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pH Calculator

Work out the pH of a solution from its hydrogen ion concentration, or reverse it to find [H+], pOH and [OH-] from a known pH. Enter just one value and the calculator fills in the rest, then tells you whether the solution is acidic, neutral or basic.

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How to use this calculator

Enter Values

Minimum 0

Minimum 0

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter EITHER the hydrogen ion concentration [H+] in mol/L OR the pH — fill in exactly one field, not both.
  2. Read off the pH (or [H+]), plus pOH, hydroxide concentration [OH-] and the acid/base classification.
  3. Use the steps panel to see each conversion, which is handy for checking homework or lab work.

How it works

pH is defined as -log10 of the hydrogen ion concentration: pH = -log10([H+]). Rearranged, [H+] = 10^(-pH). Because water self-ionises, pH and pOH always add to 14 at 25 degC, so pOH = 14 - pH and [OH-] = 10^(-pOH). A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic.

Worked example

Worked example. A solution has [H+] = 0.001 mol/L (1.000e-3). pH = -log10(0.001) = 3, so pOH = 14 - 3 = 11 and [OH-] = 10^(-11) = 1.000e-11 mol/L. A pH of 3 is well below 7, so the solution is Acidic.

Common mistakes

  • Entering both [H+] and pH at once — supply only one and let the tool derive the other.
  • Typing a hydrogen ion concentration of 0 or a negative number; concentration must be greater than 0 for the logarithm to be defined.
  • Forgetting that concentration is in mol/L (molarity), not grams per litre or percent.

Frequently asked questions

Why does pH plus pOH equal 14?

At 25 degC the ion product of water (Kw) is 1.0e-14, so [H+] times [OH-] = 1.0e-14. Taking negative logs of both sides gives pH + pOH = 14. This relationship shifts slightly at other temperatures.

Can pH be negative or above 14?

Yes, for very concentrated strong acids or bases the pH can fall below 0 or rise above 14. This calculator restricts pH entry to the common 0-14 range; for extreme concentrations enter the [H+] value directly instead.

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Cite this tool

Zerdly. (2026). pH Calculator. Retrieved from https://www.zerdly.com/tools/ph-calculator/

Tip: Enter any known values to calculate the remaining results.

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