pH & pOH Calculator
Find pH, pOH, [H⁺], and [OH⁻] from any one value, and see where a solution falls on the acid-base scale.
Calculator verified • Last updated: August 2026
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The pH Scale
The marker shows your current pH on the 0-14 scale. Reference points: battery acid (~0), lemon juice (~2), black coffee (~5), pure water (7), baking soda (~9), ammonia (~11), and bleach (~13).
pH & pOH Explained
pH measures how acidic or basic a solution is, based on the concentration of hydrogen ions dissolved in it.
pH: the measure of acidity, unitless, typically 0–14.
pOH: the measure of basicity, unitless, typically 0–14.
[H⁺]: hydrogen ion concentration, in moles per liter (mol/L).
[OH⁻]: hydroxide ion concentration, in moles per liter (mol/L).
At 25°C, pH and pOH always sum to 14, and the two ion concentrations are related by . A pH of 7 is neutral (pure water), below 7 is acidic, and above 7 is basic (alkaline).
Worked Example: Pure Water
Using the calculator's default — a hydrogen ion concentration of mol/L, characteristic of pure water — pH works out to exactly 7.00, the definition of neutral. Since pH + pOH = 14, pOH is also 7.00, and the hydroxide concentration [OH⁻] is likewise mol/L — pure water has exactly equal concentrations of both ions, which is precisely what makes it neutral rather than acidic or basic.
Why a Small pH Change Means a Big Concentration Change
Because pH is a logarithmic (base-10) scale, each single unit of pH change represents a tenfold change in hydrogen ion concentration — a solution at pH 4 isn't twice as acidic as one at pH 5, it's ten times more acidic. This is why seemingly small pH differences in things like ocean acidification or blood pH can represent dramatic underlying chemical changes, and why buffer solutions that keep pH within a narrow range are so important in biology.
A Brief History of the pH Scale
Danish chemist Søren Sørensen introduced the pH concept in 1909 while working at the Carlsberg Laboratory, developing it specifically to standardize measurements in beer brewing quality control. The "p" in pH is generally understood to stand for "power" (as in power of ten), reflecting the logarithmic definition, though Sørensen's own notation and exact intended meaning have been debated by historians of chemistry ever since.
Common pH Mistakes
Forgetting the negative sign in the pH formula is a common error — since [H⁺] is always less than 1 mol/L in practical cases, its logarithm is negative, and dropping the minus sign gives a nonsensical negative pH for an ordinary solution. Assuming pH and concentration scale linearly (treating pH 3 as "three times more acidic" than pH 9, rather than a million times more) is another frequent misconception given the logarithmic scale. Confusing pH with pOH — reporting one when the other was asked for — is a third common mix-up, especially since they're numerically related but represent different ions.
Acid-Base Chemistry Terms You Should Know
pH — a logarithmic measure of hydrogen ion concentration, indicating acidity.
pOH — a logarithmic measure of hydroxide ion concentration, indicating basicity.
Kw (Water's Ion Product) — the constant relating [H⁺] and [OH⁻] in water at 25°C.
Buffer Solution — a solution that resists pH change when small amounts of acid or base are added.
The pH + pOH = 14 relationship assumes 25°C; Kw (and therefore this relationship) shifts slightly at other temperatures.
Frequently Asked Questions
Why is the pH scale logarithmic instead of linear?
Because hydrogen ion concentrations in real solutions span an enormous range — from about 1 mol/L in strong acids down to 10⁻¹⁴ mol/L in strong bases — a linear scale would be wildly impractical. Taking the negative logarithm compresses that huge range into the convenient 0-14 scale, but it also means each whole-number step represents a tenfold change in acidity, not a fixed amount.
Why does pH + pOH always equal 14?
This comes from water's own equilibrium: [H+][OH-] = 10⁻¹⁴ at 25°C, a constant called Kw. Taking the negative logarithm of both sides turns that multiplication into addition — pH + pOH = 14 — as a direct mathematical consequence. This relationship technically only holds at 25°C, since Kw itself changes slightly with temperature.
Can pH go below 0 or above 14?
Yes — the 0-14 range is just the typical range for dilute aqueous solutions, not a hard mathematical limit. Very concentrated strong acids can have a negative pH, and very concentrated strong bases can exceed pH 14; the underlying formula, pH = -log[H+], works for any positive concentration, the 0-14 convention is just what's commonly encountered.