Balloon ceiling. How many balloons will I require?

First you need to figure out the square footage of the balloons you will be using.  Below is the square footage of typical balloons used. 

1. 9" balloon is about 0.56 sq ft (9"x9" = 81 sq in, then convert to it sq ft)
2. 11" balloon is about 0.84 sq ft (11"x11" = 121 sq in, then convert it to sq ft)
3. 16" balloon is about 1.78 sq ft (16"x16" = 256 sq in, then convert it to sq ft)

Many balloon decorators minus about 20% of the square footage of the balloon since they may not rest exactly 9, 11, or 16 inches apart as they usually nestle into each other.  Therefore, a more accuate square footage for each balloon is:

Now let's look at the square footage of the area/ceiling you wish to cover. 

1. Calculate the length by the width of the ceiling area you wish to cover. For example, a 15' by 20' room would be 300 sq ft of ceiling space to cover with balloons. 

1. Ceiling sq ft divided by balloon sq ft.  For example, if you will be using 11" balloons to fill a ceiling that is 300 sq ft you will use the equation: 300/0.84 = 357 balloons.  
2. This calculation is for one layer of balloons.  If you want balloons to overlap, you may want to multiply the total balloons needed in the calculation above by 1.5 or by 2.  

Always have extra balloons in case of popping and room for error.  Have at least 10% more balloons than you require to cover the ceiling.

1. Be aware that this only works for smooth ceilings.  Also keep balloons away from warm lights so that they don't pop. 
2. If you don't want to cover the whole ceiling, you can create a barrier (see Figure 1 below) or ring of balloons attached with tape, and then fill the centre of it with the loose balloons. You can even use this to keep the balloons away from warm ceiling lights. 
3. It is up to you if you wish to tie ribbons to the ends of the balloons, but it is encouraged if you want the balloons to float vertically, and will be easier to clean up, and will also add an element of character and colour at eye level that will force people to look up and admire the balloon ceiling. 

Figure 1. Frame of balloons if you wish to fill a specific area instead of an entire ceiling.