pH Calculator
Calculate pH from hydrogen ion concentration
pH Calculator tool
Use scientific notation (e.g., 1e-7)
Results
pH
0.00
pOH
0.00
[H⁺] (M)
0
[OH⁻] (M)
0
pH Calculator: key facts
- What it does
- Calculate pH from hydrogen ion concentration
- Category
- Science Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
Understanding the pH Calculator
pH measures how acidic or basic a solution is, calculated as the negative logarithm of hydrogen ion concentration (pH = -log₁₀[H⁺]). The pH Calculator converts between pH, pOH, hydrogen ion concentration [H⁺], and hydroxide ion concentration [OH⁻], supporting scientific notation for very small concentrations typical in chemistry.
Students solve acid-base chemistry problems, chemists analyze solution properties, and environmental scientists monitor water quality. The calculator also classifies solutions as strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic based on the pH value.
Because calculations run entirely in your browser using JavaScript, you can solve chemistry problems without any data leaving your device. The tool handles the logarithmic calculations automatically and displays results in both decimal and scientific notation.
Getting a result
- Select what you want to calculate from the dropdown: [H⁺] concentration, [OH⁻] concentration, pH, or pOH.
- Enter the known value in the input field using decimal or scientific notation.
- Click Calculate to solve for all related values automatically.
- View the results showing pH, pOH, both ion concentrations, and the acid/base classification.
- Copy any result to your clipboard using the copy buttons next to each value.
A logarithmic scale, and the ion-product relationship
pH is the negative base-ten logarithm of hydrogen ion concentration, which is why the scale compresses an enormous range into a handful of numbers. The logarithm also means each whole unit is a factor of ten: a solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4, and a hundred times that of pH 5.
The tool converts in either direction — concentration to pH, or pH back to concentration — and simultaneously reports the hydroxide side. That relationship comes from the ion product of water: at 25°C, pH and pOH always sum to 14, so establishing one fixes the other.
Concentrations are entered in scientific notation, which is unavoidable given the magnitudes involved. Neutral water at 25°C has a hydrogen ion concentration of 1×10⁻⁷ mol/L, giving pH 7 — the origin of neutrality sitting at that particular number rather than at zero.
- A hydrogen ion concentration of 1×10⁻⁷ mol/L gives pH 7.00 and pOH 7.00 — neutral.
- pH 3 corresponds to 1.00×10⁻³ mol/L of hydrogen ions and a pOH of 11.
- Lemon juice near pH 2 is roughly a hundred thousand times more acidic in hydrogen ion terms than neutral water.
Reasons to use it here
- pH calculations execute entirely in your browser using JavaScript—no data is transmitted to any server, ensuring your chemistry work remains private.
- Calculates all related values (pH, pOH, [H⁺], [OH⁻]) from a single input, saving time compared to manual logarithmic calculations.
- Handles scientific notation automatically for the very small concentrations typical in chemistry (10⁻⁷ M, 10⁻¹⁴ M, etc.).
- Provides acid/base classification (strongly acidic to strongly basic) to help interpret the practical meaning of pH values in real-world applications.
Temperature, activity, and the limits of the scale
The pH plus pOH equals 14 relationship holds at 25°C and nowhere else. The ion product of water is temperature-dependent, so at 50°C neutral water sits nearer pH 6.6 — still neutral, because hydrogen and hydroxide remain equal, but no longer at 7. Any pH measurement quoted without a temperature is incomplete.
Strictly, pH is defined in terms of hydrogen ion activity rather than concentration, and the two diverge as solutions become concentrated. For dilute solutions the approximation used here is good; in concentrated ones, ionic strength matters and the calculated figure drifts from what a meter would read.
The 0 to 14 range is a convention rather than a limit — strongly concentrated acids and bases can produce values outside it. And converting a strong acid's molarity straight to pH assumes complete dissociation, which holds for strong acids but not for weak ones, where the acid dissociation constant is needed. Handling concentrated acids or bases requires proper protective equipment and training.
Frequently Asked Questions
What is the pH formula?
The pH formula is pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in moles per liter (M). pH measures how acidic or basic a solution is on a logarithmic scale from 0 to 14.
What is the pH scale?
The pH scale ranges from 0 to 14. A pH of 7 is neutral (pure water), below 7 is acidic (lower is more acidic), and above 7 is basic (higher is more basic). Each unit represents a tenfold change in hydrogen ion concentration.
What is the relationship between pH and pOH?
pH and pOH are related by the equation pH + pOH = 14 at 25°C. pOH measures the hydroxide ion concentration using pOH = -log₁₀[OH⁻]. This relationship comes from the water autoionization constant Kw = [H⁺][OH⁻] = 10⁻¹⁴.
Can I calculate ion concentration from pH?
Yes, you can calculate hydrogen ion concentration from pH using [H⁺] = 10^(-pH). The calculator also automatically calculates the complementary [OH⁻] concentration using the water autoionization constant.
Why is the pH scale logarithmic?
The pH scale is logarithmic because hydrogen ion concentrations in solutions vary over many orders of magnitude. A logarithmic scale compresses this range into manageable numbers (0-14) while preserving the relative differences between solutions.
