Calculate H+, OH-, pH, and pOH
Convert between hydrogen ion concentration, hydroxide ion concentration, pH, and pOH instantly. This calculator assumes standard aqueous chemistry at 25 degrees Celsius, where pH + pOH = 14.00.
Tip: concentrations must be greater than 0. For pH and pOH, negative values are possible in very strong solutions, but the calculator still uses pH + pOH = 14.00 at 25 degrees Celsius.
Results
Enter one known value and click Calculate to see all related values.
Expert Guide: How to Calculate H+, OH-, pH, and pOH Correctly
Being able to calculate H+, OH-, pH, and pOH is one of the most important skills in general chemistry, analytical chemistry, biology, environmental science, and water quality work. These values tell you how acidic or basic a solution is, and they are directly tied to reaction behavior, enzyme activity, corrosion, solubility, nutrient availability, and laboratory safety. If you understand how to move between hydrogen ion concentration, hydroxide ion concentration, pH, and pOH, you can solve a wide range of chemistry problems quickly and accurately.
At 25 degrees Celsius, the key relationships are simple. The hydrogen ion concentration is written as [H+], the hydroxide ion concentration is written as [OH-], the acidity measure is pH, and the basicity measure is pOH. These quantities are connected through logarithms and the ion product of water. In pure water at 25 degrees Celsius, the product of hydrogen and hydroxide concentrations is 1.0 x 10-14. That gives the classic equation pH + pOH = 14.00.
- pH = -log10([H+])
- pOH = -log10([OH-])
- [H+] = 10-pH
- [OH-] = 10-pOH
- [H+][OH-] = 1.0 x 10-14
- pH + pOH = 14.00
What each term means
[H+] is the molar concentration of hydrogen ions in solution. More precisely in many chemistry contexts, pH is related to hydronium activity, but introductory and practical calculations often use hydrogen ion concentration. A higher [H+] means a lower pH and a more acidic solution.
[OH-] is the molar concentration of hydroxide ions. A higher [OH-] means a lower pOH and a more basic solution.
pH is a logarithmic scale for acidity. Because the scale is logarithmic, a change of 1 pH unit means a tenfold change in hydrogen ion concentration.
pOH is the logarithmic measure of hydroxide ion concentration. As pOH decreases, hydroxide concentration increases and the solution becomes more basic.
How to calculate all four values from any one known value
If you know one of the four values, you can calculate the other three. The method depends on what you start with.
- If you know [H+]: calculate pH using pH = -log10([H+]), then calculate pOH as 14.00 – pH, then calculate [OH-] as 10-pOH.
- If you know [OH-]: calculate pOH using pOH = -log10([OH-]), then calculate pH as 14.00 – pOH, then calculate [H+] as 10-pH.
- If you know pH: calculate [H+] as 10-pH, then calculate pOH as 14.00 – pH, then calculate [OH-] as 10-pOH.
- If you know pOH: calculate [OH-] as 10-pOH, then calculate pH as 14.00 – pOH, then calculate [H+] as 10-pH.
Worked examples
Example 1: Given [H+] = 1.0 x 10-3 M
- pH = -log(1.0 x 10-3) = 3.00
- pOH = 14.00 – 3.00 = 11.00
- [OH-] = 10-11 M
This solution is acidic because the pH is below 7 at 25 degrees Celsius.
Example 2: Given pH = 9.25
- [H+] = 10-9.25 = 5.62 x 10-10 M
- pOH = 14.00 – 9.25 = 4.75
- [OH-] = 10-4.75 = 1.78 x 10-5 M
This solution is basic because the pH is above 7.
Example 3: Given [OH-] = 2.5 x 10-6 M
- pOH = -log(2.5 x 10-6) = 5.602
- pH = 14.00 – 5.602 = 8.398
- [H+] = 10-8.398 = 4.00 x 10-9 M
Why the logarithm matters
Many students get confused because pH is not linear. A solution with pH 4 is not just a little more acidic than a solution with pH 5. It has ten times the hydrogen ion concentration. Likewise, a solution with pH 2 has one hundred times the hydrogen ion concentration of a solution with pH 4. This is why small pH changes can have major chemical and biological consequences.
| pH | [H+] in mol/L | Relative acidity compared with pH 7 | General interpretation |
|---|---|---|---|
| 2 | 1.0 x 10-2 | 100,000 times higher [H+] than pH 7 | Strongly acidic |
| 4 | 1.0 x 10-4 | 1,000 times higher [H+] than pH 7 | Acidic |
| 7 | 1.0 x 10-7 | Reference point at 25 degrees Celsius | Neutral water model |
| 10 | 1.0 x 10-10 | 1,000 times lower [H+] than pH 7 | Basic |
| 12 | 1.0 x 10-12 | 100,000 times lower [H+] than pH 7 | Strongly basic |
Acidic, neutral, and basic classifications
At 25 degrees Celsius, a solution is:
- Acidic when pH is less than 7 and pOH is greater than 7
- Neutral when pH is 7 and pOH is 7
- Basic when pH is greater than 7 and pOH is less than 7
Keep in mind that the exact neutral pH changes with temperature because the autoionization constant of water changes. This calculator uses the standard classroom assumption of 25 degrees Celsius, which is correct for most introductory calculations.
Common mistakes when calculating H+, OH-, pH, and pOH
- Forgetting the negative sign in the logarithm. pH and pOH use negative log base 10.
- Using natural log instead of log base 10. Chemistry pH calculations use log10.
- Confusing pH with concentration. A pH of 3 does not mean [H+] = 3 M. It means [H+] = 10-3 M.
- Applying pH + pOH = 14 at the wrong temperature without qualification. This identity is exact only when pKw = 14.00, commonly assumed at 25 degrees Celsius.
- Entering zero or negative concentrations. Concentration values for [H+] and [OH-] must be positive.
- Rounding too early. Keep several digits through intermediate steps, then round the final answer.
Real-world pH statistics and reference values
The pH scale has practical importance in medicine, agriculture, environmental science, and engineering. The following table lists widely referenced pH benchmarks and operating ranges used in real-world contexts.
| System or sample | Typical pH or range | Why it matters |
|---|---|---|
| Pure water at 25 degrees Celsius | 7.00 | Standard neutral reference in basic chemistry calculations |
| Human arterial blood | 7.35 to 7.45 | Narrow regulation range is essential for physiology |
| U.S. EPA recommended secondary drinking water range | 6.5 to 8.5 | Helps control taste, corrosion, and scaling concerns |
| Acid rain threshold often cited in environmental science | Below 5.6 | Indicates precipitation more acidic than normal atmospheric equilibrium |
| Many agricultural soils | About 5.5 to 7.5 | Nutrient availability changes strongly across this range |
| Household bleach | About 11 to 13 | Strongly basic solution with high cleaning reactivity |
Step-by-step method you can use on any chemistry problem
- Identify what quantity is given: [H+], [OH-], pH, or pOH.
- Check whether the value is physically valid. Concentrations must be positive numbers.
- If the given value is a concentration, take the negative base-10 logarithm to get pH or pOH.
- If the given value is pH or pOH, raise 10 to the negative of that value to get concentration.
- Use pH + pOH = 14.00 to find the missing logarithmic value.
- Use the classification rule to decide whether the solution is acidic, neutral, or basic.
- Round carefully according to your class or laboratory reporting rules.
How this calculator helps
This calculator automates the full chain of conversions. You enter one known quantity, choose whether it represents [H+], [OH-], pH, or pOH, and the calculator returns all related values. It also displays a chart that compares pH and pOH on the standard 0 to 14 reference scale. That visual comparison is especially useful for students who need to see how acidity and basicity balance each other numerically.
Advanced note on temperature and pKw
In more advanced chemistry, pH and pOH calculations may need a temperature-specific water ionization constant. At temperatures other than 25 degrees Celsius, neutral water does not necessarily have pH exactly equal to 7. However, many school assignments, exam problems, and quick laboratory estimates explicitly tell you to assume 25 degrees Celsius. Under that assumption, using pH + pOH = 14.00 is correct and efficient.
Authoritative resources for further reading
- U.S. Environmental Protection Agency: Alkalinity and Acidification
- U.S. Geological Survey: pH and Water
- Chemistry LibreTexts educational resource
Final takeaway
To calculate H+, OH-, pH, and pOH, remember two big ideas: pH and pOH are negative base-10 logarithms of concentration, and at 25 degrees Celsius they must add to 14.00. If you know any one of the four quantities, you can derive the other three. Once you practice a few examples, these conversions become routine. Use the calculator above for fast, accurate results, and use the formula relationships to understand what the numbers mean chemically.