pH Calculator

The pH Calculator converts between pH, pOH, hydrogen ion concentration ([H+]), and hydroxide ion concentration ([OH-]) — give it any one of the four at 25°C and it derives the other three, plus an acidity/alkalinity classification.

The relationships

At 25°C, water's autoionization constant is Kw = 1.0×10⁻¹⁴ mol²/L², which links all four quantities:

pH  = −log10([H+])        [H+]  = 10^(−pH)
pOH = −log10([OH-])       [OH-] = 10^(−pOH)
pH + pOH = 14             [H+] × [OH-] = Kw

Unlike a calculator that solves for one unknown given two knowns, pH only needs one input — the other three follow immediately from these two identities.

How to use it

1. Pick which quantity you're starting from under Given Quantity: pH, pOH, [H+], or [OH-].
2. Enter that value. Ion concentrations accept scientific notation (1e-7 works the same as 0.0000001).
3. The calculator returns all four values plus a classification — Strongly acidic, Acidic, Weakly acidic, Neutral, Weakly basic, Basic, or Strongly basic — along with a step-by-step derivation.

Example

Household ammonia measures around pOH 3:

pH = 14 − 3 = 11
[OH-] = 10^(−3) = 0.001 mol/L
[H+] = Kw ÷ [OH-] = 1.0×10⁻¹⁴ ÷ 0.001 = 1.0×10⁻¹¹ mol/L

That's a pH of 11 — Basic.

Why does pH have a Kw of 1.0×10⁻¹⁴ specifically?

That's water's ion-product constant at 25°C. It rises with temperature (to roughly 1.0×10⁻¹³ near 60°C), which is why pure water's "neutral" point isn't always exactly pH 7 at other temperatures. This calculator assumes standard 25°C conditions, as does virtually every pH reference table.

Can pH go below 0 or above 14?

Yes, mathematically — very concentrated strong acids or bases can push pH outside the familiar 0–14 range. This calculator computes the correct value either way and flags it as "Superacidic" or "Superbasic" rather than clamping it.

Privacy

All calculations run entirely in your browser. Your figures are never uploaded or stored.