pH Calculator
Compute pH from hydrogen ion concentration or the reverse, in both directions. Accepts e-notation like 1e-7 and shows the substituted pH = −log₁₀[H⁺] working plus an acidic / neutral / basic classification based on 25 °C water.
Try an example
Enter a concentration or a pH to compute the other value.
How to use
- 1
Pick a direction
Choose Concentration → pH or pH → Concentration.
- 2
Enter the value
Type the hydrogen ion concentration (e.g. 1e-7) or a pH (e.g. 2.5).
- 3
Read the value and class
Alongside the result, the substituted working and an acidic / neutral / basic verdict are shown.
Features
- Both directions — concentration to pH and back — on one page
- Accepts e-notation like 1e-7; pH shown to two decimal places
- Acidic / neutral / basic classification (25 °C basis) with substituted working
- Non-positive concentrations reported as errors; inputs are never transmitted
Use cases
Check chemistry homework
Verify pH ↔ concentration conversions and the logarithm steps.
Tidy lab report values
Convert a measured pH into a concentration, or the reverse, for reports.
Classify a solution
Enter a pH to see whether the solution reads acidic, neutral or basic.
Details
pH is defined as pH = −log₁₀[H⁺], the negative common logarithm of the hydrogen ion concentration [H⁺] in mol/L. A concentration of 1 × 10⁻⁷ mol/L gives pH = 7. To go the other way, [H⁺] = 10⁻ᵖᴴ. The tool shows the substituted working in either direction.
The classification assumes water at 25 °C: pH < 7 is acidic, pH = 7 neutral, and pH > 7 basic. The neutral point drifts away from 7 at other temperatures (about 6.3 at 80 °C), so treat it as a 25 °C guideline. Relatedly, pH + pOH = 14 holds at 25 °C.
Concentrations accept e-notation such as 1e-7. Following common practice, pH is displayed to two decimal places, and values outside the usual 0–14 window (possible for concentrated acids and bases) are still computed with a note. A concentration of 0 or less has no logarithm and is rejected as an error. Inputs are never transmitted; everything is computed locally in your browser.
FAQ
What is pH?
It is the negative common logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺] with [H⁺] in mol/L. It expresses how acidic or basic a solution is, typically on a 0–14 scale.
Why is pH 7 neutral?
Pure water at 25 °C has [H⁺] = 1 × 10⁻⁷ mol/L, whose logarithm gives pH = 7. At other temperatures the balance shifts — neutral is about 6.3 at 80 °C.
Can pH be below 0 or above 14?
Yes. Concentrated hydrochloric acid or sodium hydroxide can leave that window. The tool computes such values as-is and notes that they sit outside the usual range.
How are pH and pOH related?
At 25 °C, pH + pOH = 14. pOH is −log₁₀[OH⁻] for the hydroxide concentration: the more acidic the solution, the smaller the pH and the larger the pOH.
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Verified: Known values ([H⁺]=1e-7 → pH 7, pH 5 → 1.0×10⁻⁵), non-positive errors, the reverse direction and the English page are covered by browser tests
Did you know?
One pH unit means a tenfold change in hydrogen ion concentration: lemon juice at pH 3 is ten times as acidic as tomato juice at pH 4. Remembering "one unit = ten times" makes pH values readable at a glance.