pH Calculator
Calculate pH from hydrogen ion concentration or find H+ concentration from pH.
Shows whether a solution is acidic, basic, or neutral.
pH (potential of Hydrogen) is a logarithmic scale measuring how acidic or alkaline a solution is. It ranges from 0 to 14, with 7 being neutral at 25°C.
The pH formula: pH = −log₁₀[H⁺]
Where [H⁺] is the molar concentration of hydrogen ions in moles per liter (mol/L).
Reverse formula — finding H⁺ concentration from pH: [H⁺] = 10^(−pH)
pOH and the relationship: pOH = −log₁₀[OH⁻] pH + pOH = 14 (at 25°C)
Worked example: A solution has [H⁺] = 0.001 mol/L = 10⁻³ mol/L pH = −log₁₀(10⁻³) = −(−3) = pH 3 (acidic)
If pH = 9: [H⁺] = 10⁻⁹ = 0.000000001 mol/L pOH = 14 − 9 = 5
Why logarithmic? The log scale means each whole number step represents a 10× change in acidity. pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.
pH reference scale:
| Substance | pH |
|---|---|
| Battery acid | 0–1 |
| Lemon juice | 2–2.5 |
| Vinegar | 2.9 |
| Coffee | 4.5–5 |
| Pure water | 7.0 |
| Baking soda | 8.3 |
| Milk of magnesia | 10.5 |
| Bleach | 12–13 |
| Drain cleaner | 13–14 |
Buffer solutions resist changes in pH when small amounts of acid or base are added. They are critical in biology — human blood maintains pH between 7.35 and 7.45 very tightly.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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