pH Calculator
Convert between pH, [H+], pOH and [OH-] instantly.
This free pH calculator converts between pH and hydrogen ion concentration [H+] using the standard formula pH = -log₁₀[H+], and also derives pOH and hydroxide ion concentration [OH-] using the water dissociation relationship pH + pOH = 14 at 25°C. Enter a value in any of the fields and the rest update automatically.
pH is the standard scale used in chemistry, biology, agriculture, water treatment and food science to describe how acidic or basic a solution is. Because the pH scale is logarithmic, each whole-number change represents a tenfold change in hydrogen ion concentration — a solution of pH 4 is ten times more acidic than pH 5.
Frequently Asked Questions
What is pH?
pH is a logarithmic scale that measures how acidic or basic an aqueous solution is, based on the concentration of hydrogen ions [H+]. It typically ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral at 25°C.
How do I calculate pH from [H+]?
Use the formula pH = -log₁₀[H+], where [H+] is the hydrogen ion concentration in moles per litre (mol/L). For example, [H+] = 1 × 10⁻⁷ mol/L gives pH = 7.
How do I calculate [H+] from pH?
Use the inverse formula: [H+] = 10^(-pH). For example, a pH of 3 gives [H+] = 10⁻³ = 0.001 mol/L.
What is pOH and how is it related to pH?
pOH measures hydroxide ion concentration the same way pH measures hydrogen ion concentration. At 25°C, pH and pOH always add up to 14, so pOH = 14 - pH.
Why is the pH scale logarithmic?
Because hydrogen ion concentrations in solutions span many orders of magnitude, a logarithmic scale keeps the numbers manageable. Each single pH unit represents a tenfold change in [H+].
What pH is considered neutral?
Pure water at 25°C has a pH of exactly 7, which is considered neutral because [H+] equals [OH-].
What is the difference between acidic and basic solutions?
A solution is acidic when pH is below 7 (more H+ than OH-), and basic (alkaline) when pH is above 7 (more OH- than H+).
Does temperature affect pH calculations?
Yes. This calculator assumes the standard reference temperature of 25°C, where the ion product of water (Kw) is 1 × 10⁻¹⁴. At other temperatures, neutral pH shifts slightly away from 7.
Can pH be negative or above 14?
Yes, in very concentrated acids or bases. Extremely strong acids can have a pH below 0, and extremely strong bases can exceed pH 14, though most everyday solutions fall between 0 and 14.
What are some common pH examples?
Stomach acid is around pH 1.5–3.5, black coffee is about pH 5, pure water is pH 7, seawater is about pH 8.1, and household bleach is around pH 12–13.