pH Calculator
pH, pOH, [H⁺] and [OH⁻] from any one of them
Acidic
Working
- pH = −log₁₀[H⁺] = −log₁₀(1 × 10⁻³) = 3
- pOH = 14 − pH = 11
- [OH⁻] = 10⁻¹⁴ ÷ [H⁺] = 1 × 10⁻¹¹ mol/L
pH of everyday substances
| Substance | pH |
|---|---|
| Battery acid | < 1 |
| Stomach acid | 1.5–3.5 |
| Lemon juice | 2–2.6 |
| Vinegar | 2.4–3.4 |
| Cola | 2.5 |
| Coffee | 5 |
| Milk | 6.5–6.8 |
| Pure water | 7 |
| Blood | 7.35–7.45 |
| Sea water | 8.1 |
- 1Choose what you know[H⁺], [OH⁻], pH or pOH
- 2Enter the valueConcentrations as a × 10ⁿ
- 3See all fourWith working and the pH scale
How is pH calculated?
pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre. Each pH unit is a tenfold change in acidity.
pH = −log₁₀[H⁺] · [H⁺] = 10^(−pH)
pOH = −log₁₀[OH⁻] · pH + pOH = 14 (at 25 °C)
[H⁺] × [OH⁻] = 10⁻¹⁴
Drinking water pH
India's drinking water standard (BIS IS 10500) sets an acceptable pH of 6.5 to 8.5. RO-purified water is often slightly acidic, around 6 to 7.
Strong acids and bases
Strong acids such as HCl and strong bases such as NaOH ionise fully, so [H⁺] or [OH⁻] equals their molar concentration: 0.01 M HCl has pH 2, and 0.01 M NaOH has pH 12. Weak acids like acetic acid ionise only partly, so their pH is higher than this suggests.
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Frequently asked questions
What is the pH of 0.001 M HCl?
3. HCl is a strong acid, so [H⁺] = 1 × 10⁻³ mol/L and pH = −log₁₀(10⁻³) = 3.
How much more acidic is pH 3 than pH 5?
100 times. Each step on the pH scale is a factor of 10 in hydrogen ion concentration.
Can pH be negative or above 14?
Yes, for very concentrated strong acids or bases, such as 10 M HCl (pH about −1). Most everyday solutions are between 0 and 14.
Last reviewed: October 2026