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pH Calculator

Chemistry

Calculate pH from hydrogen ion concentration [H⁺], pOH, or hydroxide ion concentration [OH⁻]. Instantly find acid/base strength with step-by-step working.

Reviewed by the thecalcu.com team · Last updated July 14, 2026

0.001 mol/L
mol/L

pH

3
pOH
11
[OH⁻] Concentration
0
Solution Type
Acidic

This calculator computes your pH, pOH, [OH⁻] Concentration, Solution Type from the values you enter.

Inputs
H⁺ Concentration [H⁺]
Outputs
pHpOH[OH⁻] ConcentrationSolution Type

What is a pH?

The pH Calculator converts hydrogen ion concentration [H⁺] to pH using the fundamental acid-base formula pH = −log₁₀([H⁺]). It also returns pOH, hydroxide ion concentration [OH⁻], and classifies the solution as acidic, neutral, or basic, all from a single input value.

pH is the most widely used measurement of acidity or alkalinity in chemistry, biology, environmental science, agriculture, and industry. The scale runs from 0 (most acidic) to 14 (most basic), with 7 representing neutral pure water at 25°C. Because pH is a logarithmic scale, each unit change represents a ten-fold change in hydrogen ion concentration, a solution at pH 3 has 10× more H⁺ ions than one at pH 4, and 100× more than one at pH 5. This logarithmic compression is precisely why very small [H⁺] values (like 10⁻⁷ mol/L for pure water) map neatly to the familiar 0–14 range.

The concept was introduced by Danish biochemist Søren Peder Lauritz Sørensen in 1909 while studying enzyme activity at the Carlsberg Laboratory. He needed a convenient way to express [H⁺] without writing out unwieldy exponential values, and the negative logarithm notation proved so useful that it has been standard in chemistry ever since.

In the Indian school curriculum, pH is introduced in NCERT Class 10 Science (Chapter 2) at a conceptual level and then treated mathematically in Class 11 Chemistry Chapter 7 (Equilibrium). Practical applications range from soil testing in agriculture to water quality assessment for BIS IS 10500 drinking water standards. Industrial uses include food preservation (jam pH < 4 to inhibit bacterial growth), pharmaceutical formulation (drug stability often depends on pH), and effluent treatment plant monitoring required under CPCB discharge norms.

For the reverse calculation, finding [H⁺] when you know pH, use the Hydrogen Ion Concentration Calculator. For buffer solutions where acid and conjugate base concentrations determine pH, see the Buffer pH Calculator.

Why Use a pH Calculator?

The formula pH = −log₁₀([H⁺]) requires taking a base-10 logarithm of a very small number, which is quick on a scientific calculator but error-prone when [H⁺] is in scientific notation. The most common mistake is a sign error: students and technicians frequently compute log(0.001) = −3 and forget the negative sign, reporting pH = −3 instead of 3.

Key use cases:

  • Lab practicals: Confirm the pH of a prepared acid solution from the [H⁺] you know went into the preparation.
  • Titration calculations: Find the pH at any point in a strong acid-strong base titration when the excess [H⁺] is known.
  • NEET and JEE problem-solving: Verify intermediate steps in acid-base equilibrium problems where [H⁺] is calculated first and pH is the final answer.
  • Industrial QC: Convert instrument-measured [H⁺] or conductivity-derived hydrogen ion activity to pH for reporting against regulatory standards.

The Molarity Calculator is useful when you need to first convert a mass of dissolved acid to a molar concentration before calculating pH.

Who Should Use This Calculator?

Class 10 and 11 chemistry students who are learning the pH scale and need to verify calculated values or build intuition about how [H⁺] changes with pH. Seeing that 0.001 mol/L = pH 3 and 0.0000001 mol/L = pH 7 makes the logarithmic relationship concrete.

JEE Main and Advanced aspirants who need fast, accurate pH values during problem practice. Acid-base and equilibrium questions are a consistent part of the JEE chemistry syllabus, and pH often appears as either the final answer or an intermediate step.

NEET students studying biological systems where pH is critical, blood pH (7.35–7.45), gastric acid (pH 1.5–3.5), and enzyme activity windows all require understanding the [H⁺] ↔ pH relationship. Use alongside the Hydrogen Ion Concentration Calculator for reverse lookups.

Environmental science students and professionals monitoring water quality for CPCB discharge limits (pH 5.5–9.0 for effluents) or BIS drinking water standards (pH 6.5–8.5).

Laboratory technicians in pharmaceutical, food, and chemical industries who verify solution pH from known preparation concentrations before instrument measurement.

What Insights Does the pH Calculator Give You?

pH is the primary highlighted output, the dimensionless measure of hydrogen ion concentration. A pH of 3 means the solution has [H⁺] = 10⁻³ = 0.001 mol/L. Use this value to classify the solution, compare acidity across samples, or as an input to downstream equilibrium and buffer calculations.

pOH is the complementary alkalinity measure. At 25°C, pOH = 14 − pH. A pH of 3 gives pOH = 11, confirming the solution is strongly acidic (low [OH⁻]). pOH is directly useful in base concentration problems, if you know pOH, [OH⁻] = 10^(−pOH).

[OH⁻] Concentration shows the hydroxide ion concentration in mol/L, derived from Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴. This output is needed for neutralisation calculations, base strength comparisons, and checking whether [OH⁻] meets treatment targets.

Solution Type labels the result as Acidic (pH < 7), Neutral (pH = 7), or Basic/Alkaline (pH > 7). This quick classification is useful when interpreting results at a glance or when presenting results to a non-specialist audience.

How to use this pH calculator

  1. Determine [H⁺] concentration, measure or calculate the hydrogen ion concentration of your solution in mol/L. For a strong acid like HCl, [H⁺] equals the molar concentration of the acid. For a weak acid, calculate [H⁺] using Ka and the acid concentration.
  2. Enter H⁺ Concentration [H⁺], type the molar concentration into the H⁺ Concentration [H⁺] field (unit: mol/L). Enter small values in decimal form: for 10⁻⁵ mol/L, enter 0.00001.
  3. Read pH, the highlighted output shows the pH. A result of 5 means the solution is mildly acidic with [H⁺] = 10⁻⁵ mol/L.
  4. Read pOH, the pOH output shows the complementary alkalinity value. Confirm that pH + pOH = 14 to verify the calculation.
  5. Read [OH⁻] Concentration, note the hydroxide ion concentration in mol/L. This is useful if you need to calculate a neutralisation dose or check alkalinity targets.
  6. Check Solution Type, the text output confirms whether the solution is Acidic, Neutral, or Basic. For buffer or equilibrium problems, take the pH value to the Buffer pH Calculator or Henderson-Hasselbalch Calculator for further analysis.
Show formula & methodology ↓Show less ↑

Formula & Methodology

pH formula:

> pH = −log₁₀([H⁺])

Where:
- [H⁺] = molar concentration of hydrogen ions (mol/L)
- log₁₀ = base-10 logarithm

Derived outputs:

> pOH = 14 − pH  (at 25°C, from Kw = 10⁻¹⁴)

> [OH⁻] = Kw ÷ [H⁺] = 10⁻¹⁴ ÷ [H⁺]

Worked example 1, Dilute HCl:

A student prepares 0.01 mol/L HCl (strong acid, fully dissociated):
- [H⁺] = 0.01 mol/L
- pH = −log₁₀(0.01) = −(−2) = 2
- pOH = 14 − 2 = 12
- [OH⁻] = 10⁻¹⁴ ÷ 10⁻² = 10⁻¹² mol/L
- Classification: Acidic

Worked example 2, Vinegar (acetic acid approximation):

Commercial vinegar has an approximate [H⁺] of 0.00158 mol/L:
- pH = −log₁₀(0.00158) = −(−2.80) = 2.80
- pOH = 14 − 2.80 = 11.20
- This confirms vinegar is moderately acidic, consistent with its use as a food preservative

Worked example 3, BIS drinking water check:

BIS IS 10500 requires drinking water pH between 6.5 and 8.5. At pH 6.5:
- [H⁺] = 10⁻⁶˙⁵ = 3.162 × 10⁻⁷ mol/L
- At pH 8.5: [H⁺] = 10⁻⁸˙⁵ = 3.162 × 10⁻⁹ mol/L
- The acceptable [H⁺] range spans two orders of magnitude, illustrating why pH is more practical than [H⁺] for communicating water quality standards

Use the pKa Calculator when the acid dissociation constant is known and you need to find [H⁺] for a weak acid before calculating pH.

Frequently Asked Questions

What is pH?
pH (potential of hydrogen) is a logarithmic scale that measures the concentration of hydrogen ions [H⁺] in a solution, indicating how acidic or basic that solution is. The scale runs from 0 to 14, where pH 7 is neutral (pure water at 25°C), pH below 7 is acidic, and pH above 7 is basic (alkaline). Because the scale is logarithmic, a pH of 3 is ten times more acidic than a pH of 4, and one hundred times more acidic than a pH of 5.
What is the formula for calculating pH?
The pH formula is pH = −log₁₀([H⁺]), where [H⁺] is the molar concentration of hydrogen ions in mol/L. For example, a solution with [H⁺] = 0.001 mol/L has pH = −log₁₀(0.001) = −(−3) = 3. The negative logarithm ensures that higher hydrogen ion concentrations give lower (more acidic) pH values, which is the convention established by Søren Peder Lauritz Sørensen in 1909.
What is the pH scale and what do the values mean?
The pH scale runs from 0 to 14 (and beyond in extreme concentrations). Values 0–6.9 are acidic, with 0 representing concentrated strong acids like battery acid. pH 7 is neutral (pure water). Values 7.1–14 are basic (alkaline), with 14 representing concentrated strong bases like caustic soda (NaOH). Common references: stomach acid ≈ 1.5–3.5, cola drinks ≈ 2.5–3.5, milk ≈ 6.5–6.8, seawater ≈ 8.1–8.3, baking soda solution ≈ 8.3.
What is pOH and how is it related to pH?
pOH is the negative logarithm of the hydroxide ion concentration: pOH = −log₁₀([OH⁻]). At 25°C, pH and pOH are complementary: pH + pOH = 14, a relationship derived from the ionic product of water (Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴). When pH is 3, pOH is 11, confirming the solution is acidic. The pH Calculator outputs pOH automatically from the pH result.
What is the difference between pH and pOH?
pH measures acidity (hydrogen ion concentration) while pOH measures alkalinity (hydroxide ion concentration). They are always linked: pH + pOH = 14 at 25°C. A solution with low pH has high pOH and vice versa. Most laboratory and environmental contexts report pH rather than pOH, but pOH is sometimes more convenient when working with basic solutions, for example, calculating the normality of a base in a titration.
What is the difference between a strong acid and a weak acid in terms of pH?
A strong acid (like HCl, H₂SO₄, HNO₃) dissociates completely in water, all molecules release H⁺ ions. This means the [H⁺] equals the molar concentration of the acid, and pH can be calculated directly. A weak acid (like acetic acid, citric acid) dissociates only partially, so [H⁺] is much less than the acid's molar concentration and requires the acid dissociation constant (Ka) to calculate precisely. Enter the actual [H⁺] into this calculator regardless of acid type.
What is the [OH⁻] concentration output and when do I use it?
The [OH⁻] Concentration output shows the hydroxide ion concentration in mol/L, calculated from Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴. This value is useful when you need the hydroxide concentration for a downstream calculation, for example, finding the concentration of a base in a neutralisation reaction, or checking whether a soil sample has excess alkalinity that needs treatment. For a pH of 11, [OH⁻] = 1 × 10⁻³ mol/L = 0.001 M.
How do I calculate pH from pOH instead of [H⁺]?
If you know pOH, calculate pH using pH = 14 − pOH (at 25°C). For example, pOH = 5 → pH = 9. You can then convert pH to [H⁺] using [H⁺] = 10^(−pH) with the [Hydrogen Ion Concentration Calculator](/hydrogen-ion-concentration-calculator/), or enter the [H⁺] value directly into this pH Calculator if you prefer the reverse direction.
What is the pH of drinking water in India?
The Bureau of Indian Standards (BIS IS 10500) specifies that drinking water should have a pH between 6.5 and 8.5. The permissible limit extends to 6.5–9.2 in absence of an alternate source. Most municipal water in India has a pH of 6.8–7.8. Water with pH below 6.5 is mildly corrosive to pipes and can leach heavy metals; water above 8.5 may have a bitter taste and indicates excess mineral content or treatment chemicals.
What pH is used in agriculture and irrigation water in India?
The Indian Council of Agricultural Research (ICAR) recommends irrigation water with pH between 6.0 and 8.5 for most crops. Soils with pH below 5.5 are acid soils requiring lime treatment, while pH above 8.5 indicates alkaline or saline conditions requiring gypsum or organic amendment. Paddy tolerates pH 5.0–7.5, while wheat grows best at pH 6.0–7.5. Knowing the pH of irrigation water or soil extract is essential before applying fertiliser to avoid nutrient lock-up.
Can pH be negative or greater than 14?
Yes, pH can be negative or exceed 14 for extremely concentrated acid or base solutions. A 10 mol/L HCl solution has pH = −1; a 10 mol/L NaOH solution has pOH = −1, so pH = 15. These values are outside the standard 0–14 range and are rarely encountered in routine laboratory or industrial practice. The pH Calculator accepts very small [H⁺] values (down to 1 × 10⁻¹⁴) for pH up to 14; for extreme concentrations, apply the formula directly.
How is pH taught in India's chemistry syllabus?
pH is introduced in NCERT Class 11 Chemistry, Chapter 7 (Equilibrium), as part of the acid-base equilibrium topic. Students learn the pH scale, the formulae pH = −log[H⁺] and pOH = −log[OH⁻], and the relationship pH + pOH = 14. It is also covered in NCERT Class 10 Science (Chapter 2, Acids, Bases and Salts) at an introductory level. The topic appears in JEE Main, JEE Advanced, and NEET in problems involving strong acid/base solutions, buffer calculations, and titration equivalence points.
Also known as
pH from H+ concentrationacid base calculatorhydrogen ion pHpH pOH calculatorpH of solution