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pH Converter

Science

Convert between pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-] in mol/L — for chemistry, water testing, and lab use.

Reviewed by the thecalcu.com team · Last updated July 12, 2026

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What is a pH?

A pH Converter translates between the four interlinked ways chemists and lab technicians express acidity and basicity: pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-]. pH itself is a logarithmic measure, a scale from roughly 0 (strongly acidic) to 14 (strongly basic), with 7 as neutral, that compresses hydrogen ion concentrations spanning fourteen orders of magnitude into a compact, easy-to-read number.

This is distinct from the Concentration Converter, which handles ratio-based concentration units like ppm and mg/L, and the Molar Concentration Converter, which converts molarity units like mol/L and mmol/L, neither of those tools works on a logarithmic scale the way pH does. This converter is purpose-built for the specific mathematical relationship between pH, pOH, and ion concentration.

Why Use a pH Converter?

Lab reports, water quality tests, and chemistry coursework sometimes express acidity as a raw hydrogen ion concentration (in mol/L) rather than the more familiar pH value, or ask you to derive pOH from a measured pH reading. Converting between these manually requires working with logarithms and water's ion product constant, which is easy to get wrong under time pressure.

Two concrete situations this solves: converting a water testing kit's pH reading into [H+] concentration for a lab report that requires molar concentration units, and calculating the pOH of a solution when only the pH has been measured, to determine its relative basicity. Both conversions apply standard chemistry formulas instantly.

Who Should Use This Converter?

  • Chemistry students checking pH-to-concentration homework problems or verifying manual logarithm calculations.
  • Lab technicians and water quality testers converting between pH meter readings and molar ion concentrations for reporting purposes.
  • Aquarium and hydroponics hobbyists translating pH test strip readings into concentration terms when following technical dosing guides.
  • Environmental science students studying acid rain, soil acidity, or water treatment processes that reference both pH and ion concentration.
  • Educators demonstrating the logarithmic relationship between pH and hydrogen ion concentration with instant, verifiable examples.

What Insights Does the pH Converter Give You?

Converting to pH or pOH gives you the standard, human-readable acidity/basicity scale used in almost all everyday and lab contexts, easy to compare at a glance (lower pH = more acidic). Converting to Hydrogen Ion Concentration [H+] or Hydroxide Ion Concentration [OH-] gives you the underlying molar concentration values (in mol/L) needed for further chemical calculations, such as titration or reaction rate equations.

Because pH is logarithmic, small pH differences represent large concentration differences, a pH change of just 1 unit means the hydrogen ion concentration changes by a factor of 10. Converting to [H+] makes this scale explicit and useful for calculations that need the actual concentration rather than the compressed pH number.

How to use this pH calculator

  1. Select your starting unit from the From dropdown, pH, pOH, [H+], or [OH-].
  2. Enter the value you want to convert, such as 4 for a pH reading.
  3. Select the unit you want to convert to from the To dropdown, such as Hydrogen Ion Concentration [H+] (mol/L).
  4. Read the converted result, which updates instantly as you change the value or units.
  5. Use the swap (⇅) button to reverse the conversion direction if needed.
  6. Cross-check the result against a known reference (like pure water at pH 7) to confirm the conversion looks correct for your context.

Formula & Methodology

All conversions route through hydrogen ion concentration [H+] in mol/L as the canonical unit, using water's ion product constant at 25°C (Kw = 1×10⁻¹⁴ mol²/L²):

pH = −log₁₀([H+])  |  [H+] = 10^(−pH)
pOH = 14 − pH  |  [OH-] = Kw ÷ [H+]

Worked example: Converting a pH of 3 to hydrogen ion concentration, [H+] = 10^(−3) = 0.001 mol/L. Converting that same solution's pOH: pOH = 14 − 3 = 11, and its hydroxide ion concentration: [OH-] = 1×10⁻¹⁴ ÷ 0.001 = 1×10⁻¹¹ mol/L, confirming the solution is strongly acidic.

Frequently Asked Questions

pH is a logarithmic scale, ranging roughly from 0 to 14, that measures how acidic or basic (alkaline) a water-based solution is, based on its hydrogen ion concentration [H+]. A pH of 7 is neutral (pure water), values below 7 are acidic, and values above 7 are basic.
Hydrogen ion concentration is calculated as [H+] = 10^(-pH), which is the mathematical inverse of the pH definition, pH = -log10([H+]). This converter applies that formula directly, so entering a pH of 4 returns [H+] = 10⁻⁴ mol/L.
At 25°C, pH and pOH always add up to 14, so pOH = 14 - pH, and pH = 14 - pOH. This relationship comes from water's ion product constant (Kw = 1×10⁻¹⁴), and this converter uses it to convert between pH and pOH directly.
Select pH as your From unit, enter your pH value, and choose pOH as the To unit, the tool calculates pOH = 14 - pH automatically. For example, a pH of 3 converts to a pOH of 11.
Select Hydrogen Ion Concentration [H+] (mol/L) as your From unit, enter the concentration value (such as 0.0001), and choose pH as the To unit. The tool applies pH = -log10([H+]) automatically, giving a pH of 4 for that example.
[H+] is the concentration of hydrogen ions, which determines acidity, while [OH-] is the concentration of hydroxide ions, which determines basicity, the two are always linked by water's ion product, [H+] × [OH-] = 1×10⁻¹⁴ at 25°C. This converter calculates either one from the other, or from pH/pOH, using that relationship.
Hydrogen ion concentrations in real solutions range across many orders of magnitude, from about 1 mol/L in strong acids down to 10⁻¹⁴ mol/L in strong bases, which is impractical to work with directly. The logarithmic pH scale compresses this huge range into small, easy-to-compare numbers from roughly 0 to 14.
The Bureau of Indian Standards (IS 10500) and WHO guidelines recommend a drinking water pH range of 6.5 to 8.5, which keeps water close to neutral without being noticeably acidic or basic. Water testing kits typically report pH directly, which this converter can cross-check against [H+] concentration for lab reporting.
Battery acid measures around pH 0-1, lemon juice around pH 2, pure water is neutral at pH 7, seawater is mildly basic around pH 8, and household ammonia is strongly basic at around pH 11-12. This converter is useful for translating any of these reference pH values into [H+] or [OH-] concentration for lab calculations.
No, this converter uses the standard 25°C value for water's ion product constant (Kw = 1×10⁻¹⁴), which is the conventional reference temperature for pH and pOH calculations. At significantly different temperatures, Kw shifts slightly, which would change the exact pH-pOH relationship, but 25°C is the standard used in most lab and educational contexts.
Also known as
ph to h+ concentrationph to poh converterhydrogen ion concentration calculatorpoh to phacidity converterph scale converter