pH Calculator — pH, pOH, and Hydrogen Ion Concentration

pH is one of those measurements that looks simple on a 0–14 scale but hides a logarithmic relationship that catches people off guard. A pH of 3 isn't "twice as acidic" as pH 6 — it's a thousand times more acidic. The CalcHub pH Calculator handles the conversions between pH, pOH, and ion concentrations so you don't have to wrestle with negative logarithms by hand.

The Relationships

Everything flows from these three equations:

pH = −log₁₀[H⁺]

pOH = −log₁₀[OH⁻]

pH + pOH = 14 (at 25°C)

So if you know the hydrogen ion concentration, you can find pH. If you know pH, you can find pOH. The calculator handles all directions.

The pH Scale at a Glance

pH Range	Classification	Examples
0 – 2	Strongly acidic	Battery acid, stomach acid
2 – 4	Weakly acidic	Lemon juice, vinegar
4 – 6	Mildly acidic	Coffee, rain
7	Neutral	Pure water
8 – 10	Mildly basic	Seawater, baking soda
10 – 12	Weakly basic	Milk of magnesia
12 – 14	Strongly basic	Bleach, drain cleaner

How to Use the Calculator

1. Open the pH Calculator.
2. Choose your input type: pH, pOH, [H⁺], or [OH⁻].
3. Enter the value.
4. The tool returns all four related values simultaneously.

If you're working with a weak acid or base and need to find pH from molarity, you'll need the Ka or Kb — look for the acid dissociation mode in the tool.

Worked Examples

From concentration to pH: [H⁺] = 0.001 mol/L = 1 × 10⁻³ M pH = −log(10⁻³) = 3 From pH to [H⁺]: pH = 9.5 [H⁺] = 10⁻⁹·⁵ = 3.16 × 10⁻¹⁰ mol/L Finding pOH: pH = 4.2 pOH = 14 − 4.2 = 9.8 [OH⁻] = 10⁻⁹·⁸ = 1.58 × 10⁻¹⁰ mol/L

Tips

• The 14 rule only applies at 25°C. At higher temperatures, the neutral pH shifts slightly below 7 (water ionizes more). For most classroom problems, assume 25°C.
• pH below 0 is possible. Extremely concentrated strong acids can have negative pH values. The scale doesn't technically stop at 0.
• Buffers resist pH change. A buffer solution containing a weak acid and its conjugate base resists changes in pH when small amounts of acid or base are added.

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Why is pH a logarithmic scale?

Because [H⁺] concentrations in real solutions span an enormous range — from 1 mol/L in strong acids to 10⁻¹⁴ mol/L in strong bases. A logarithmic scale compresses this 10-trillion-fold range into a tidy 0–14 number that's easy to compare and communicate.

Can I calculate pH for a weak acid with this tool?

For strong acids, pH ≈ −log[acid] since they fully dissociate. For weak acids you need to account for partial dissociation using Ka. The tool has a weak acid mode where you enter concentration and Ka to get the actual [H⁺] and pH.

What does neutral pH actually mean?

Neutral means [H⁺] = [OH⁻]. In pure water at 25°C, both equal 10⁻⁷ mol/L, giving pH = 7. Neutral doesn't mean "safe" — it just means equal concentrations of both ions.

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Related calculators: Molarity Calculator · Concentration Converter · Stoichiometry Calculator