While airlines may set the fares and fees for air travel, the decisions made by passengers also come with costs. Every item on board makes a plane heavier, which burns more fuel. An airliner’s cost of operating rises with every laptop (33 cents per flight), pillow (6 cents), or magazine (5 cents) you bring along.

Want your flight to burn less fuel? Start by emptying your bladder before boarding. MIT aeronautical engineers Luke Jensen and Brian Yutko used a set of typical U.S. and European flight conditions to analyze how specific items add up on three major carriers (United, American, and Ryanair) over a normal day. Uncertainties abound, such as the price of fuel or the cost of an unexpected detour.

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