pH Calculator
Enter [H⁺] (or [OH⁻], or pH/pOH, depending on mode); the tool instantly computes pH, pOH, [H⁺] and [OH⁻], and shows whether the solution is acidic, neutral or basic.
Input Data
Results
At a glance:pH is a measure of how acidic or basic a solution is, defined as pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion (more precisely hydronium H₃O⁺) molar concentration (mol/L). At 25 °C, by the water ion product Kw = [H⁺][OH⁻] = 1×10⁻¹⁴, we also have pH + pOH = 14 and pOH = −log₁₀[OH⁻]. A solution with pH < 7 is acidic, pH = 7 neutral (pure water), pH > 7 basic. The scale is logarithmic, so each pH unit means a tenfold change in [H⁺] (pH 3 is ten times more acidic than pH 4) — why even small pH changes matter, e.g. in blood (normal pH 7.35–7.45). Complementary to the molarity and dilution calculators.
Formula
pH = −log₁₀[H⁺]; pOH = −log₁₀[OH⁻].
pH + pOH = 14 (25 °C, Kw = 1×10⁻¹⁴).
Acidic pH < 7; neutral pH = 7; basic pH > 7.
[H⁺] = 10^(−pH); [OH⁻] = 10^(−pOH).
$$pH = -\log_{10}[H^+]$$$$pH + pOH = 14 \quad (25^{\circ}C)$$How to Use
- Select the known quantity (pH, pOH, [H⁺] or [OH⁻]).
- Enter the value; the tool gives pH, pOH, [H⁺] and [OH⁻].
- Read whether the solution is acidic, neutral or basic from pH.
Common solutions and their pH (25 °C)
| Solution | pH | Nature |
|---|---|---|
| Stomach acid | ≈ 1–2 | Strongly acidic |
| Lemon juice | ≈ 2–3 | Acidic |
| Pure water | 7 | Neutral |
| Blood | 7.35–7.45 | Slightly basic |
| Bleach | ≈ 12–13 | Strongly basic |
Each pH unit means a tenfold change in [H⁺]; at 25 °C pH + pOH = 14.
Case Studies
From [H⁺] = 1×10⁻³
pH = −log₁₀(1×10⁻³) = 3.
pOH = 14 − 3 = 11; [OH⁻] = 10⁻¹¹ mol/L.
pH < 7, so the solution is acidic (e.g. a weak acid).
From pH = 9
pOH = 14 − 9 = 5.
[H⁺] = 10⁻⁹, [OH⁻] = 10⁻⁵ mol/L.
pH > 7, so the solution is basic (e.g. dilute ammonia).
FAQ
What does pH mean?
pH = −log₁₀[H⁺]; a low pH means high [H⁺] (acidic), high pH means low [H⁺] (basic). It is logarithmic, so each unit is a tenfold change in hydrogen-ion concentration.
Why is pH = 7 neutral?
At 25 °C, pure water has [H⁺] = [OH⁻] = 1×10⁻⁷ mol/L, so pH = pOH = 7. This is the neutral point (shifts with temperature).
What is the relation between pH and pOH?
At 25 °C, pH + pOH = 14 (from Kw = 1×10⁻¹⁴). Given one, you get the other by subtraction.
Is pH always 0–14?
Not always. Very concentrated strong acids can have pH slightly below 0, and concentrated strong bases above 14, but for most everyday and lab solutions pH is within 0–14.
How is pH measured?
With a pH meter (glass electrode) or pH indicator paper/strips. For biology and chemistry labs, pH affects reaction rate, enzyme activity and solubility, so it is monitored closely.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.