pH Calculator

Calculate pH from hydrogen ion concentration, or convert pH to [H⁺], [OH⁻], and pOH. Use the tabs below to switch between calculation modes.

pH Value

pH scale: 0 (most acidic) to 14 (most basic). pH 7 = neutral at 25 °C.

Quick examples:

What Is pH?

pH is a logarithmic scale used to measure the acidity or basicity (alkalinity) of an aqueous solution. The term stands for "potential of hydrogen" — specifically, it measures the negative base-10 logarithm of the hydrogen ion (H⁺) concentration:

pH = −log₁₀([H⁺])

Because the scale is logarithmic, each one-unit change in pH represents a tenfold change in hydrogen ion concentration. A solution with pH 3 has ten times more H⁺ ions than a solution with pH 4, and one hundred times more than pH 5.

The pH Scale at a Glance

pH 0–6.9: Acidic. Concentrations of H⁺ exceed OH⁻. Common examples: battery acid (pH ~0), gastric acid (~2), lemon juice (~2.3), vinegar (~2.9), coffee (~5).
pH 7.0: Neutral. H⁺ and OH⁻ concentrations are equal at 25 °C (1×10⁻⁷ mol/L each). Pure water is the classic neutral reference.
pH 7.1–14: Basic (alkaline). OH⁻ exceeds H⁺. Common examples: baking soda (~8.3), seawater (~8.1), milk of magnesia (~10.5), bleach (~12.5), NaOH 1 M (pH 14).

pH and pOH: The Relationship

At 25 °C, the product of hydrogen ion and hydroxide ion concentrations is always the water dissociation constant K_w = 1×10⁻¹⁴. This gives the fundamental relationship:

pH + pOH = 14

So if you know the pH, pOH = 14 − pH, and vice versa. Similarly, [OH⁻] = K_w / [H⁺] = 10⁻¹⁴ / [H⁺].

How to Use This Calculator

pH → Concentration: Enter any pH value between 0 and 14. The calculator returns [H⁺], [OH⁻], and pOH, plus an acid/base classification.
Concentration → pH: Enter the hydrogen ion concentration in mol/L (for example, 0.0001 for a 10⁻⁴ M solution). The calculator computes pH using pH = −log₁₀([H⁺]).

pH in Everyday Science

pH measurements are critical in chemistry, biology, medicine, environmental science, and agriculture. Blood must stay in the narrow range of pH 7.35–7.45 for normal cellular function. Soil pH affects nutrient availability for plants. Aquatic ecosystems are sensitive to acid rain, which can lower lake pH below 5 — lethal to many fish species.

For chemistry work involving chemical formulas and molar quantities, our Molar Mass Calculator can help you determine the molar mass of acids and bases. When studying gas-phase reactions, the Ideal Gas Law Calculator (PV = nRT) is a related tool for pressure and volume calculations.

Frequently Asked Questions

Can pH be negative or above 14?

Yes, though it's uncommon in practice. Extremely concentrated acids (e.g., 10 M HCl) can have a negative pH, and very concentrated bases can exceed pH 14. The 0–14 range applies to typical dilute aqueous solutions at 25 °C.

Why does the neutral pH change with temperature?

The water dissociation constant K_w increases with temperature, so pH 7 is only neutral at exactly 25 °C. At 37 °C (body temperature), neutral pH is closer to 6.81. This calculator uses the 25 °C standard.

What is the difference between a strong acid and a weak acid in terms of pH?

Strong acids (like HCl and H₂SO₄) dissociate completely in water, so for a 0.1 M strong acid, [H⁺] ≈ 0.1 mol/L giving pH ≈ 1. Weak acids (like acetic acid) only partially dissociate, producing a higher pH for the same molar concentration. Calculating the pH of weak acids requires the Henderson-Hasselbalch equation and the acid dissociation constant Ka.

How do I read scientific notation concentrations?

A concentration like 3.16×10⁻⁸ mol/L means 3.16 divided by 10⁸, or 0.0000000316 mol/L. For pure water at pH 7, [H⁺] = 1×10⁻⁷ mol/L = 0.0000001 mol/L. This calculator displays concentrations in scientific notation for readability.

What are some common pH values to know?

Battery acid: 0–1; gastric acid: 1.5–2; lemon juice: 2–2.5; vinegar: 2.5–3; orange juice: 3–4; tomato juice: ~4; coffee: 4.5–5; pure water: 7; seawater: ~8.1; baking soda: ~8.3; antacids: ~10; ammonia: ~11; bleach: ~12–12.5; drain cleaner: ~14.