You choose your quantity of balloons. Size: 11" balloons helium filled with long string attached. 1 - 11" balloon is about 0.84 sq ft Square footage of the ceiling let's look at the square footage of the area/ceiling you wish to cover. Calculate the length by the width of the ceiling area you wish to cover. For example, a 15' by 20' room would have 300 sq ft of ceiling space to cover with balloons. Calculate how many balloons you need Ceiling sq ft divided by balloon sq ft. Using 11" balloons to fill a ceiling that is 300 sq ft you will use the equation: 300/0.84 = 357 balloons. 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. TIPS: 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. Be aware that this only works for smooth ceilings. Also, keep balloons away from warm lights so that they don't pop. Pricing: 75 or under = $2.65 each 76 -150 = $2.55 each 151 - 250 = $2.45 each 251 -500 = $2.35 each 501+ = $2.25 Upcharge may apply for chrome, pearl, metallic, printed, or agate latex. To get a quote for other quantities and for delivery pricing fill out this form