pH Calculator
Calculate pH, pOH and OH⁻ concentration from H⁺ (hydrogen ion) concentration in mol/L. Find out if a solution is acidic, basic or neutral.
How to use this tool
- Enter h⁺ concentration [h⁺] in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your ph and the full breakdown beneath it.
pH is defined as pH = −log₁₀[H⁺]. A solution with [H⁺] = 10⁻⁷ mol/L is neutral (pH 7). Below 7 is acidic; above 7 is basic. pOH = 14 − pH at 25 °C.
Formula
pH = −log10[H+]
pOH = 14 − pH
[OH−] = 10−pOH
Where [H+] = hydrogen ion concentration in mol/L. The scale runs 0 (most acidic) to 14 (most basic).
How it works
This calculator converts hydrogen ion concentration to pH using the negative base-10 logarithm. It then derives pOH from the water equilibrium constant (Kw = 10−14 at 25 °C), which gives pH + pOH = 14, and back-calculates the hydroxide ion concentration. The pH + pOH = 14 relationship assumes a temperature of 25 °C; at other temperatures, Kw changes and the neutral point shifts.
Worked example
- Given: [H+] = 0.001 mol/L = 10−3 mol/L.
- Calculate pH: pH = −log10(0.001) = −(−3) = 3.0.
- Calculate pOH: pOH = 14 − 3 = 11.0.
- Since pH = 3 < 7, the solution is acidic.
pH = 3.0; pOH = 11.0; Character = Acidic
Common mistakes to avoid
- Entering concentration in units other than mol/L -- if H+ is given in mmol/L, divide by 1000 first before entering.
- Assuming pH 7 is always neutral -- strictly, pH 7 is neutral only at 25 degrees C; at higher temperatures the neutral point shifts.
- Confusing [H+] with [OH-] -- entering the hydroxide concentration instead of the hydrogen ion concentration gives a pH that is 14 units off.
Key terms
- pH
- A logarithmic scale measuring hydrogen ion concentration. pH 7 is neutral; below 7 is acidic; above 7 is basic. Each unit represents a 10-fold change in [H+].
- pOH
- The negative base-10 logarithm of the hydroxide ion concentration. At 25 °C, pH + pOH = 14.
- Hydrogen ion concentration [H+]
- The molar concentration of hydrogen ions (protons) in solution, typically expressed in mol/L. Pure water at 25 °C has [H+] = 10−7 mol/L.
- Water autoionisation constant (Kw)
- The product [H+][OH−] = 10−14 at 25 °C. This relationship links pH and pOH to sum to 14 at that temperature.
- Acidic / Basic / Neutral
- A solution is acidic when pH < 7, neutral when pH = 7, and basic (alkaline) when pH > 7, at 25 °C.
Frequently asked questions
- What everyday substances have known pH values?
- Lemon juice ~2, coffee ~5, pure water 7, baking soda ~9, bleach ~12.
- Does pH change with temperature?
- Yes. Pure water at 25 °C has pH 7, but at 100 °C it has pH ~6.1 — still neutral because both H⁺ and OH⁻ concentrations rise equally.