Minimum Hair Length for a Body Wave
A: A body wave is usually simply a "large curl perm" and pretty much any length of hair can be given a body wave, although smaller rods would be needed for shorter lengths.
The purpose of a body wave is to give some texture and "body" or movement to the hair. It's generally used on medium-to-fine, straight hair types, or hair that seems limp. The process will increase the diameter of the hair shaft, making the hair look and feel thicker and can help the hair hold curl more easily and style much more readily.
Since the process isn't meant to actually add "curls" as this would be done with traditional perming methods, the size of the tool used is determined by the length of the hair being rolled. The tool should be large enough that the hair will only go around it 1-1/2 to 2 times as it is being rolled end-to-scalp. If you are using the largest perm tool available you may wrap the hair more times than the above listed.
A nice rule of thumb is that if you can roll the hair on small standard wet rollers or on small hot-rollers, you should easily be able to book a body wave.
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See also:
Hair lengths
Could I possibly put a perm in my very short hair without rods?
How much does hair shrink up when you get a perm?
Perm rods and how to know what size you need