pH Calculator - Acids, Bases & Buffer Solutions
Calculate pH, pOH, [H+], and [OH−] for strong and weak acids and bases, plus buffer solutions using the Henderson-Hasselbalch equation.
Choose your solution type, enter the concentration and any dissociation constants, and get the pH along with pOH and ion concentrations instantly.
pH Calculator - Acids, Bases & Buffer Solutions
Calculate pH, pOH, [H+], and [OH−] for strong and weak acids and bases, plus buffer solutions using the Henderson-Hasselbalch equation.
About the pH calculator
pH is the standard measure of how acidic or basic an aqueous solution is. Defined as the negative base-10 logarithm of the hydrogen-ion concentration, pH = −log₁₀[H⁺], it compresses a huge range of concentrations into a convenient scale that usually runs from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic (alkaline). Because the scale is logarithmic, each whole-number step represents a tenfold change in hydrogen-ion concentration — a solution at pH 3 is ten times more acidic than one at pH 4 and a hundred times more acidic than pH 5.
The pH calculator handles the five most common solution types you encounter in general and analytical chemistry. For a strong acid that dissociates completely, the hydrogen-ion concentration equals the acid concentration, so pH = −log₁₀(C). For a strong base, the hydroxide concentration equals the base concentration, giving pOH = −log₁₀(C) and pH = 14 − pOH. These two cases need only the concentration in moles per litre.
Weak acids and bases ionise only partially, so they require their dissociation constant. For a weak acid the calculator uses the equilibrium approximation [H⁺] = √(Ka · C), where Ka = 10^(−pKa); for a weak base it uses [OH⁻] = √(Kb · C). Enter the pKa or pKb alongside the concentration and the pH calculator returns the pH from the resulting ion concentration. This square-root approximation assumes the amount of acid or base that dissociates is small relative to the starting concentration, which holds well for typical dilute, weakly ionising solutions.
Buffer solutions — mixtures of a weak acid and its conjugate base — resist changes in pH and are calculated with the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]). Enter the pKa of the acid and the molar concentrations of the acid (HA) and its conjugate base (A⁻); when the two are equal the pH simply equals the pKa, which is why buffers are most effective near their pKa. Buffers are essential in biochemistry, cell culture, fermentation, and any process that must hold a stable pH.
Alongside pH, the calculator reports pOH, [H⁺], and [OH⁻] so you have a complete picture of the solution's equilibrium. Remember that the simple formulas here assume ideal behaviour at 25 °C, where the ion product of water Kw = 1×10⁻¹⁴. Very concentrated solutions, extreme pH values, temperature changes, and activity effects can shift real measurements slightly, so treat the output as an accurate teaching and planning estimate rather than a substitute for a calibrated pH meter.
pH calculator examples
Click any example button under the calculator to load these classic solutions.
| Solution | pH | How it's calculated |
|---|---|---|
| 0.01 mol/L HCl (strong acid) | pH 2.00 | HCl dissociates completely, so [H⁺] = 0.01 mol/L and pH = −log₁₀(0.01) = 2.00. |
| 0.1 mol/L acetic acid, pKa 4.75 (weak acid) | pH 2.88 | Ka = 10⁻⁴·⁷⁵, so [H⁺] = √(Ka × 0.1) ≈ 1.33×10⁻³ mol/L and pH ≈ 2.88. |
| 0.05 mol/L NaOH (strong base) | pH 12.70 | pOH = −log₁₀(0.05) = 1.30, so pH = 14 − 1.30 = 12.70. |
| Acetic acid / acetate buffer, 0.1 / 0.1 mol/L, pKa 4.76 | pH 4.76 | Henderson-Hasselbalch: pH = 4.76 + log₁₀(0.1/0.1) = 4.76. Equal amounts make pH equal the pKa. |
How to use the pH calculator
- Select the solution type: strong acid, weak acid, strong base, weak base, or buffer solution.
- For acids and bases, enter the molar concentration in mol/L; for weak species, also enter the pKa or pKb.
- For a buffer, enter the acid's pKa and the molar concentrations of the acid (HA) and its conjugate base (A⁻).
- Click Calculate to see the pH, pOH, [H⁺], and [OH⁻] of your solution.
- Click Reset to clear the inputs, or load an example to see a worked calculation.
pH calculator FAQ
What is pH and how is it calculated?
pH measures acidity as the negative base-10 logarithm of the hydrogen-ion concentration: pH = −log₁₀[H⁺]. A pH below 7 is acidic, 7 is neutral, and above 7 is basic. The calculator finds [H⁺] for your solution type and applies this formula.
How do I calculate the pH of a weak acid?
For a weak acid, use [H⁺] = √(Ka × C), where Ka = 10^(−pKa) and C is the concentration. Then pH = −log₁₀[H⁺]. Enter the concentration and pKa and the calculator does this automatically.
What is the Henderson-Hasselbalch equation?
It calculates buffer pH: pH = pKa + log₁₀([A⁻]/[HA]), where [A⁻] is the conjugate base concentration and [HA] is the acid concentration. When the two are equal, pH equals the pKa, which is where a buffer works best.
What's the difference between pH and pOH?
pOH measures hydroxide concentration: pOH = −log₁₀[OH⁻]. In water at 25 °C, pH + pOH = 14, so once you know one you can find the other. The calculator reports both for every solution.
Why does my calculated pH differ from a meter reading?
The formulas assume ideal behaviour at 25 °C and use approximations for weak acids and bases. Temperature, ionic strength (activity), very high or low concentrations, and impurities shift real readings, so a calibrated pH meter may differ slightly from the ideal value.
Can pH be negative or above 14?
Yes. The 0–14 range covers most everyday solutions, but very concentrated strong acids can have a negative pH and very concentrated strong bases can exceed 14. The calculator will return those values when the concentrations call for them.