How pH works
pH measures how acidic or basic a water-based solution is using a logarithmic scale. It is defined as the negative base-10 logarithm of the hydrogen-ion concentration, so each whole step on the scale represents a tenfold change in acidity.
Enter a hydrogen-ion concentration to get the pH, or enter a pH directly. Either way the tool also reports pOH (14 − pH) and labels the solution as acidic, neutral, or basic.
Reading the result
On the 0–14 scale, lower numbers are more acidic and higher numbers are more basic, with 7 marking neutral.
- pH below 7: acidic, such as vinegar or lemon juice.
- pH of 7: neutral, like pure water at room temperature.
- pH above 7: basic, such as baking soda or soapy water.
Practical tips
Because the scale is logarithmic, a solution at pH 4 is ten times more acidic than one at pH 5 and a hundred times more acidic than one at pH 6. Small pH differences represent large concentration differences.
Caveats and common mistakes
The hydrogen-ion concentration must be greater than zero, since the logarithm of zero is undefined. The neat 0–14 range and the pOH = 14 − pH relationship assume conditions near 25°C; very concentrated or very hot solutions can fall outside it.
Formula
pH = −log10([H⁺]); [H⁺] = 10^(−pH); pOH = 14 − pH