pH Calculator
pH7.0000
pOH7.0000
[H+] (mol/L)1.0000e-7
[OH-] (mol/L)1.0000e-7
The pH scale measures how acidic or basic a water-based solution is, based on its hydrogen ion concentration. This calculator converts between hydrogen ion concentration and pH, and also reports pOH and hydroxide ion concentration. You can start from a known molar concentration of hydrogen ions or from a known pH value.
Formula
pH = -log₁₀[H⁺]; pOH = 14 - pH; [H⁺] = 10^(-pH)
- [H⁺]
- Hydrogen ion concentration in moles per litre (mol/L)
- pH
- Acidity measure, where lower is more acidic
- pOH
- Basicity measure, equal to 14 - pH
How it works
- Choose your input mode. In "from H+" mode you enter the hydrogen ion concentration in moles per litre; in "from pH" mode you enter a pH value between 0 and 14.
- From a concentration the tool computes pH as the negative base-10 logarithm of [H+]. From a pH it computes [H+] as 10 raised to the negative pH. In both cases pOH is 14 minus pH and [OH-] follows from pOH, assuming a standard temperature where the ion product of water is 14.
Worked example
A solution has a hydrogen ion concentration of 0.001 mol/L (1 × 10⁻³).
- pH = -log₁₀(0.001) = -(-3) = 3.
- pOH = 14 - 3 = 11.
- [OH-] = 10^(-11) = 1 × 10⁻¹¹ mol/L.
pH = 3 (acidic), pOH = 11, and hydroxide concentration ≈ 1 × 10⁻¹¹ mol/L.
Frequently asked questions
- What pH values count as acidic, neutral, or basic?
- On the 0 to 14 scale, a pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). Each whole-number step represents a tenfold change in hydrogen ion concentration.
- Why does pH plus pOH equal 14?
- At about 25 °C the ion product of water means pH and pOH always sum to 14. This calculator assumes that standard condition, so once it knows the pH it sets pOH to 14 minus pH.
- Can pH be outside the 0 to 14 range?
- In principle very concentrated strong acids or bases can give pH values below 0 or above 14. This calculator restricts the "from pH" mode to 0 through 14, which covers nearly all everyday solutions.