pH Calculator

Start by entering a pH value between 0 and 14. You can type the number directly or use the color slider to adjust it visually. The slider matches the universal pH indicator color scale, so you can quickly match the color on your test strip.

Next, select what you are testing. Choosing pool, aquarium, soil, or drinking water adjusts the ideal range shown on the scale and gives you specific advice about what your reading means in that context.

The results show your hydrogen ion and hydroxide ion concentrations in scientific notation, a classification of your reading from strongly acidic to strongly alkaline, and a list of common substances near your pH value for reference. You can also expand the comparison tool to see the fold difference between two pH values.

pH stands for "potential of hydrogen" and measures the concentration of hydrogen ions (H⁺) in a solution. The scale runs from 0 to 14, where 0 is the most acidic, 14 is the most alkaline, and 7 is neutral. Pure water at 25°C has a pH of exactly 7.0 because its hydrogen and hydroxide ion concentrations are equal.

The key thing most people do not realize is that pH is logarithmic. This means that a pH of 5 is not slightly more acidic than a pH of 6. It is ten times more acidic. A pH of 4 is one hundred times more acidic than pH 6. This is why small changes in pool or aquarium pH can have such a large effect on water chemistry.

Pool and hot tub owners test pH regularly because it directly affects how well chlorine sanitizes the water. When pH drifts above 7.6, chlorine becomes significantly less effective at killing bacteria. When it drops below 7.2, the water becomes corrosive enough to damage metal fixtures and irritate skin and eyes.

Gardeners use soil pH to understand why certain plants thrive while others struggle. Most vegetables prefer slightly acidic to neutral soil (pH 6.0 to 7.0) because that range allows roots to absorb the most nutrients. Blueberries and azaleas prefer much more acidic conditions, which is why they often need sulfur amendments in alkaline soils.

Aquarium hobbyists monitor pH because fish and aquatic plants have evolved to live within specific ranges. Most freshwater tropical fish do well between pH 6.5 and 7.5, but African cichlids prefer higher pH around 7.8 to 8.5. Sudden pH swings are more dangerous than a pH that is slightly outside the ideal range, because fish can acclimate gradually but not to rapid shifts.

Litmus paper and pH test strips are the most affordable option. You dip a strip into the liquid, wait the recommended time (usually 15 to 30 seconds), and compare the color change against the chart included with the strips. These are accurate enough for pool and soil testing but may not distinguish differences smaller than 0.5 pH units.

Liquid drop test kits use a reagent that changes color when added to a water sample. You compare the resulting color against a reference chart. These are slightly more accurate than strips and are popular with pool and aquarium owners because they can detect changes of about 0.2 pH units.

Digital pH meters provide the most precise readings, often to 0.01 pH units. They require calibration with buffer solutions before each use and proper storage in electrode storage solution. For most home applications like pool or aquarium testing, a mid range meter around $20 to $40 provides more than enough accuracy.