pH Calculator

Find pH, pOH, [H+] and [OH-] for chemistry homework and lab problems. Pick the value you already know, type it in — 1e-3 and 10^-3 both work — and press Calculate pH. You get all four values, an acidic / neutral / basic classification, and the formula with each step of the working. Everything runs in your browser at 25 °C, where pH + pOH = 14. No account, no upload, nothing saved.

pH and pOH formulas

The pH scale measures acidity using hydrogen ion concentration. Common formulas include:

• pH = -log10([H+])
• pOH = -log10([OH-])
• pH + pOH = 14 at 25 °C
• [H+] = 10^-pH
• [OH-] = 10^-pOH

The two logarithmic formulas run in one direction and the two powers of ten run back the other way, which is the whole trick to this topic: any one of the four values fixes the other three. Concentrations are in moles per litre, so M and mol/L mean the same thing and can be used interchangeably.

The relationship pH + pOH = 14 comes from the ion product of water, Kw = [H+] × [OH-] = 1.0 × 10^-14 at 25 °C. Taking negative logarithms of both sides turns that product into a sum, which is why subtraction from 14 is all that stands between pH and pOH. Kw changes with temperature, so the sum is only exactly 14 at 25 °C — this calculator assumes that standard condition throughout.

The calculator above applies these formulas directly. It parses the value you type, converts it, classifies the solution, and prints each line of working. The arithmetic is ordinary logarithms in your browser and no AI model is involved, so the same input always returns the same answer.

Acidic, basic, and neutral solutions

A solution is acidic when pH is below 7, neutral around 7, and basic when pH is above 7 at 25 °C. Very small changes in pH can represent large changes in hydrogen ion concentration because the pH scale is logarithmic.

One pH unit is a factor of ten in [H+]. A solution at pH 3 holds ten times the hydrogen ion concentration of one at pH 4 and a hundred times that of one at pH 5, which is why lemon juice near pH 2 and black coffee near pH 5 feel so different despite sitting only three units apart on the scale.

Neutral means [H+] = [OH-], not simply "pH 7". At 25 °C both concentrations equal 1.0 × 10^-7 M and the pH does come out at 7, but the defining condition is the equality, not the number. Because decimal arithmetic can turn an exactly neutral solution into 6.999999, the calculator treats anything that displays as 7.00 as neutral rather than reporting it as a very weak acid.

Values outside 0-14 are unusual but not wrong. Concentrated strong acids and bases genuinely produce negative pH or pH above 14, so the calculator shows a warning and still returns the result instead of refusing to answer.

What the pH calculator accepts

Enter the value in whichever form your textbook prints it:

• Plain decimals — 0.001, 4.5, 7
• Scientific notation — 1e-3, 2.5e-9
• The textbook power-of-ten form — 10^-3, 1.0 × 10^-3

pH and pOH accept any number, including negatives. Concentrations must be greater than zero: a concentration of 0 or a negative concentration has no logarithm, so the calculator rejects it with a message rather than printing something meaningless. Empty and unreadable fields are never silently calculated from.

This tool covers strong acid and strong base problems where the ion concentration is already known or given. Weak acid and weak base equilibria, buffers and Henderson-Hasselbalch, titration curves, and temperature-adjusted Kw are outside its scope — for those, work the problem through with the AI chemistry solver.