pH Calculator
A Logarithmic Scale Hiding in Plain Sight
pH compresses an enormous range of hydrogen ion concentrations — spanning fourteen orders of magnitude, from battery acid to drain cleaner — into a simple number most people can reason about. Because the scale is logarithmic, a difference of one pH unit means a tenfold change in acidity, which is easy to overlook if you only ever read the number and never touch the underlying math.
The Formula
[H+] = 10-pH
The calculator runs whichever direction you need: give it a hydrogen ion concentration in mol/L and it returns pH, or give it a pH and it returns the corresponding [H+].
Where This Calculation Matters
- Water treatment and aquariums — pH determines chemical dosing, and small numeric shifts represent large real changes in acidity.
- Food and beverage production — fermentation, brewing, and preservation all depend on holding pH within a target band.
- Clinical chemistry — blood pH is tightly regulated near 7.4, and even small deviations are clinically significant.
- Agricultural soil testing — soil pH governs which nutrients are available to plant roots.
The pH Scale in Everyday Substances
| Substance | Approximate pH | Character |
|---|---|---|
| Battery acid | ~0 | Strongly acidic |
| Gastric (stomach) acid | 1.5–3.5 | Strongly acidic |
| Lemon juice | ~2 | Acidic |
| Vinegar | ~2.5 | Acidic |
| Black coffee | ~5 | Weakly acidic |
| Pure water | 7.0 | Neutral |
| Human blood | ~7.4 | Weakly alkaline |
| Baking soda solution | ~9 | Alkaline |
| Household ammonia | ~11 | Strongly alkaline |
| Bleach | ~12.5 | Strongly alkaline |
A Worked Example
A solution with [H+] = 0.0001 mol/L (that is, 10-4 mol/L) gives pH = -log10(0.0001) = 4.0. Drop the concentration by a factor of ten to 0.00001 mol/L and the pH rises by exactly one unit, to 5.0 — the logarithmic relationship in action.
How to Use This Calculator
- Choose the direction of the calculation: converting a hydrogen ion concentration to pH, or a pH value back to concentration.
- For the first mode, enter the [H+] Concentration (mol/L).
- For the second mode, enter the pH Value directly.
- Select Calculate to get the converted value.
Related Calculations
Working with a weak acid and its conjugate base rather than a strong acid? The Henderson-Hasselbalch Calculator handles buffer pH directly. For titrations to a known endpoint, see the Acid-Base Titration Calculator.