Let's see... I logged it all out when I made my first trial infant tutu. I did it in 3 layers which made the knots really big. I didn't care for the huge knots, but other people told me they preferred it to the smaller 2 layer knots.

3 layer Infant Tutu of 12" circumference/aprox 5" length:

each layer = 1 foot

3 layers per knot = 1 yard

2 knots per inch X 12 inches = 24 knots

24 knots X 1 yard = 24 yards

Hope that made sense, lol!

Basically I multiply length of layer X # of layers X # of knots per inch.

In a two layer tutu, I get 2.5 knots per inch.

2 layer Infant Tutu of 12" circumference/aprox 5" length:

each layer = 1 foot

2 layers per knot = 2 feet

2.5 knots per inch X 12 inches = 30 knots

30 knots X 2 feet = 60 feet or 20 yards

This might not be the same for everyone since skill plays a part in how large or small the knots are. But if you figure your own # of knots per inch, you should be able to pre-formulate how much tulle you need ahead of time.