What a Helium Balloon Can Actually Lift
The upward force comes from the air the balloon pushes aside. Air pressure presses on the balloon from every direction, and the pressure is slightly greater underneath than above. That difference creates an upward buoyant force equal to the weight of the displaced air.
But the whole balloon has weight, too: helium, the rubber or foil envelope, and anything attached to it. The balloon rises only when the displaced air weighs more than all of those together. The leftover force—called net lift—is what can carry a string, a card, or another small load. Add too much weight, and the balloon may hover low or stay on the ground even though it is filled with helium.
Why the Lift Fades
Helium atoms are small enough to slip through many balloon materials, including latex, over time. As helium leaks out, the balloon loses volume and displaces less air, so the upward force shrinks. The envelope and its attachments, meanwhile, still weigh about the same. Eventually their weight wins, and the balloon sinks.
A balloon’s rise is therefore a balance, not a tug-of-war between helium and gravity: it must push aside enough air to support its entire load. The same rule explains why a large balloon can carry more than a small one—and why even a helium balloon cannot lift an unlimited string of decorations.
