pH Calculator
Enter hydrogen ion concentration to get pH, or pH to get [H+] and pOH.
General chemistry reference, not medical advice about biological pH.
pH = −log₁₀[H+] = −log₁₀(1.000e-3) = 3.000. pOH = 14 − pH = 11.000 (14.00 is exact only at 25°C, where Kw = 1.0×10⁻¹⁴).
The pH Formula
pH = −log10[H+], where [H+] is the molar concentration of hydrogen ions. The negative log compresses a huge range of possible [H+] values -- from about 1 M in a strong acid down to 10^-14 M in a strong base -- into a manageable 0-to-14 scale.
At 25°C, pOH = 14 − pH, and [H+][OH-] = 1.0 x 10^-14 always holds. Knowing any one of pH, pOH, [H+] or [OH-] lets you find the other three.
Worked example
A solution has [H+] = 2.5 × 10⁻⁵ M. Find its pH and pOH.
pH = −log10(2.5 × 10⁻⁵) = 4.602.
pOH = 14 − 4.602 = 9.398 -- since pH is under 7, this solution is acidic.
Common mistake
Forgetting the negative sign, or forgetting that pH and [H+] move in opposite directions -- a HIGHER [H+] (more acidic) always gives a LOWER pH number, not a higher one.
Keep going
- pH tells you the acidity of one solution; a titration uses a whole curve of pH values to find an unknown solution's concentration by neutralizing it with a known one. Titration Calculator
Frequently Asked Questions
What does pH actually measure?
pH measures the concentration of hydrogen ions [H+] in a solution, on a logarithmic scale: pH = −log10[H+]. Because it's logarithmic, each single pH unit represents a 10x change in [H+] -- a solution at pH 3 has 10 times more H+ than one at pH 4.
Why is the pH scale usually 0 to 14?
0 to 14 covers the most common range for aqueous solutions at 25°C, where pH + pOH = 14 (since Kw, water's self-ionization constant, is 1.0 x 10^-14 at that temperature). Very strong acids or bases can technically fall outside 0-14, but most everyday and lab solutions don't.
Why does pOH = 14 - pH only hold at 25°C?
Because that relationship comes from Kw = [H+][OH-] = 1.0 x 10^-14, and Kw itself changes with temperature (it's larger at higher temperatures). At 25°C specifically, 14 is the right number; at other temperatures the true neutral pH shifts slightly.
What pH is neutral?
7, at 25°C -- the point where [H+] and [OH-] are exactly equal. Below 7 is acidic (more H+ than OH-); above 7 is basic/alkaline (more OH- than H+).
Is this calculator meant for medical or health use?
No -- this is a general chemistry pH calculation for coursework and lab reference, not medical advice about blood, skin, or any biological pH.
Related Calculators
- Molarity Calculator — Solutions & Concentration
- Dilution Calculator — Solutions & Concentration
- Molality Calculator — Solutions & Concentration
- Molar Mass Calculator — Atomic & Molecular