pH Calculator
Compute pH, pOH, [H+], and [OH-] for any aqueous solution in either direction.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this ph calculator
- Pick what you have: [H+], [OH-], pH, or pOH.
- Type the value. For concentrations, scientific notation like 1e-7 is fine.
- Press Calculate to see pH, pOH, [H+], and [OH-] together, plus a quick acid-base classification.
- Use Copy to grab a one-line summary or Reset to start over.
About this ph calculator
pH is a logarithmic measure of hydrogen ion concentration: pH = −log₁₀[H+]. Each whole-number step is a tenfold change in concentration, which is why a pH 3 solution is 10,000× more acidic than a pH 7 solution, not 2.3× more. The pH calculator lets you enter any one of the four related quantities — pH, pOH, [H+], or [OH-] — and computes the rest using the water self-ionisation relation [H+][OH-] = 10⁻¹⁴ at 25 °C (pH + pOH = 14).
Worked example: lemon juice has roughly [H+] = 1 × 10⁻³ mol/L. Plugging that in gives pH = −log₁₀(10⁻³) = 3. The tool also returns pOH = 14 − 3 = 11 and [OH-] = 10⁻¹¹ mol/L — heavily acidic, as you would expect. Conversely, household ammonia at pH 11.5 yields pOH = 2.5, [H+] = 10⁻¹¹·⁵ ≈ 3.2 × 10⁻¹² mol/L, and [OH-] = 10⁻²·⁵ ≈ 3.2 × 10⁻³ mol/L — strongly basic. Note: pH = 14 − pOH only holds at 25 °C; the water ion product Kw changes with temperature.
FAQ
- What pH counts as acidic vs basic?
- Below 7 is acidic, above 7 is basic, exactly 7 is neutral (at 25 °C in pure water). The tool labels strong vs slight on either side of 7.
- Why does pH go negative or above 14?
- For very concentrated strong acids ([H+] > 1 mol/L) pH is negative; for very concentrated strong bases pH can exceed 14. The 0–14 range is just the convenient span for typical aqueous solutions.
- Does the relation pH + pOH = 14 always hold?
- Only at 25 °C in pure water, where Kw = 10⁻¹⁴. At higher temperatures Kw is bigger, neutral pH drifts below 7, and pH + pOH < 14.
- How do I enter very small concentrations?
- Use scientific notation: 1e-7 means 1 × 10⁻⁷. The input field accepts that format directly.
- Is this for strong or weak acids?
- It works for any aqueous solution as long as you supply the actual [H+] or pH. For a weak acid, [H+] depends on Ka and concentration — compute that first, then use this tool to get pH and [OH-].
- Is the pH calculator free?
- Yes — free, no signup, no ads inside the tool, and everything runs in your browser.