How many helium balloons does it take to lift a kilogram?.

Mónica, answering your question:

This depends on the size (shape, too) and weight of the balloons. A good rule of thumb is that a liter of Helium will lift around a gram - or 1000 liters of gaseous Helium to lift a kilogram. (This is calculated subtracting the helium density from the air density of 1.204 g/L - 0.179 g/L) = 1.025 g/L net lift.

Estimating a not-to-small party balloon (11-inches in diameter) to hold about the same net lifting helium (Except for the weight of the balloon) as a 10-inch sphere of standard pressure (1 atm): It will lift about 8.6 grams since the volume of a sphere this size is about 8.6 liters. A 100-pack of my inexpensive commercial 11-inch party ballons weigh very close to 12 ounces which comes out to about 3.4 grams each.

So the net lift of one 11-party balloon is the helium lift minus the balloon's weight:

8.6g-3.4g = 5.2 grams.

Let's call it 5 grams even as you will probably have things like string or a tape of some sort to tie or connect things to the balloon. A kilogram is 1000 grams, so 1000/5 = 200 balloons will be required to lift the kilogram.

If you use bigger balloons the weight of the balloons and strings which in this example consumes nearly 42% of the helium's lift can be cut down to only about 10-15% loss. But those would be 5 or 6 foot ballons which cost up to $15 each. With 100-200 of those filled with helium, a full grown man, his lawn chair, and many gallons of water can float up to nearly 15,000 feet and possibly the next state/country, as Kent Couch of Oregon has demonstrated. That is a 75-100 kilogram payload. Each ballon has a net lift of between 1 kilogram and 2 1/2 kilograms depending on the optimal fill amount.