pH Calculator: what this tool is for
The online pH calculator accurately determines the acidity or alkalinity of aqueous solutions for strong acids, strong bases, weak acids, weak bases, as well as directly from oxonium ion [H₃O⁺] or hydroxide ion [OH⁻] concentrations.
pH (potential of hydrogen) is the fundamental standard measurement of proton activity in aqueous chemistry. Defined by Søren Sørensen in 1909 as pH = -log10([H₃O⁺]), it underpins chemical equilibrium, biochemistry, pharmacology, and environmental sciences.
Unlike simplistic calculators that rely on crude approximations, this tool solves the exact quadratic equations derived from charge neutrality and water auto-ionization (Kw = 1.00 × 10⁻¹⁴ at 25 °C), preventing mathematical distortions on ultra-dilute solutions.
For every calculation, the tool instantly provides pH, pOH, exact molar concentrations of [H₃O⁺] and [OH⁻], the dissociation percentage (α), and dynamically plots the position on an interactive 0 to 14 pH scale.
All calculations run entirely client-side in your web browser: no experimental data is transmitted over the network, ensuring zero latency and complete research privacy.
No file stored
Nothing is saved on our servers, during or after processing.
Local processing
The computation runs in your browser whenever the technology allows it.
Instant result
No waiting and no export queue: the result appears live.
Free, no account
No sign-up, no daily limit and no intrusive ads.
What this tool gives you
Exact Quadratic Equation Solver
Rigourously solves second-degree equations for weak acids, weak bases, and dilute solutions without misleading simplifications.
Complete 6-Mode Coverage
Calculate the pH of strong acids, strong bases, weak acids (pKa), weak bases (pKb or pKa), or directly from [H₃O⁺] and [OH⁻].
Interactive Color-Coded pH Scale
Instantly see where your solution falls on the 0–14 universal indicator spectrum (red for strong acid, green for neutral, purple for alkaline).
Common Reagent Presets
One-click presets for hydrochloric acid (HCl), acetic acid (vinegar), formic acid, sodium hydroxide (NaOH), and aqueous ammonia (NH₃).
Real-world examples
Standard theoretical pH calculations for common acidic and basic aqueous solutions at 25 °C.
Calculating the pH of a weak acid: 0.1 M Acetic Acid
For acetic acid (CH₃COOH) at C = 0.1 mol/L with pKa = 4.76 (Ka = 1.74 × 10⁻⁵), solving the quadratic equation yields [H₃O⁺] = 1.31 × 10⁻³ mol/L. The resulting pH is pH = -log10(1.31 × 10⁻³) = 2.88, with a dissociation degree α = (1.31 × 10⁻³ / 0.1) × 100 = 1.31 %.
Calculating the pH of a strong base: 0.01 M Sodium Hydroxide
For sodium hydroxide (NaOH) at 0.01 mol/L, complete dissociation gives [OH⁻] = 0.01 mol/L. Thus pOH = -log10(0.01) = 2.00, yielding pH = 14.00 - 2.00 = 12.00.
How to use pH Calculator
Select the chemical species type
Choose the relevant mode: strong acid, weak acid, strong base, weak base, or direct [H₃O⁺] / [OH⁻] concentration.
Enter the initial concentration
Type the solution concentration and choose the appropriate unit (mol/L, mmol/L, or µmol/L).
Provide the equilibrium constant
For weak acids or weak bases, specify the pKa or pKb value (e.g., 4.76 for acetic acid).
Review and copy the results
Inspect the calculated pH, pOH, ion concentrations, and copy the full summary directly to your clipboard.
Tips and mistakes to avoid
Temperature dependence of Kw
The standard 0–14 pH scale assumes Kw = 10⁻¹⁴ at 25 °C. At physiological temperature (37 °C), neutral water pH is approximately 6.81.
Evaluating weak acid dissociation
If the dissociation degree α < 5%, the common approximation pH ≈ 0.5(pKa - log C) holds well. For higher dissociation, only the quadratic formula is mathematically sound.
Universal pKa and pKb relationship
In aqueous solution at 25 °C, for any conjugate acid/base pair: pKa + pKb = 14.00.
Avoid : Ignoring water auto-ionization in ultra-dilute acids
Calculating pH for 10⁻⁸ M strong acid with the naive formula -log(10⁻⁸) would suggest a basic pH of 8. The exact Kw quadratic equation correctly gives pH = 6.98.
Avoid : Confusing pH with titratable total acidity
pH only measures free [H₃O⁺] ions at equilibrium, not the reserve capacity of undissociated weak acid released during titration.
Frequently asked questions
Answers to the questions people ask about pH Calculator.
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Updated on August 2026. See all tools