The answer is 5.0. Milady states the pH range of hair and skin runs 4.5 to 5.5, with an average of 5 on the pH scale — comfortably on the acidic side of neutral (7).
That average matters because it's the benchmark barbers use to judge products: pH-balanced shampoos and conditioners are formulated to sit near 5 so they don't disturb the hair's natural acidity, while pure water (pH 7, neutral) is actually about 100 times more alkaline than hair and skin and can cause hair to swell up to 20 percent.
Milady Standard Barbering, 6th ed., Ch. 7 (Basics of Chemistry), p. 188: "The pH range of hair and skin is 4.5 to 5.5, with an average of 5, on the pH scale."
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