pH is a logarithmic measure of how acidic or basic a solution is, defined as pH = −log₁₀[H⁺] where [H⁺] is the hydrogen-ion concentration in mol/L. The Toolenza calculator returns pH from concentration (and the reverse), plus the corresponding pOH so you have both halves of the acid/base picture at once.

The scale, in plain English

• pH 0–3 — strongly acidic. Stomach acid, lemon juice, vinegar.
• pH 4–6 — weakly acidic. Coffee, beer, rainwater.
• pH 7 — neutral. Pure water at 25 °C.
• pH 8–11 — weakly basic. Sea water (~8.1), baking soda solution, milk of magnesia.
• pH 12–14 — strongly basic. Bleach, lye, drain cleaner.

Because pH is logarithmic, each unit is 10× the previous in H⁺ concentration. pH 4 is 100× more acidic than pH 6. Coffee (pH ~5) is roughly 100× more acidic than pure water.

Where this calculator actually gets used

• Pool and spa chemistry. Pool water should sit at pH 7.2–7.8. Below that the water is corrosive (eats pump seals, irritates eyes); above that, chlorine disinfection becomes ineffective. Test strips give a colour you read against a chart; the calculator converts measured [H⁺] back-and-forth when you're calibrating.
• Aquariums. Freshwater tropical fish want pH 6.5–7.5; African cichlids want 7.8–8.5; reef tanks need 8.1–8.4 (the same as ocean water). A 0.3 pH unit drop overnight will kill sensitive species.
• Soil and gardening. Blueberries thrive at pH 4.5–5.5; tomatoes at 6.0–6.8; lavender at 7.0–8.0. Soil-test kits report pH directly; the calculator helps if you only have [H⁺].
• Chemistry homework / titration. The bridge between concentration and pH is the most common stumbling block in intro chem; the calculator does the conversion both ways so you can check your work.

Limits

The simple formula pH = −log₁₀[H⁺] assumes the activity coefficient is 1, which only holds for dilute solutions (< 0.1 M). For concentrated acids/bases, the measured pH differs from the calculated value because ions interact. For practical chemistry below 0.1 M, the calculator is accurate to better than 0.05 pH units.