Three numbers appear on a pack of hair extensions: length, weight, and unit count. This page explains what each one measures — because two of them are measured differently by different manufacturers, and that is where most ordering surprises come from.
The numbers themselves, per line and per pack, are not published here. They have no approved source, and inventing a specification is the one thing worse than not having one. What you can do is know what to ask the agent.
The three numbers
What is actually measured
Length
The distance from the bond to the tip, in centimetres or inches. ⚠️ The measuring point is not standard across the market: some manufacturers measure from the top of the bond and some from its bottom, and the difference can be two or three centimetres.
What to ask: exactly where the measurement starts, and whether it is taken on straight hair or on textured hair.
Weight
The weight of the hair in grams, per pack. This is what settles density — two packs of the same length at different weights give a completely different result.
What to ask: whether the stated weight is per pack or per single unit, and whether it includes the bond itself or only the hair.
Unit count
How many pieces are in the pack. ⚠️ This is the number that reads differently in every method: a tape "unit" sometimes means a single tab and sometimes a pair closed on each other.
What to ask: what counts as a unit — one or a pair — and how many units a full head takes in the method you work in.
The one rule that holds without a spec sheet
Length is what the client sees, but weight is what decides whether the result looks real. Long length at low weight gives long, see-through hair, and see-through hair is the most common complaint after an application. When you order more length, the weight has to rise with it.
Where this actually gets settled: against samples. Your area agent comes with samples of all three lines, and what you can see and feel in your hand is worth more than a number on a page.
What is missing to finish this page
Until then: ask the agent · compare bonding methods · shade guide