but that day may never come. Until then, check the exact restrictions for your forthcoming flight, and observe them. With low-cost flights, the penalty fees for not following directions can add up.