This pH calculator converts between pH, pOH, hydrogen ion concentration [H+] and hydroxide concentration [OH−] in whichever direction you need, and solves buffer pH with the Henderson–Hasselbalch equation. Enter any one value to get the rest.
What Is pH?
pH measures how acidic or basic a solution is, on a scale from 0 to 14 at room temperature. Below 7 is acidic, 7 is neutral, above 7 is basic (alkaline). The scale is logarithmic: each whole pH unit is a tenfold change in hydrogen ion concentration, so a pH 3 lemon juice is ten times more acidic than a pH 4 tomato juice. This calculator sits in our Chemistry Calculators collection.
The pH Formulas
- pH = −log₁₀[H+] — example (verified): [H+] = 0.001 mol/L → pH = 3.00
- [H+] = 10−pH — pH 5 → [H+] = 1.0 × 10−5 mol/L
- pH + pOH = 14 (at 25 °C) — a solution with pOH 4 has pH 10, basic
- pOH = −log₁₀[OH−]
- Buffer (Henderson–Hasselbalch): pH = pKa + log([A−]/[HA]) — pKa 4.76 with [A−] 0.20 M and [HA] 0.10 M → 4.76 + log(2) = pH 5.06 (verified)
Common pH Values
| Substance | Approx. pH |
|---|---|
| Battery acid | ~0–1 |
| Stomach acid | ~1.5–2 |
| Lemon juice | ~2–3 |
| Black coffee | ~5 |
| Pure water | 7 |
| Human blood | 7.35–7.45 |
| Seawater | ~8.1 |
| Household bleach | ~12–13 |
Notes for Students
The pH + pOH = 14 relationship assumes 25 °C; at other temperatures the neutral point shifts slightly. Concentrations here are ideal-dilute values — very concentrated solutions deviate because activity replaces concentration. For the mass side of chemistry, pair this with our Molar Mass Calculator and Percent Composition Calculator.
Related Tools
- Molar Mass Calculator — molecular weight from a formula
- Percent Composition Calculator
- Chemistry Calculators — the full chemistry collection
Frequently Asked Questions
How do I calculate pH from concentration?
pH = −log₁₀[H+]. For [H+] = 0.001 M, that is −log(10−3) = 3.
What is the pH of pure water?
7 at 25 °C — [H+] and [OH−] are equal at 10−7 mol/L.
Can pH be negative or above 14?
Yes, for very concentrated strong acids or bases (e.g., 10 M HCl has a pH near −1). The 0–14 range covers typical dilute solutions.
What is the Henderson–Hasselbalch equation used for?
Estimating the pH of a buffer — a mixture of a weak acid and its conjugate base — from the pKa and the ratio of the two concentrations.
Is rainwater acidic?
Slightly: unpolluted rain sits around pH 5.6 because dissolved CO₂ forms weak carbonic acid. “Acid rain” refers to lower values from sulfur and nitrogen oxides.
Is this pH calculator free?
Yes — free, instant, no sign-up.