pH Calculator
Acid, base, or neutral — with the numbers.
pH Calculator
Convert between pH, pOH, hydrogen-ion, and hydroxide-ion concentration, with the acidity clearly labelled.
A number from -1 to 15. 7 is neutral at 25 °C.
The conversion assumes 25 °C, where the ion product of water is 1.0 × 10⁻¹⁴. At other temperatures, neutral pH is not exactly 7.
7pH
Neutral
- pOH
- 7
- [H⁺]
- 1.00 × 10−⁷ mol/L
- [OH⁻]
- 1.00 × 10−⁷ mol/L
Embed this calculator
Put this calculator on your own site with the snippet below. The frame contains the tool only, reports its own height, and carries a visible link back to the full page. Embedding is free; keep the attribution link as it is, and do not wrap the frame in a way that hides it.
Preparing the embed code for this page...
What This Calculator Does
The pH Calculator converts between pH, pOH, hydrogen-ion concentration, and hydroxide-ion concentration. Enter whichever value you have and it returns the rest, with the solution labelled acidic, neutral, or basic.
How It Works
pH is the negative base-10 logarithm of hydrogen-ion activity, approximated here by concentration in moles per litre. pOH is 14 minus pH at 25 °C, where the ion product of water is 1.0 × 10⁻¹⁴. Hydrogen and hydroxide concentrations follow from the logarithmic definitions and always multiply to that ion product.
Example
A solution with [H⁺] = 1 × 10⁻³ mol/L has pH 3, so it is acidic. pOH is 11 and [OH⁻] is 1 × 10⁻¹¹ mol/L. A pH of 10 gives pOH 4 and [H⁺] = 1 × 10⁻¹⁰ mol/L, which is basic.
Formula
pH = −log₁₀[H⁺]pOH = 14 − pHValid in water at 25 °C.
[H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴Reading the Results
pH
The negative log of hydrogen-ion concentration. Each unit is a tenfold change, so pH 4 is ten times more acidic than pH 5.
[H⁺] and [OH⁻]
Concentrations in moles per litre. They are reciprocals through the ion product of water, which is why one rises as the other falls.
Sources and References
IUPAC — Compendium of Chemical Terminology (Gold Book)
Definition of pH as negative logarithm of hydrogen-ion activity
Strictly, pH measures activity rather than concentration; the two diverge in concentrated solutions.
USGS Water Science School — pH
pH scale and typical natural waters
A practical reference for interpreting environmental and drinking-water pH values.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Why is pH 7 neutral?
At 25 °C pure water has equal hydrogen and hydroxide concentrations of 1 × 10⁻⁷ mol/L, giving pH 7. Neutral pH shifts with temperature because the ion product of water changes.
Can pH be negative or above 14?
Yes, for concentrated strong acids or bases. A 2 M strong acid has a nominal pH near −0.3. The scale is not bounded, but the simple concentration approximation breaks down badly at those strengths.
What is the difference between pH and pOH?
They measure opposite ions. pH tracks hydrogen ions, pOH tracks hydroxide ions, and in water at 25 °C they add to 14. Reporting pH alone is conventional.
Does temperature matter?
Yes. The ion product of water rises with temperature, so the pH of neutrality falls below 7 as water heats. Buffers also change pH with temperature, which is why calibrations specify it.