dopecalc

pH Calculator

Calculate pH from hydrogen ion concentration [H+] or determine [H+] from pH. Also shows pOH = 14 − pH.

Calculate pH from hydrogen ion concentration [H+], or find [H+] from pH. Results show both pH and pOH.

Scientific notation (1e-7) or decimal

Enter pH to reverse-calculate [H+]

About this Calculator

Calculate pH from hydrogen ion concentration [H+] or determine [H+] from pH. Also shows pOH = 14 − pH.

Formula & Calculations

Formula

pH = −log₁₀[H+], [H+] = 10^(−pH), pOH = 14 − pH
Where:
  • pH=Measure of acidity (0–14 scale, 7 = neutral)
  • [H+]=Hydrogen ion concentration in mol/L
  • pOH=Measure of basicity, pOH = 14 − pH at 25°C

Assumptions

  • Calculations assume aqueous solution at 25°C.
  • Kw = [H+][OH−] = 1 × 10⁻¹⁴ at 25°C.
  • Values outside the 0–14 range are mathematically possible but uncommon.

Calculation Examples

Example 1

Inputs:[H+] = 1 × 10⁻⁷ M
Result:pH = 7 (neutral), pOH = 7

pH = −log(10⁻⁷) = 7. At 25°C, pOH = 14 − 7 = 7.

Example 2

Inputs:[H+] = 0.001 M
Result:pH = 3 (acidic), pOH = 11

pH = −log(0.001) = 3.

Example 3

Inputs:pH = 4.5
Result:[H+] = 3.162 × 10⁻⁵ M

[H+] = 10⁻⁴·⁵ = 3.162 × 10⁻⁵ mol/L.

Frequently Asked Questions

What does pH measure?

pH measures the acidity or basicity of an aqueous solution on a logarithmic scale from 0 (very acidic) to 14 (very basic). A pH of 7 is neutral. Each unit change represents a 10-fold change in H+ concentration.

What is pOH and how does it relate to pH?

pOH measures hydroxide ion concentration. At 25°C, pH + pOH = 14. A solution with pH 3 has pOH 11, meaning it's acidic. pH and pOH are complementary.