Free pH Calculator
Use this pH scale calculator to convert between pH, pOH, hydrogen ion concentration [H+], and hydroxide concentration [OH-].
What Is pH?
pH is a logarithmic measure of acidity or basicity. It tells you how concentrated hydrogen ions are in a solution. Lower pH values indicate a higher hydrogen ion concentration (more acidic), while higher pH values indicate lower hydrogen ion concentration (more basic or alkaline).
The pH scale is commonly presented from 0 to 14 for dilute water-based solutions, but strong acids and bases can fall outside this range in real lab conditions.
Core Formulas Used in This pH Calculator
1) pH from hydrogen ion concentration
pH = -log10[H+]
2) pOH from hydroxide concentration
pOH = -log10[OH-]
Then convert to pH using pH = 14 - pOH (at 25°C).
3) From pH to concentrations
- [H+] = 10-pH
- [OH-] = 10pH-14
How to Use the Calculator
- Select a calculation type from the dropdown menu.
- Enter your known value in the displayed field.
- Click Calculate to get the result instantly.
- Use Clear to reset and start another conversion.
Quick pH Interpretation Guide
| pH Range | Classification | Typical Example |
|---|---|---|
| 0 to < 7 | Acidic | Lemon juice, vinegar |
| 7 | Neutral | Pure water (idealized) |
| > 7 to 14 | Basic (alkaline) | Baking soda solution, soap solution |
Why pH Matters
Water quality and treatment
Drinking water, wastewater treatment, and aquariums all rely on pH control. Poor pH balance can damage pipes, reduce disinfectant performance, or stress aquatic life.
Agriculture and soil science
Soil pH affects nutrient availability. Even with adequate fertilizer, crops may struggle if pH is too high or too low for root uptake.
Hydroponics and gardening
In hydroponic systems, pH drift can rapidly impact nutrient absorption. Frequent monitoring helps prevent deficiency symptoms and growth slowdown.
Laboratory and manufacturing settings
Chemical reaction rates, product stability, and quality control often depend on precise pH management. This is critical in pharmaceuticals, food science, and biotechnology.
Worked Examples
Example A: Find pH from [H+]
If [H+] = 1.0 × 10-3 mol/L, then pH = 3. This is acidic.
Example B: Find pH from [OH-]
If [OH-] = 1.0 × 10-4 mol/L, then pOH = 4 and pH = 10. This is basic.
Example C: Find concentrations from pH
If pH = 9, then [H+] = 1.0 × 10-9 mol/L and [OH-] = 1.0 × 10-5 mol/L.
Common Mistakes to Avoid
- Entering zero or negative concentration values (not physically valid for this calculation).
- Mixing concentration units (the formulas expect mol/L).
- Assuming pH + pOH = 14 at all temperatures (this relation changes slightly with temperature).
- Rounding too early in multistep calculations.
FAQ
Can pH be negative or above 14?
Yes. In very concentrated solutions, pH may fall below 0 or rise above 14.
Does this calculator work for buffered systems?
It computes direct mathematical conversions, not full equilibrium behavior of complex buffer mixtures.
Is this accurate for all temperatures?
This version assumes 25°C. For high-precision work at other temperatures, use temperature-adjusted equilibrium constants.