What Is pH and How Is It Calculated?
pH is a logarithmic measure of how acidic or basic a water-based solution is, based on the concentration of hydrogen ions it contains. The scale runs from 0 to 14: values below 7 are acidic, 7 is neutral, and values above 7 are basic (alkaline).
The pH formula — part of the Sørensen scale, introduced by Danish chemist Søren Sørensen at the Carlsberg Laboratory in 1909 — is: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per litre (mol/L). The related value pOH uses hydroxide ion concentration: pOH = −log₁₀[OH⁻], and at 25°C, pH + pOH = 14.
According to the International Union of Pure and Applied Chemistry (IUPAC), pure water at 25°C has a hydrogen ion concentration of exactly 1.0 × 10⁻⁷ mol/L, giving it a pH of 7.0 — the reference point for neutrality.
How to Use This pH Calculator
Choose whether you want to calculate from a known pH value or from an ion concentration, then enter the number. Every other value updates instantly.
- pH Value: A number typically between 0 and 14, though very strong acids or bases can fall outside this range.
- Ion Concentration: The molarity (mol/L) of either hydrogen ions (H⁺) or hydroxide ions (OH⁻) in the solution — accepts standard decimals or scientific notation like
1e-4.
The pH scale is a universal chemistry standard with no metric/imperial equivalent, so this calculator applies identically to labs, classrooms, pool testing, and water-quality checks anywhere in the world. The pH + pOH = 14 relationship assumes a solution at 25°C, since the ion product of water (Kw) shifts slightly at other temperatures.
What Common pH Values Mean
Most everyday substances fall into a well-documented pH range. The table below lists typical values reported by water-quality and food science references.
| Substance | Typical pH | Classification |
|---|---|---|
| Battery acid | 0.0 – 1.0 | Strongly acidic |
| Stomach (gastric) acid | 1.5 – 3.5 | Strongly acidic |
| Lemon juice | ~2.0 | Acidic |
| Vinegar | ~2.9 | Acidic |
| Black coffee | ~5.0 | Weakly acidic |
| Pure water | 7.0 | Neutral |
| Human blood | 7.35 – 7.45 | Weakly basic |
| Baking soda solution | ~9.0 | Basic |
| Household ammonia | ~11.5 | Basic |
| Bleach | ~12.6 | Strongly basic |
For example, a 0.001 M solution of a fully-dissociating strong acid like HCl has [H⁺] = 0.001 mol/L, giving pH = −log₁₀(0.001) = 3 — squarely in the acidic range shown above.
Frequently Asked Questions
What is a neutral pH?
A neutral pH is exactly 7.0, the value of pure water at 25°C, where the hydrogen ion concentration [H⁺] equals the hydroxide ion concentration [OH⁻] at 1.0 × 10⁻⁷ mol/L each.
How accurate is this pH calculator?
This calculator applies the exact pH = −log₁₀[H⁺] relationship at 25°C and assumes full dissociation for the concentration mode, which holds for strong acids and bases like HCl or NaOH. Weak acids and buffers dissociate only partially, so their real-world pH depends on the acid dissociation constant (Ka) via the Henderson-Hasselbalch equation rather than concentration alone.
What is the difference between pH and pOH?
pH measures hydrogen ion (H⁺) concentration while pOH measures hydroxide ion (OH⁻) concentration, and at 25°C the two always add up to 14 — so a solution with a low pH (acidic) has a correspondingly high pOH, and vice versa.
How do I calculate pH from concentration?
Take the negative base-10 logarithm of the hydrogen ion concentration in mol/L: pH = −log₁₀[H⁺]. This calculator performs that step automatically when you select "Ion Concentration" as the input mode.