pH Calculator: pH, pOH, Weak Acids and Buffers
In short: pH calculator: find pH from [H+], strong or weak acid and base concentration, buffer pH (Henderson–Hasselbalch), pOH and pKa. It is free, works in your web browser and needs no download.
Calculate pH, pOH, hydrogen ion concentration, hydroxide concentration, strong acid/base solution pH, dilution pH, buffer pH, and pKa values.
Presets
pH Calculator Formulas
pH = -log10[H+]
pH + pOH = 14 at 25 C
[H+] = 10^-pH
pH = pKa + log10(base / acid)
When to Use Each pH Calculator Mode
- pH level calculator: use [H+] to pH or pH to [H+] modes.
- pOH and pH calculator: use pOH / [OH-] mode.
- Solution pH calculator: use strong acid/base mode for simple fully dissociated acids and bases.
- Buffer pH calculator: use Henderson-Hasselbalch mode for weak acid/conjugate base systems.
- pH to pKa calculator: use pKa from pH mode when base/acid ratio is known.
How to use the ph calculator
- Pick a mode: [H+] to pH, Strong acid/base, Weak acid/base, Buffer pH, Dilution or pKa from pH.
- Enter concentrations in mol/L. Scientific notation is accepted (
1.8e-5). - Read pH, pOH and the ion concentration. Results update as you type.
- Use the formula line under the result to show your working in a lab report.
Worked examples
0.010 M HCl (strong acid)
[H+] = 0.010 M
pH = −log(0.010) = 2.00
0.10 M acetic acid, Ka = 1.8 × 10−5
x²/(0.10 − x) = 1.8 × 10−5 → x = 1.33 × 10−3 M
pH = 2.88 (1.3% ionised)
0.10 M NH3, Kb = 1.8 × 10−5
[OH−] = 1.33 × 10−3 M, pOH = 2.88
pH = 11.12
Acetate buffer, 0.10 M each
pH = 4.76 + log(0.10/0.10)
pH = 4.76
Which pH formula should I use?
| Situation | Formula |
|---|---|
| Known [H+] | pH = −log[H+] |
| Strong acid, concentration C | [H+] = C × (H+ per formula); e.g. 0.05 M H2SO4 ≈ 0.10 M H+ |
| Strong base | pOH = −log[OH−], pH = 14 − pOH |
| Weak acid (Ka) | x²/(C − x) = Ka; if x < 5% of C, x ≈ √(KaC) |
| Buffer | pH = pKa + log([A−]/[HA]) (Henderson–Hasselbalch) |
All modes assume 25 °C (Kw = 1.0 × 10−14) and ideal dilute solutions. For very dilute strong acids and bases, the calculator includes water's own H+/OH−, so 10−8 M HCl correctly gives pH 6.98 rather than 8.
Preparing a buffer? Work out the masses with the solution prep calculator, then draw the acid–base structures in MolDraw.
Solution prep calculatorFrequently asked questions
How do you calculate pH?
pH = −log10[H+]. For 0.01 M HCl, [H+] = 0.01 M, so pH = 2.00. To go back, [H+] = 10^−pH.
How do you calculate the pH of a weak acid?
Use Ka and the concentration: solve x²/(C − x) = Ka for x = [H+]. For 0.10 M acetic acid (Ka = 1.8 × 10⁻⁵), [H+] = 1.33 × 10⁻³ M and pH = 2.88.
What is the relationship between pH and pOH?
At 25 °C, pH + pOH = 14 because Kw = [H+][OH−] = 1.0 × 10⁻¹⁴.
How do you calculate the pH of a buffer?
Use the Henderson–Hasselbalch equation, pH = pKa + log10([A−]/[HA]). Equal acid and base concentrations give pH = pKa.
Why is the pH of 10⁻⁸ M HCl not 8?
An acid cannot make water basic. At very low concentrations the H+ from water (10⁻⁷ M) matters. Including it gives pH 6.98, which is what this calculator reports.
Does diluting an acid ten times raise the pH by one?
For a strong acid, yes, until the pH approaches 7. A tenfold dilution of pH 3 gives pH 4, but further dilution levels off at 7 rather than passing it.
Related chemistry tools
Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).
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