Perm and Hair Shrinkage
A: This is a good question. Not many people realize that a perm changes the hair's apparent length. They imagine that if their hair is shoulder length normally, they can get it permed into a curly style and it will still be shoulder length, only curlier. They get a rather rude shock when they discover that the hair that was brushing their shoulders before the perm now hangs well above them.
The longer and thicker the hair is, the heavier it is. This weight pulls on the curl, stretching it down and preventing it from shrinking as much as it would if the hair were shorter or finer. Fine hair, being lighter, springs up more easily. This is why many people with naturally curly or fine hair find that their hair looks much shorter than intended after a fresh cut or perm.
The firmness of the curl created is also referred to as the “strength” of the curl. Different perm formulas offer differing firmnesses. Alkaline perms create stronger, firmer bonds and tighter curls, while acid perms offer softer, looser waves. However, alkaline perms are primarily formulated for coarse, thick, or resistant hair, whereas fine or damaged hair is usually treated with acid perms to prevent severe chemical damage. Shorter or finer hair can use a milder formula because the hair is light enough that the curls hold their shape easily.
The size of the wrapping tool - or perm rod - is another major factor. Larger rods create wider, softer waves that retain more vertical length, while smaller rods create tight coils that pull the hair upward significantly. The designation of rod size is relative to the length of the hair being permed and how many times the strand winds around the tool.
Generally, you can expect to lose about 20% to 30% of the hair’s apparent length in an average perm with standard medium-sized rods. Small rods can drop length by up to 50%. This percentage can vary depending on the factors above, your hair's natural elasticity, and how you style it afterward.
However, rather than thinking of it as lost length, think of it as length converted into volume. The hair isn't gone, it's simply resting in a different position.
©Hairfinder.com
See also:
Perms
How much will hair stretch?
How long does hair have to be to get a body wave?