pH Calculator (Acid/Base Test)

Enter the hydrogen ion concentration [H+] or a pH value to calculate pH, pOH, [H+], [OH-], and determine whether a solution is acidic, neutral, or basic.

pH Scale Reference

pH Range Common Example
0–2 Gastric acid, lemon juice
3–4 Vinegar, soda (carbonated drinks)
5–6 Coffee, rainwater
7 (Neutral) Pure water
8–9 Seawater, baking soda solution
10–11 Soapy water, ammonia solution
12–14 Bleach, sodium hydroxide solution

Usage Tips

  • Since pH is a logarithmic scale, a difference of just 1 in pH means a tenfold change in hydrogen ion concentration. A solution with pH 4 is 10 times more acidic than pH 5, and 100 times more acidic than pH 3.
  • This tool's calculations assume a temperature of 25°C (77°F). Because the ion product of water (Kw) changes with temperature, the relationship pH + pOH = 14 strictly holds only at 25°C.
  • [H⁺] concentration values are often very small, so it's easier to enter and check them using scientific notation (e.g., 1.0×10⁻⁷).
  • To find the approximate pH of everyday liquids, refer to the "pH Scale Reference" table below.

Frequently Asked Questions

pH (potential of Hydrogen) is a measure of the hydrogen ion concentration [H⁺] in an aqueous solution, defined as pH = -log₁₀[H⁺]. It ranges from 0 to 14, where 7 is neutral, values below 7 are acidic, and values above 7 are basic (alkaline) — the further from 7, the stronger the effect.

In an aqueous solution at 25°C, the relationship pH + pOH = 14 always holds (based on the ion product of water, Kw = 1.0×10⁻¹⁴). If you know the pH, you can find the pOH simply by subtracting it from 14, and vice versa.

Because pH is a logarithmic scale, a change of 1 in pH corresponds to a tenfold change in hydrogen ion concentration. For example, a solution with pH 3 has 100 times (10 squared) the hydrogen ion concentration of a solution with pH 5, making it that much more acidic.

In pure water, a small fraction of water molecules dissociate, producing equal concentrations of [H⁺] and [OH⁻] (1.0×10⁻⁷ mol/L each at 25°C). Calculating from this concentration gives pH = 7, which is defined as the neutral reference point where [H⁺] and [OH⁻] are balanced.
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Side Note — Where Does the Notation "pH" Come From?

The notation "pH" was devised in 1909 by the Danish chemist Søren Sørensen. It is said to derive from "power of Hydrogen," or from "potentia Hydrogenii" (Latin for "power of hydrogen"), reflecting the original Danish and French terminology. He worked at a laboratory that monitored the brewing quality of beer (the Carlsberg Laboratory), and the need for a simple index of acidity to control fermentation was the impetus behind the concept of pH.

The original definition of pH dealt directly with hydrogen ion concentration, but because such values are often impractically small (like 0.0000001), taking the logarithm compressed them into the more manageable range of roughly 0 to 14. This idea — using logarithms to tame values that differ by overwhelming orders of magnitude — is widely applied in other scientific fields as well, such as earthquake magnitude and sound loudness (decibels).

Today, pH test strips and pH meters (devices that calculate pH from an electrode's potential difference) are widely used, and the pH index continues to appear far beyond the chemistry lab — in water quality testing, food processing, pool water management, soil diagnostics, and many other everyday contexts.

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