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pH Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Based on NIST Chemistry WebBook and NIST SP 330. Last verified 14 June 2026.

pH is the measure of the hydrogen ion (H+) concentration in a solution, defined as pH = -log10([H+]). Because the scale is logarithmic, each unit of pH represents a 10-fold change in H+ concentration. The scale runs from 0 (most acidic) to 14 (most basic) at 25 degrees C, with 7 being neutral (pure water). The complementary scale, pOH, measures hydroxide ion (OH-) concentration: pOH = -log10([OH-]). At 25 degrees C, pH + pOH = 14, because the water ionisation constant Kw = [H+][OH-] = 1 x 10^-14. This calculator supports two modes: enter a hydrogen ion concentration in mol/L to compute pH, pOH, and [OH-]; or enter a pH value directly to compute pOH, [H+], and [OH-]. Both modes provide a classification of the solution from strongly acidic through neutral to strongly basic. A color-coded reference table of the full pH scale (0 to 14) with examples is included. All formulas follow NIST SP 330 and standard physical chemistry definitions.

pH: -- (--)

Enter a concentration or pH value below. Source: NIST Chemistry WebBook, as at 14 June 2026.

Enter hydrogen ion concentration in mol/L (e.g. 0.001 for pH 3)
pH --
pOH --
[H+] (mol/L) --
[OH-] (mol/L) --
Classification --

pH formulas

The core pH relationships at 25 degrees C, where Kw = 1.0 x 10-14:

pH = -log10([H+])
pOH = -log10([OH-])
pH + pOH = 14 (at 25 degrees C)
[H+] = 10^(-pH)
[OH-] = 10^(-pOH) = Kw / [H+]
Kw = [H+] x [OH-] = 1.0 x 10^-14 at 25 degrees C

Worked example: Mode A (from [H+])

  1. Given: [H+] = 0.001 mol/L = 1.0 x 10-3 mol/L
  2. pH = -log10(0.001) = -(-3) = 3.00
  3. pOH = 14 - 3 = 11.00
  4. [OH-] = 10-11 = 1.0 x 10-11 mol/L
  5. Classification: Acidic (pH 3 to 6)

Worked example: Mode B (from pH)

  1. Given: pH = 9.00
  2. pOH = 14 - 9 = 5.00
  3. [H+] = 10-9 = 1.0 x 10-9 mol/L
  4. [OH-] = 10-5 = 1.0 x 10-5 mol/L
  5. Classification: Basic (pH 8 to 11)

pH scale reference table

Values at 25 degrees C. [H+] shown as approximate power of 10. Kw = 1.0 x 10-14.

pH [H+] approx. Classification Example
0 10⁻⁰ Strongly acidic Battery acid
1 10⁻¹ Strongly acidic Stomach acid (gastric juice)
2 10⁻² Strongly acidic Lemon juice, vinegar
3 10⁻³ Acidic Orange juice
4 10⁻⁴ Acidic Tomato juice, acid rain
5 10⁻⁵ Acidic Coffee, beer
6 10⁻⁶ Weakly acidic Milk, saliva
7 10⁻⁷ Neutral Pure water (25°C)
8 10⁻⁸ Weakly basic Seawater, baking soda
9 10⁻⁹ Basic Baking soda solution
10 10⁻¹⁰ Basic Milk of magnesia
11 10⁻¹¹ Basic Ammonia solution
12 10⁻¹² Strongly basic Bleach, soapy water
13 10⁻¹³ Strongly basic Oven cleaner
14 10⁻¹⁴ Strongly basic Liquid drain cleaner (NaOH)

pH calculator: frequently asked questions

What is pH?

pH is a logarithmic scale that measures the concentration of hydrogen ions (H+) in a solution. It is defined as pH = -log10([H+]), where [H+] is the molar concentration of hydrogen ions in mol/L. The scale runs from 0 to 14 at standard conditions (25 degrees C). Pure water at 25 degrees C has a pH of exactly 7 (neutral). Values below 7 indicate acidic solutions; values above 7 indicate basic (alkaline) solutions. The scale is logarithmic: a solution at pH 4 has 10 times more H+ ions than one at pH 5.

What pH is neutral?

At 25 degrees C (298.15 K), pure water is neutral at pH 7.00. This is because at that temperature, the water self-ionisation constant Kw = [H+][OH-] = 1.0 x 10^-14, so in neutral water [H+] = [OH-] = 1.0 x 10^-7 mol/L, giving pH = -log10(10^-7) = 7. Note that at higher temperatures, Kw increases, so the neutral pH is slightly lower (for example, at 37 degrees C, neutral pH is approximately 6.81). The 7 value applies specifically at 25 degrees C.

What is the pH of common substances?

Some well-known pH values at approximately 25 degrees C: stomach acid (gastric acid) is approximately pH 1 to 2; lemon juice is approximately pH 2 to 3; coffee is approximately pH 4 to 5; pure water is pH 7; blood is approximately pH 7.35 to 7.45; baking soda (sodium bicarbonate solution) is approximately pH 8 to 9; milk of magnesia is approximately pH 10; bleach (sodium hypochlorite solution) is approximately pH 12 to 13; oven cleaner (NaOH solution) can reach pH 13 to 14. These are approximate and vary with concentration and temperature.

What does the logarithmic pH scale mean?

Because pH = -log10([H+]), each unit change in pH represents a 10-fold change in hydrogen ion concentration. A solution at pH 3 has 10 times more H+ than a solution at pH 4, and 100 times more than pH 5. This means small pH changes can represent large changes in acidity. For example, going from pH 7 to pH 5 increases H+ concentration by a factor of 100. This logarithmic compression makes the pH scale useful for describing a very wide range of [H+] values (from roughly 10 mol/L at pH -1 to 10^-15 mol/L at pH 15).

What is a buffer solution?

A buffer solution resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal concentrations. The Henderson-Hasselbalch equation describes the pH of a buffer: pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant of the weak acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. Buffers are critical in biological systems (blood is buffered near pH 7.4 by bicarbonate and carbonic acid) and in laboratory and industrial processes.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. For educational use only.