Tap a substance, drag the marker, or pick any pH from −3 to 17. The scale doesn't actually stop at 0 and 14 — concentrated acids and bases live in the shaded zones.
The conventional pH scale runs from 0 to 14, but that range is set by the dissociation of water at standard conditions, not by any limit on the formula pH = −log₁₀[H⁺]. Concentrated strong acids genuinely have negative pH; concentrated strong bases genuinely have pH above 14. For solutions so concentrated that activity diverges from concentration (most superacids and saturated bases), chemists use the Hammett acidity function H₀ (or H₋ for the basic side). Substances flagged "Hammett scale" in this widget use those values — they're the rigorous extension of pH beyond the 0-14 conventional range.