How much shorter will my hair be when I dread it? - Dreadlocks FAQ's
How much length you'll loose when you backcomb and start your dreadlocks depends on a few things. In general we see about a 20-30% loss of length during backcombing. If you're not sure how to figure a percentage just ask google: "twenty percent of 12" (without the quotes) You'll get back an answer of 2.4 and that's the number of inches you'll likely loose when you backcomb. So if your hair is 7" long before you backcomb, it will be about 5.6" after you backcomb it. If its 3" before it will be 2.4" after.
Two important factors that effect how much length you'll loose are the section size you choose and the texture of your hair. Very curly or layered hair styles tend to loose a lot more length than straight hair (that all falls together and ends at the same length). This is because we start backcombing at the roots, working our way towards the tips. When we run out of hair we can't continue backcombing. If the hair is very layerd we begin running out when we get to the shorter hair. This really isn't a big problem, it just means you'll loose more length when you first backcomb. Also the larger you make the sections the more the hair is required per inch of dreads to fill the width as you go. That's why making really phat dreadlocks (large sections) means you'll loose more length. So if your hair is very short to begin with you don't want to go with huge sections. =] Drying time is a big issue too with larger dreads.
You'll also loose some length as the dreads tighten and mature because they shrink up from the bottom as they're maturing. This makes them wider and shorter. The amount they will shrink has a lot to do with how they were backcombed and the section size. Generally tighter backcombing will shrink a bit less and larger sections with shrink more than smaller ones. The amount of shrinkage varies from about 10-30%. It occurs over time as your hair is growing, so very often it will seem that your dreads are not growing longer at all. The truth is they are growing longer but shrinking at the same time so it looks like they are just getting wider instead of longer.