pH Calculator

Reading acidity in water

Read pH and ion concentration in one view

This pH calculator starts from either a pH value or hydrogen ion concentration [H⁺], then shows pOH, hydroxide ion concentration [OH⁻], and the acid/base range. [H⁺] means the hydrogen ion amount treated as being in one liter of solution, while pOH reads the same solution from the [OH⁻] side.

Start with either pH or concentration

The tool updates as you type. Do not look for a separate calculate button; enter the value you already know in the pH field, concentration field, or slider first.

If you know the pH value enter a number from 0 to 14 in the pH field or move the slider.
If you know [H⁺] concentration enter a mol/L value in the concentration field. The tool reads forms such as 1e-7, 1×10^-7, and 1×10⁻⁷.
If you need a reference point load lemon juice, vinegar, milk, pure water, seawater, or soapy water from the sample buttons.
If you need to move the result use Copy pH or the copy buttons for pH, pOH, [H⁺], and [OH⁻].

pH and [H⁺] describe the same solution from different angles

A lower pH means stronger acidity. [H⁺] is the concentration used in the calculation, so when [H⁺] changes by a factor of 10, the pH moves by 1 because the scale is logarithmic.

pH value

This is the main input. The result card displays it to two decimals, and 7.00 is treated as near neutral.

[H⁺] concentration

Enter hydrogen ion concentration in M, meaning mol/L. Very small values are easier to type in forms such as 1e-7.

Slider and samples

Use these when you want to check the direction of the acid/base range. Samples are learning references, not replacements for measuring a real solution.

At 25°C, pH and pOH add up to 14

The calculator uses the common 25°C aqueous-solution relation. Under that condition, the ion product of water is treated as Kw = [H⁺]×[OH⁻] = 1.0×10⁻¹⁴, which gives pH + pOH = 14.

From pH to concentration[H⁺] = 10^-pH

From concentration to pHpH = -log₁₀([H⁺])

pOHpOH = 14 - pH

[OH⁻][OH⁻] = Kw / [H⁺]

IUPAC pH definitionUse this when you need to confirm that pH is tied to hydrogen ion activity.pH and pOH relationUse this to review the 25°C relationship between [H₃O⁺], pOH, and Kw.

Two values are enough to see the flow

If you enter pH 3.00, the tool shows [H⁺] as 1.00×10⁻³ M and pOH as 11.00. It classifies that state as a strong acid.

pH 3.00 input[H⁺] 1.00×10⁻³ M · pOH 11.00 · [OH⁻] 1.00×10⁻¹¹ MThe solution leans strongly acidic.

[H⁺] 1×10⁻⁵ M inputpH 5.00 · pOH 9.00 · [OH⁻] 1.00×10⁻⁹ MThis reads as a weak acid range.

Keep examples and real samples separate

Lemon juice and soapy water examples are there to teach the direction of the pH scale. Real sample pH can change with temperature, concentration, additives, and the measuring device.

Read the range together with the limits

The first result card gives the current pH and acid/base range. The pOH, [H⁺], and [OH⁻] values below it are the same calculation expressed in different forms, so copy only the field your problem or report needs.

The tool calls pH 6–8 near neutral, but the exact neutral point can move with temperature and solution conditions.
In concentrated or salty solutions, activity can matter more than concentration.
For lab reporting, confirm the value with a calibrated pH meter or test strip instead of using the calculation alone.
When handling strong acids or bases, safety gear and dilution order come before the calculation.

Questions that often come up in pH calculations

Why does this tool use 25°C as the basis?

Because the familiar pH + pOH = 14 relation comes from the 25°C ion product of water, Kw = 1.0×10⁻¹⁴. At a different temperature, Kw and neutral pH can shift.

Why does [H⁺] 1×10⁻⁵ M give pH 5.00?

pH is the negative base-10 logarithm of hydrogen ion concentration. Since 1×10⁻⁵ M is 10 to the power of -5, -log₁₀(10⁻⁵) = 5.

Can pH be below 0 or above 14?

Yes, it is possible in theory and in some concentrated solutions. This tool keeps the default screen at pH 0–14 because that is the common learning and comparison range.

Can I copy this value straight into a lab report?

It is fine for conversion practice or rough interpretation. For an actual sample report, include the measured value with temperature, calibration, and sample conditions.