pH Calculator
Find pH and pOH from hydrogen ion concentration.
Hydrogen ion concentration in moles per litre.
Results update as you type.
How it works
pH tells you how acidic or basic a solution is, and this pH calculator converts a hydrogen ion concentration in moles per litre into a pH value, its matching pOH, and a plain-language classification of acidic, neutral, or basic. Because hydrogen ion concentrations range across many powers of ten, the logarithmic pH scale packs them neatly between 0 and 14, where 7 is neutral, lower numbers are acidic, and higher numbers are basic. Getting pH right matters everywhere from the lab bench to daily life: it governs enzyme activity in the body, the safety of drinking water, the health of soil and aquariums, and the outcome of countless chemical reactions. A reliable pH calculator removes the tricky logarithm arithmetic and the easy-to-miss sign, so you can move straight from a measured concentration to an interpretable result. The formula works because pH is the negative logarithm of hydrogen ion concentration, turning tiny fractional numbers into a readable scale, while pOH completes the picture since the two always sum to 14 in water at room temperature. Chemistry students, lab technicians, and environmental scientists all lean on a pH calculator to check acidity quickly and accurately.
pH = −log₁₀[H⁺] · pOH = 14 − pH
pH is the negative base-ten logarithm of hydrogen ion concentration because those concentrations span many orders of magnitude, and a logarithm compresses that huge range into a convenient 0-to-14 scale. The negative sign makes acidic solutions, which have high concentrations, correspond to low pH numbers. pOH sums with pH to 14 at 25°C because water's self-ionisation fixes the product of hydrogen and hydroxide ion concentrations.
Source:Standard chemistry (pH scale)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter [h⁺] concentration.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
pH updates instantly as you type — no button to press.
Frequently asked questions
What does the pH scale actually measure?
The pH scale measures the concentration of hydrogen ions in a solution on a logarithmic scale, indicating how acidic or basic it is. It typically runs from 0 to 14, with 7 being neutral, values below 7 acidic, and values above 7 basic. Because it is logarithmic, each whole-number step represents a tenfold change in hydrogen ion concentration, so pH 4 is ten times more acidic than pH 5.
How are pH and pOH related?
In water at 25°C, pH and pOH always add up to 14. This comes from the self-ionisation of water, where the product of hydrogen and hydroxide ion concentrations is fixed at 10⁻¹⁴. So if you know one, you immediately know the other: a solution with pH 3 has pOH 11. The calculator computes pOH by subtracting the pH from 14 automatically for you.
What concentration gives a neutral pH?
A neutral solution at 25°C has a hydrogen ion concentration of 1×10⁻⁷ moles per litre, which produces a pH of exactly 7. At this concentration the hydrogen and hydroxide ions are perfectly balanced, so the solution is neither acidic nor basic. Pure water sits at this point. Entering 0.0000001 mol/L into the calculator returns a pH of 7 and a classification of neutral.
Can pH be negative or above 14?
Yes, though it is uncommon in everyday settings. Very concentrated strong acids can have hydrogen ion concentrations above 1 mol/L, producing a negative pH, while extremely concentrated strong bases can push pH above 14. The 0-to-14 range is simply the span covering most aqueous solutions, not a hard limit. The calculator handles values outside that window correctly using the same logarithmic formula.
Why is the pH scale logarithmic instead of linear?
Hydrogen ion concentrations in real solutions vary across an enormous range, from around 1 mol/L in strong acids to 10⁻¹⁴ mol/L in strong bases. A linear scale would be unusable across so many orders of magnitude. The logarithmic pH scale compresses that range into simple numbers between 0 and 14, making comparisons easy while still capturing the vast underlying differences in acidity between solutions.