🧫 pH Calculator
Enter a pH, pOH, hydrogen-ion or hydroxide-ion concentration, and get all four values plus whether the solution is acidic, neutral or basic.
pH
3.00
pOH
11.00
[H⁺]
1.000e-3 M
[OH⁻]
1.000e-11 M
pH = −log₁₀[H⁺]; pH + pOH = 14 and [H⁺][OH⁻] = 1×10⁻¹⁴ at 25 °C. 🔒 Computed in your browser.
How the ph calculator works
The relationships are pH = −log₁₀[H⁺], pOH = −log₁₀[OH⁻], pH + pOH = 14 (at 25 °C), and [H⁺][OH⁻] = 1×10⁻¹⁴. From any one input the tool derives the other three and classifies the solution: pH < 7 acidic, = 7 neutral, > 7 basic. Working backward, [H⁺] = 10^(−pH), so each whole pH unit is a tenfold change in acidity, pH 3 is ten times more acidic than pH 4.
The 14 and 1×10⁻¹⁴ relationships hold at 25 °C (where the water ionisation constant Kw = 1×10⁻¹⁴). At other temperatures Kw shifts slightly.
Frequently asked questions
How do I calculate pH?
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol/L. For [H⁺] = 1×10⁻³ M, pH = 3. Enter any one of pH, pOH, [H⁺] or [OH⁻] and the tool finds the rest.
What is the relationship between pH and pOH?
At 25 °C, pH + pOH = 14. So pOH = 14 − pH, and you can get one from the other.
How are [H⁺] and [OH⁻] related?
Their product is the water ionisation constant: [H⁺][OH⁻] = 1×10⁻¹⁴ at 25 °C. So [OH⁻] = 1×10⁻¹⁴ ÷ [H⁺].
What pH is acidic, neutral or basic?
At 25 °C: pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). The tool shows the verdict.
Does temperature affect pH?
Yes, the “14” and 1×10⁻¹⁴ values are for 25 °C. At higher temperatures water’s Kw increases, so neutral pH is slightly below 7. This tool uses the standard 25 °C values.
How much more acidic is pH 3 than pH 5?
Because pH is a base-10 logarithm, each unit is a factor of ten. pH 3 has [H⁺] = 10⁻³ and pH 5 has 10⁻⁵, so pH 3 is 100 times more acidic.
How do I get [H⁺] back from a pH?
Take the antilog: [H⁺] = 10^(−pH). For pH 8.4, [H⁺] = 10⁻⁸·⁴ ≈ 4.0×10⁻⁹ M. Enter the pH and the tool returns [H⁺] and [OH⁻] automatically.
What is the pH of a strong acid like 0.01 M HCl?
A strong acid ionises completely, so [H⁺] equals the concentration: pH = −log(0.01) = 2. For weak acids that ionise only partially, use the weak-acid pH tool instead.
Can pH be negative or above 14?
Yes, the 0-14 range is just the common span in dilute water. A concentrated strong acid (say 10 M H⁺) gives a negative pH, and a very concentrated base can exceed 14. The −log relationship still holds.