How to use the ph calculator
- Select Concentration to pH or pH to concentration from the input mode dropdown.
- Enter the hydrogen ion concentration in mol/L, or the pH value you want to convert.
- Read the calculated pH, pOH, ion concentrations, and acidity classification below.
- Change the input mode to perform the reverse calculation at any time.
How the pH formula works
The pH of a solution is defined as the negative base-10 logarithm of the hydrogen ion activity. For dilute solutions, activity is approximated by molar concentration:
pH = -log10([H+])
To go the other direction, raise 10 to the power of negative pH:
[H+] = 10-pH
For example, if the hydrogen ion concentration is 0.001 mol/L (which is 10-3), then pH = -log10(10-3) = 3. This ph calculator applies these formulas directly so you do not need to handle the logarithms yourself.
The relationship between pH and pOH
In aqueous solutions at 25 degrees Celsius, water undergoes autoionization described by the equilibrium constant Kw:
Kw = [H+][OH-] = 1.0 x 10-14
Taking the negative logarithm of both sides gives the familiar relationship:
pH + pOH = 14
This means that once you know the pH, you immediately know the pOH and therefore the hydroxide ion concentration. The pH calculator above uses this constant to display all four values together. Keep in mind that Kw changes with temperature: at 37 degrees Celsius it is approximately 2.4 x 10-14, so neutral pH shifts slightly below 7.
Common pH reference values
The table below lists typical pH values for everyday substances. Because the scale is logarithmic, each whole number step represents a tenfold change in hydrogen ion concentration.
| Substance | Approximate pH | Classification |
|---|---|---|
| Battery acid | 0.5 | Strongly acidic |
| Gastric acid | 1.5 | Strongly acidic |
| Lemon juice | 2.0 | Strongly acidic |
| Vinegar | 3.0 | Acidic |
| Coffee | 5.0 | Weakly acidic |
| Pure water | 7.0 | Neutral |
| Blood | 7.4 | Weakly basic |
| Baking soda solution | 8.3 | Basic |
| Soapy water | 10.0 | Basic |
| Household bleach | 12.5 | Strongly basic |
Can pH be negative or above 14?
A common misconception is that the pH scale is strictly limited to 0 through 14. In reality, those boundaries are just convenient markers for dilute aqueous solutions. A 10 M solution of hydrochloric acid has a hydrogen ion concentration of roughly 10 mol/L, giving a pH of about -1. Likewise, concentrated sodium hydroxide can push pH well above 14.
At extreme concentrations, the simple formula pH = -log([H+]) becomes less accurate because ion activity deviates significantly from molar concentration. Activity coefficients must be applied for precise work. However, for most laboratory and educational purposes, the standard formula used by this ph calculator remains appropriate.
Why pH matters in practice
Measuring and controlling hydrogen ion concentration is essential across many fields. In medicine, blood pH is tightly regulated between 7.35 and 7.45; deviations indicate acidosis or alkalosis, which can be life-threatening. In agriculture, soil pH determines nutrient availability for crops, and farmers use lime or sulfur to adjust it. Swimming pool operators test pH daily because values outside 7.2 to 7.8 cause skin irritation and reduce chlorine effectiveness.
In chemistry labs, buffer solutions resist changes in pH when small amounts of acid or base are added. Calculating the required ratio of conjugate acid to base relies on the Henderson-Hasselbalch equation, which itself is derived from the same logarithmic principles this tool uses.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Pure water | Input mode: conc; Hydrogen ion concentration (mol/L): 1e-7 | pH: 7; pOH: 7; [OH⁻]: 1e-7 mol/L; Classification: Neutral |
| Stomach acid | Input mode: ph; pH value: 1.5 | [H⁺]: 0.0316227766017 mol/L; pOH: 12.5; [OH⁻]: 3.16227766017e-13 mol/L; Classification: Strongly acidic |
| Blood plasma | Input mode: ph; pH value: 7.4 | [H⁺]: 3.98107170553e-8 mol/L; pOH: 6.6; [OH⁻]: 2.51188643151e-7 mol/L; Classification: Neutral |
| Household bleach | Input mode: ph; pH value: 12.5 | [H⁺]: 3.16227766017e-13 mol/L; pOH: 1.5; [OH⁻]: 0.0316227766017 mol/L; Classification: Strongly basic |
FAQ
What does pH stand for?
The term comes from the French 'pouvoir hydrogene,' meaning hydrogen power. It refers to the negative logarithm of the hydrogen ion concentration in a solution.
Is a pH of 7 always neutral?
At 25 degrees Celsius, pure water has a pH of exactly 7, which is neutral. At higher temperatures the autoionization constant increases, so neutral pH drops slightly below 7.
How do I convert pH back to concentration?
Use the formula [H+] = 10^(-pH). For example, a pH of 4 gives a hydrogen ion concentration of 10^(-4) or 0.0001 mol/L. This ph calculator performs that conversion automatically.
Does this tool store my data?
No. All calculations run entirely in your browser. No values are sent to any server, and nothing is stored after you close the page.
What is the difference between pH and pOH?
pH measures hydrogen ion concentration while pOH measures hydroxide ion concentration. At 25 degrees Celsius they always add up to 14.
Why is the pH scale logarithmic?
Hydrogen ion concentrations in nature span many orders of magnitude, from over 1 mol/L in strong acids to 10^(-14) mol/L in strong bases. A logarithmic scale compresses this enormous range into manageable numbers.