Last updated: June 2026
Use this pH calculator to connect pH, pOH, hydrogen ion concentration and hydroxide ion concentration. It is written for O-Level, IGCSE, A-Level and Singapore H2 Chemistry students who need to understand acid-base calculations clearly.
What is pH?
pH tells us how acidic or alkaline a solution is. A solution with pH less than 7 is acidic, a solution with pH 7 is neutral, and a solution with pH greater than 7 is alkaline.
However, pH is not just a simple scale. It is a logarithmic scale. This means a change of 1 pH unit represents a tenfold change in hydrogen ion concentration.
For example, a solution with pH 2 has a hydrogen ion concentration ten times greater than a solution with pH 3.
pH formula
The main formula is:
pH = −log10[H+]
where [H+] is the hydrogen ion concentration in mol dm−3.
The reverse formula is:
[H+] = 10−pH
This is useful when the question gives pH and asks for hydrogen ion concentration.
Hydrogen Ion Concentration Calculator
Hydrogen ion concentration, written as [H+], tells us how much H+ is present in a solution. It is directly related to pH.
The formula connecting pH and hydrogen ion concentration is:
pH = −log10[H+]
To find hydrogen ion concentration from pH, rearrange the formula:
[H+] = 10−pH
For example, if pH = 3.00:
[H+] = 10−3.00 = 1.00 × 10−3 mol dm−3
If pH = 5.00:
[H+] = 10−5.00 = 1.00 × 10−5 mol dm−3
pOH and hydroxide ion concentration
For alkaline solutions, it is often useful to calculate pOH first.
pOH = −log10[OH−]
At 25°C:
pH + pOH = 14
This means that once you know pOH, you can find pH by subtracting pOH from 14.
Hydroxide ion concentration
Hydroxide ion concentration, written as [OH−], tells us how much OH− is present in an alkaline solution.
The formula connecting pOH and hydroxide ion concentration is:
pOH = −log10[OH−]
To find hydroxide ion concentration from pOH:
[OH−] = 10−pOH
At 25°C, you can connect pH and pOH using:
pH + pOH = 14
Strong acids and strong bases
For a strong monoprotic acid such as hydrochloric acid, HCl, the acid dissociates completely in water. This means the hydrogen ion concentration is the same as the acid concentration.
For example, 0.0100 mol dm−3 HCl gives:
[H+] = 0.0100 mol dm−3
pH = −log10(0.0100) = 2.00
For a strong alkali such as sodium hydroxide, NaOH, the hydroxide ion concentration is the same as the alkali concentration.
For example, 0.0100 mol dm−3 NaOH gives:
[OH−] = 0.0100 mol dm−3
pOH = −log10(0.0100) = 2.00
pH = 14.00 − 2.00 = 12.00
Worked examples
Example 1: Finding pH from hydrogen ion concentration
Calculate the pH of a solution with [H+] = 1.0 × 10−3 mol dm−3.
pH = −log10[H+]
pH = −log10(1.0 × 10−3)
pH = 3.00
Example 2: Finding hydrogen ion concentration from pH
Calculate [H+] when pH = 4.50.
[H+] = 10−pH
[H+] = 10−4.50
[H+] = 3.16 × 10−5 mol dm−3
Example 3: Finding pH of a strong alkali
Calculate the pH of 0.0200 mol dm−3 NaOH at 25°C.
Since NaOH is a strong alkali:
[OH−] = 0.0200 mol dm−3
pOH = −log10(0.0200) = 1.70
pH = 14.00 − 1.70 = 12.30
Common exam traps
Frequently Asked Questions
What does a pH calculator calculate?
A pH calculator can convert between pH and hydrogen ion concentration. It can also connect pH, pOH and hydroxide ion concentration for acid-base calculations.
What is the formula for pH?
The formula is pH = −log10[H+], where [H+] is the hydrogen ion concentration in mol dm−3.
How do I find hydrogen ion concentration from pH?
Use [H+] = 10−pH. For example, if pH = 3.00, then [H+] = 1.0 × 10−3 mol dm−3.
How do I calculate the pH of a strong base?
First calculate pOH using pOH = −log10[OH−]. Then use pH = 14 − pOH at 25°C.
Can this calculator handle weak acids and buffers?
The basic pH calculator is mainly for strong acids, strong bases and direct pH-to-concentration conversions. Weak acids and buffers require additional equilibrium ideas, so they are better handled using separate weak acid and buffer pH tools.