pH Calculator
Give this calculator any one of pH, pOH, the hydrogen ion concentration or the hydroxide ion concentration, and it returns the other three at 25 degrees Celsius. It works from the definitions pH = -log10[H+] and pOH = -log10[OH-], together with pH + pOH = 14 and the water ion product [H+][OH-] = 1.0 x 10^-14. A colour-coded label tells you whether the solution is acidic, neutral or basic.
How to calculate pH
- Open the I know dropdown and choose pH, pOH, [H+] or [OH-].
- Type your single value into the number box, using mol/L for a concentration.
- Read the four result cards for pH, pOH, [H+] and [OH-].
- Check the acidic, neutral or basic verdict shown under the cards.
- Click Copy to copy all four values plus the nature line.
Examples
From a hydrogen ion concentration
I know [H+] = 0.001 mol/L
pH 3, pOH 11, [H+] 0.001, [OH-] 1.000e-11, acidic
Neutral water
I know pH = 7
pOH 7, [H+] 1.000e-7, [OH-] 1.000e-7, neutral
From a pOH value
I know pOH = 2
pH 12, pOH 2, [H+] 1.000e-12, [OH-] 0.01, basic
Frequently asked questions
What is pH?
pH measures how acidic or basic a solution is. It equals the negative base-10 logarithm of the hydrogen ion concentration, pH = -log10[H+], so a smaller [H+] means a higher pH.
How are pH and pOH related?
At 25 degrees Celsius pH plus pOH equals 14, because the ion product of water [H+][OH-] is 1.0 x 10^-14. Enter a pOH of 2 and the tool reports a pH of 12.
What pH counts as acidic, neutral or basic?
At 25C a pH below 7 is labelled acidic, exactly 7 is neutral, and above 7 is basic. The verdict is colour coded, with acidic in rose, neutral in green and basic in blue.
How do I get [H+] from pH?
Raise ten to the power of the negative pH: [H+] = 10^(-pH) in mol/L. Entering pH 3 returns [H+] of 0.001, and the tool works in either direction.
Can I start from a concentration instead of pH?
Yes. Set the I know dropdown to [H+] or [OH-] and enter the concentration in mol/L. Scientific notation such as 1e-11 is accepted for very dilute solutions.
How are the numbers rounded?
pH and pOH are rounded to six decimal places, and the concentrations are kept to nine significant figures so tiny values like 1e-11 are not flattened to zero. The display shows plain digits between 0.001 and 1000 and switches to scientific notation outside that range.
Does temperature affect the result?
Yes. These relations assume 25 degrees Celsius. At other temperatures the water ion product Kw shifts, so neutral pH moves away from exactly 7, which this tool does not model.
Why do I only enter one value?
The four quantities are linked by fixed equations, so any one of them determines the rest at 25C. The engine expects exactly one input and derives the other three from it.
Is my input private?
Yes. The calculation runs entirely in your browser, so nothing you type is uploaded. The tool is free, private and works offline once loaded.
Learn more
- pH, pOH and the ion product of water
How pH and pOH connect through the water ion product Kw, why they add to 14 at 25C, and how to read concentrations from either scale.
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