pH Calculator — Determine the Acidity or Basicity of Any Solution
With this pH calculator, you can determine the pH of a solution in several ways: from the concentration of an acid or base, from the mass and volume of a dissolved compound, or directly from ion concentration ([H⁺], [OH⁻], or pOH). Supports both metric (g, mL, L) and US customary (oz, lb, fl oz, cup, pint, quart, gallon) measurement units.
The pH Scale
The pH scale is a numeric scale (0–14) used to define how acidic or basic an aqueous solution is. The value is logarithmically and inversely related to the concentration of hydrogen ions:
pH = −log₁₀([H⁺])
- pH < 7 — acidic solution
- pH = 7 — neutral (pure water at 25 °C)
- pH > 7 — basic (alkaline) solution
The scale can go beyond 0–14 for extremely concentrated solutions. Our bodies maintain a near-neutral pH: blood pH ≈ 7.4, stomach acid pH ≈ 1–2.
How to Use the pH Calculator
1. From the Concentration of an Acid
Select an acid from the list of 22 common acids (or choose Custom and enter your own Ka value) and enter the molar concentration in mol/L. The calculator will compute pH, pOH, [H⁺], and [OH⁻].
2. From the Concentration of a Base
Same approach for bases. For strong bases (NaOH, KOH, etc.) [OH⁻] = C. For weak bases the quadratic equilibrium formula is applied.
3. From the Mass and Volume of an Acid
Choose the acid, enter the mass (g, kg, mg, oz, or lb) and the total volume of the solution (mL, L, fl oz, cup, pint, quart, or gallon). The calculator converts these to molar concentration first, then finds the pH.
4. From the Mass and Volume of a Base
Identical procedure for a base — select the compound, enter mass and volume in your preferred units.
5. From Ion Concentration
Provide any one of the following and all others are calculated:
- [H⁺] — hydrogen ion concentration (mol/L)
- pOH — the negative logarithm of [OH⁻]
- [OH⁻] — hydroxide ion concentration (mol/L)
pH Formula
The fundamental relationships used by this calculator:
pH = −log₁₀([H⁺])pOH = −log₁₀([OH⁻])pH + pOH = 14at 25 °C- Weak acid:
[H⁺] = (−Ka + √(Ka² + 4·Ka·C)) / 2 - Weak base:
[OH⁻] = (−Kb + √(Kb² + 4·Kb·C)) / 2 - Molar concentration from mass/volume:
C = (m / M) / V
pH Classification Used in This Calculator
| pH Range | Classification | Examples |
|---|---|---|
| 0 – 2 | Strongly acidic | Battery acid, gastric acid |
| 2 – 5 | Acidic | Lemon juice (2.2), vinegar (2.9), coffee (5.0) |
| 5 – 6.5 | Weakly acidic | Milk (6.3–6.6), rain (5.6) |
| 6.5 – 7.5 | Neutral | Pure water (7.0), blood (7.4) |
| 7.5 – 9 | Weakly basic | Seawater (8.1), baking soda (8.3) |
| 9 – 12 | Basic | Borax (9.2), milk of magnesia (10.5) |
| 12 – 14 | Strongly basic | Bleach (12.5), lye / NaOH (14) |
Frequently Asked Questions
What is pH?
pH stands for potential of hydrogen (or power of hydrogen). It measures the concentration of hydrogen ions in a solution. A higher [H⁺] means a lower pH and a more acidic solution.
What is the difference between Ka and Kb?
Ka is the acid dissociation constant — how strongly an acid releases protons. Kb is the base dissociation constant — how readily a base accepts protons. Larger Ka = stronger acid; larger Kb = stronger base. For a conjugate acid-base pair: Ka × Kb = Kw = 10⁻¹⁴.
Why does pH + pOH = 14?
At 25 °C, water has an autoionization constant Kw = [H⁺][OH⁻] = 10⁻¹⁴. Taking −log₁₀ of both sides gives pH + pOH = 14.
Can pH be negative?
Yes! Extremely concentrated strong acids (e.g., 12 mol/L HCl) produce [H⁺] > 1 mol/L, giving pH < 0. The scale shown is 0–14 for typical conditions, but the formula has no restriction.