# pH Calculator

URL: https://speedycalculator.com/calculators/chemistry/ph/
Last updated: 2026-10-30

> Calculate pH, pOH, [H⁺] and [OH⁻] for a strong or weak acid or base from its concentration, or from the mass dissolved in a volume, using built-in Ka and Kb values for 40 common compounds – or convert directly from [H⁺], [OH⁻] or pOH.

## Inputs

- Calculate the pH from
- Acid
- Acid dissociation constant Ka
- Base
- Base dissociation constant Kb
- Molar mass of the acid (g/mol, kg/mol, Da (u), kDa)
- Molar mass of the base (g/mol, kg/mol, Da (u), kDa)
- Concentration (mol/L (M), mmol/L (mM), µmol/L (µM), nmol/L (nM), mol/m³)
- Mass of compound (g, mg, µg, kg, oz, lb)
- Volume of solution (L, mL, µL, m³, dm³, cm³, US gal, fl oz (US))
- Hydrogen-ion concentration [H⁺] (mol/L (M), mmol/L (mM), µmol/L (µM), nmol/L (nM), mol/m³)
- Hydroxide-ion concentration [OH⁻] (mol/L (M), mmol/L (mM), µmol/L (µM), nmol/L (nM), mol/m³)
- pOH

## Results

- pH
- Character
- [H⁺]
- Hydrogen-ion concentration [H⁺]
- pOH
- Hydroxide-ion concentration [OH⁻]
- Compound
- Dissociation constant
- Degree of dissociation
- Concentration of the compound
- Molar mass
- Amount dissolved

## Formula

```
pH = −log₁₀[H⁺]  ·  pOH = −log₁₀[OH⁻]  ·  pH + pOH = 14 (25 °C)
Strong acid: [H⁺] ≈ n·c  ·  Weak acid: Ka = x² ÷ (c − x)
The calculator solves the full charge balance [H⁺] = [A⁻] + [OH⁻], so very dilute solutions come out right too.
```

## Explanation

pH measures how acidic or basic a water solution is on a logarithmic scale: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion (strictly hydronium) concentration in mol/L. Pure water at 25 °C has [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M and a pH of 7. Every unit down the scale means ten times more H⁺ – lemon juice at pH 2 is about 100,000 times more acidic than pure water.

Strong acids such as HCl and HNO₃ dissociate completely, so [H⁺] is simply their concentration. Weak acids such as acetic acid only partly dissociate, and the extent is set by the acid dissociation constant Ka (or pKa = −log Ka). A 0.1 M acetic acid solution is only about 1.3% ionised, so its pH is 2.88 rather than 1. Bases work the same way through [OH⁻] and Kb.

Choose a mode: an acid or base from the list (pKa values at 25 °C from standard tables) with its concentration, or with the mass dissolved and the solution volume, or simply convert a known [H⁺], [OH⁻] or pOH into pH. For polyprotic acids only the first dissociation is used, apart from sulfuric acid, whose second proton is included.

### How to use the pH Calculator

- **Calculate the pH from**
- **Acid**
- **Acid dissociation constant Ka**
- **Base**
- **Base dissociation constant Kb**
- **Molar mass of the acid**
- **Molar mass of the base**
- **Concentration**
- **Mass of compound**
- **Volume of solution**
- **Hydrogen-ion concentration [H⁺]**
- **Hydroxide-ion concentration [OH⁻]**
- **pOH**

Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.

### How it works

For acids and bases the calculator finds [H⁺] that satisfies the charge balance of the solution exactly – including the water autoionisation (Kw = 1.0 × 10⁻¹⁴) – instead of using the square-root shortcut, so the result is correct for weak, strong and very dilute solutions alike. Then pH = −log[H⁺], pOH = 14 − pH and [OH⁻] = Kw ÷ [H⁺].

### Worked example

0.1 M acetic acid, pKa 4.756 (Ka = 1.754 × 10⁻⁵). Solving Ka = x² ÷ (0.1 − x) gives x = [H⁺] = 1.316 × 10⁻³ M, so **pH = 2.88**, pOH = 11.12 and only 1.32% of the acid is ionised. Compare 0.01 M sulfuric acid: the first proton is fully released (0.01 M) and the second partly (pKa₂ 1.99), giving [H⁺] = 0.0142 M and pH 1.85 rather than 1.70. In mass mode, 2 g of NaOH (39.997 g/mol) in 500 mL is 0.050 mol in 0.5 L = 0.100 M, so pH = 13.00.

### pH of everyday substances

SubstanceApproximate pHBattery acidbelow 1Stomach acid1.5–3.5Lemon juiceabout 2Coffeeabout 5Pure water (25 °C)7.0Human blood7.35–7.45Seawaterabout 8.1Household ammoniaabout 11–12Bleach12–13

### Tips

- Use the final volume of the solution, not the volume of solvent added, for molarity.
- For dilutions, C₁V₁ = C₂V₂ only holds when both concentrations use the same unit.
- Add acid to water, never water to acid.
- Everything is calculated in your browser.

### Common mistakes to avoid

- Mixing w/w, w/v and v/v percentages.
- Forgetting that pH is logarithmic – one unit is a tenfold change in [H⁺].
- Using pKa where Ka is asked for (or the other way round).

## Worked example

Inputs — Calculate the pH from: Concentration of an acid; Acid: Acetic acid – pKa 4.76; Concentration: 0.1 mol/L (M).

Results — **pH = 2.881**; Character = Strongly acidic; [H⁺] = 1.316 × 10⁻³ mol/L; Hydrogen-ion concentration [H⁺] = 0.001316; pOH = 11.119; Hydroxide-ion concentration [OH⁻] = 7.6011e-12.

## Key terms

- **pH**: Negative base-10 logarithm of the hydrogen-ion concentration.
- **pOH**: Negative logarithm of the hydroxide-ion concentration; pH + pOH = 14 at 25 °C.
- **Ka / pKa**: Acid dissociation constant and its negative logarithm; measures acid strength.
- **Kb / pKb**: Base dissociation constant and its negative logarithm.
- **Kw**: Ion product of water, 1.0 × 10⁻¹⁴ at 25 °C.

## FAQ

**How do I calculate pH from concentration?**
For a strong monoprotic acid, pH = −log₁₀(c): 0.01 M HCl has pH 2. For a weak acid, solve Ka = x²/(c − x) for x = [H⁺] first – 0.1 M acetic acid gives pH 2.88.

**What is the pH of 0.1 M formic acid?**
With Ka = 1.78 × 10⁻⁴, [H⁺] = 4.13 × 10⁻³ M and the pH is 2.38.

**What is the pH of 0.1 M ammonia?**
Kb = 1.78 × 10⁻⁵ gives [OH⁻] = 1.32 × 10⁻³ M, pOH 2.88 and pH 11.12.

**Why is the pH of 10⁻⁸ M HCl not 8?**
An acid cannot make water basic. Water itself supplies 10⁻⁷ M H⁺, and the exact charge balance gives pH 6.98 – this calculator includes that.

**Does pH + pOH always equal 14?**
Only at 25 °C, where Kw = 1.0 × 10⁻¹⁴. At 37 °C, pKw is about 13.6, so neutral water has pH 6.8.

**Can pH be negative?**
Yes – concentrated strong acids above 1 M have pH below 0, although at such concentrations activity effects make the simple formula approximate.

**What is the difference between Ka and pKa?**
pKa = −log₁₀ Ka. A smaller pKa means a stronger acid; each unit lower is ten times stronger.

**How accurate is the result for polyprotic acids?**
The first dissociation dominates for most of them, so the pH is usually within a few hundredths. Sulfuric acid's significant second proton (pKa₂ 1.99) is included.

**What formula does the pH Calculator use?**
pH = −log₁₀[H⁺]  ·  pOH = −log₁₀[OH⁻]  ·  pH + pOH = 14 (25 °C).


## Sources

- [OpenStax – Chemistry 2e](https://openstax.org/details/books/chemistry-2e)
- [IUPAC Compendium of Chemical Terminology (Gold Book)](https://goldbook.iupac.org/)
- [LibreTexts Chemistry](https://chem.libretexts.org/)