pH Calculator
PH calculator — enter pH, pOH, [H⁺] or [OH⁻] and get the other three at 25 °C, plus whether the solution is acidic or basic. Curriculum-aligned.
pH Calculator
Enter one value — pH, pOH, [H⁺] or [OH⁻] — and leave the rest blank. Concentration is in mol/L (M).
- Curriculum
- English (Singapore) — Singapore Examinations and Assessment Board (SEAB)
- Built against
- SEAB GCE A-Level Chemistry 9729 (2023–2026) — Ionic Equilibria
- Unit system
- SI primary; US/imperial readout below
- First published
- 1 Jun 2026
- Last updated
- 1 Jun 2026
View authoritative scientific sources
- IUPAC Gold Book — definition of pH
- NIST — ionization constant of water (Kw)
- CRC Handbook of Chemistry and Physics
- pH — Encyclopædia Britannica
⚠️ Educational use only — see full disclaimer
EDUCATIONAL USE DISCLAIMER
This calculator is provided for educational and reference purposes only. It is not a substitute for instruction from a qualified teacher, your prescribed textbook, or your school's official curriculum materials.
When preparing for examinations, always cross-check our calculations and notation against your current syllabus and your teacher's guidance. Syllabus conventions and accepted notation vary between curricula and may change between examination years.
If you believe any calculation, notation, or curriculum reference in this tool is inaccurate, please let us know via the feedback button. We review feedback promptly and update tools when verified corrections are needed.
RECATOOLS accepts no liability for academic, examination, professional, or research outcomes arising from use of this tool.
How to Use the pH Calculator
Pick your curriculum
Use the curriculum pills above to match your syllabus. Terminology, the worked example and the whole page follow your selection.
Enter one value
Type any one of pH, pOH, [H⁺] or [OH⁻]. Concentration is in moles per litre (mol/L); pH and pOH have no units.
Read the result
The calculator gives the other three values at 25 °C and tells you whether the solution is acidic, neutral or basic.
Check against your syllabus
The Tool Information block shows exactly which syllabus this is built against. Spot something off? Use the feedback button — we review every report.
The pH Scale, in Your Curriculum's Words
pH = −log₁₀[H⁺]
Worked example (SEAB style): A solution has [H⁺] = 1.0 × 10⁻³ mol/L. Determine its pH and pOH.
Given: [H⁺] = 1.0 × 10⁻³ mol/L
Applying pH = −log[H⁺]:
pH = −log(1.0 × 10⁻³) = 3.00; pOH = 14 − 3.00 = 11.00 → acidic
The pH scale measures the acidity of an aqueous solution as pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol per litre. The matching pOH scale is −log₁₀[OH⁻], and at 25 °C the two are linked by pH + pOH = 14 because the ionic product of water Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. Enter any one quantity and the calculator gives the other three.
pH 7 is neutral at 25 °C; below it is acidic, above it is basic. Concentration is reported in SI scientific units (mol/L) only — there is no imperial equivalent. All calculation happens in your browser — nothing is uploaded.
Each single pH unit is a tenfold change in hydrogen-ion concentration. pH 3 is ten times as acidic as pH 4.
10 Facts About the pH Scale
pH = −log₁₀[H⁺]; the scale was introduced by Søren Sørensen in 1909.
The "p" stands for the power (potenz) of hydrogen.
At 25 °C, pH + pOH = 14 because Kw = 1.0 × 10⁻¹⁴.
Each whole pH unit is a tenfold change in [H⁺].
pH 7 is neutral at 25 °C; below is acidic, above is basic.
Pure water self-ionises: [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M.
pH can go below 0 or above 14 for very concentrated acids/bases.
Kw rises with temperature, so neutral pH is below 7 above 25 °C.
Blood is tightly buffered near pH 7.4.
This calculator runs in your browser — your working stays private.
Frequently Asked Questions
- pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol per litre. The matching pOH scale is −log₁₀[OH⁻]. Enter any one of pH, pOH, [H⁺] or [OH⁻] and the calculator gives the other three plus whether the solution is acidic or basic.
- pH and pOH have no units (they are logarithms). The ion concentrations [H⁺] and [OH⁻] are in moles per litre (mol/L or M). Molar concentration has no imperial equivalent, so results are reported in SI scientific units only, often in scientific notation because the values are so small.
- Yes. Enter any one of the four quantities. From [OH⁻] the calculator finds pOH = −log[OH⁻], then pH = 14 − pOH at 25 °C, and hence [H⁺]. Only one input is allowed at a time.
- At 25 °C, water self-ionises so that the ionic product Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. Taking −log₁₀ of both sides gives pH + pOH = 14. This relationship shifts slightly at other temperatures because Kw is temperature-dependent.
- The chemistry — pH = −log[H⁺] — is identical worldwide. What changes is the terminology and notation. For example, mainland Chinese textbooks write the base-10 logarithm as "lg" (pH = −lg[H⁺]), while English and Malay textbooks write "log" — but the calculated value is the same.
- pH and pOH are shown to two decimal places, following standard exam practice. Concentrations are shown to the significant figures matching your input, capped at five, in scientific notation.
- pH 7 is neutral only at 25 °C. Because Kw rises with temperature, the neutral point falls below pH 7 at higher temperatures, even though [H⁺] still equals [OH⁻] there.
- The Tool Information block lists the exact syllabus for your selected curriculum (e.g. SEAB A-Level Chemistry 9729 or SPM Kimia 4541). It is a study aid, not a substitute for your official syllabus or teacher.
- No. Every calculation runs in your browser; nothing you type is uploaded. It works offline once the page has loaded.
- Completely free, no account or usage limit. It runs entirely in your browser and collects no data.
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