pH Calculator – pH, pOH, [H+] and [OH-] Free
This pH calculator turns any one known value into the full set: pH, pOH, hydrogen ion concentration [H+] and hydroxide ion concentration [OH-]. Pick what you know, type it in, and the tool uses pH = -log[H+] and pH + pOH = 14 at 77 °F (25 °C) to fill in the rest. It also finds the pH of a strong acid, a strong base, or a weak acid from its concentration and acid dissociation constant Ka, using the exact quadratic instead of the rough square-root shortcut. Use it for chemistry homework, lab prep or checking pool and aquarium numbers.
pH Calculator
Find pH, pOH, hydrogen ion and hydroxide ion concentration, including weak acids.
How to use the pH Calculator
- Choose what you know: [H+], [OH-], pH, pOH, a strong acid or base, or a weak acid.
- Enter the value in mol/L (M), or the pH number.
- For a weak acid, also enter Ka.
- Read pH, pOH, both concentrations and whether the solution is acidic, neutral or basic.
pH formulas
pH = -log₁₀[H+], pOH = -log₁₀[OH-], and pH + pOH = 14 at 25 °C. For a weak acid HA with concentration C, [H+] = (-Ka + √(Ka² + 4·Ka·C)) / 2.
Features
- Any one value gives all four
- Strong acid and strong base modes
- Weak acid pH from Ka with the exact quadratic
- Scientific notation for tiny concentrations
- Acidic, neutral or basic label
- Copy and print results
Frequently asked questions
How do you calculate pH?
pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = -log[H+]. For [H+] = 0.001 M, pH = 3.
What is the relationship between pH and pOH?
At 25 °C (77 °F), pH + pOH = 14, because [H+] × [OH-] = 1.0 × 10^-14.
What pH is neutral?
Pure water at 25 °C has a pH of 7. Below 7 is acidic and above 7 is basic (alkaline).
How do I find the pH of a weak acid?
Enter the acid concentration and its Ka. The calculator solves x² / (C - x) = Ka for x = [H+], then takes -log x. For 0.1 M acetic acid (Ka 1.8 × 10^-5), pH is about 2.88.
Can pH be negative or above 14?
Yes, for very concentrated strong acids or bases, but the simple formulas used here are most accurate for dilute solutions.