Education
pH Calculator
Pick a mode and enter what you know. The calculator returns pH, pOH, [H+], and [OH-] for the same solution using pH = -log10[H+] and the 25 C identity pH + pOH = 14.
Mode
Use scientific notation like 1e-7 for very small values. · e.g. 1e-4 or 0.0001
Assumptions
- Aqueous solutions at 25 C, where Kw = 1.0 x 10^-14 and pH + pOH = 14.
- Strong acid/strong base behavior unless your input already reflects the equilibrium concentration of H+ or OH-.
- Activity coefficients are assumed to be 1 (ideal dilute solution).
Educational only. Not lab, medical, or water-quality advice.
pH
4.00
Acidic
At 25 C, Kw = [H+][OH-] = 10^-14. So pH + pOH = 14 only at 25 C; at other temperatures the constant shifts and so does the sum.
Examples
[H+] = 1e-4 mol/L
pH = 4 (acidic)
pH = 8.5
[H+] ≈ 3.16 x 10^-9 mol/L
[OH-] = 1e-3 mol/L
pOH = 3, pH = 11 (basic)
pH = 5
pOH = 9 (at 25 C)
How it works
The pH definition runs in both directions: take a base-10 log to go from concentration to pH, raise 10 to a negative power to go back. The pOH side uses the same relations, and at 25 C the two are tied together by the water ion product.
pH · pH = -log10[H+]
Reverse · [H+] = 10^(-pH)
25 C identity · pH + pOH = 14
Related chemistry calculators
- Molarity calculator for the M, n, V, m relations that produce the H+ or OH- concentrations.
- Molality calculator when you need a temperature-independent concentration measure.
- Scientific calculator for the log and exponent steps without leaving the formula.
- Log calculator for base-10 logs of any positive number.
- All education calculators.
Frequently asked questions
pH is the negative base-10 logarithm of the hydrogen ion concentration in a solution. Lower pH means more H+ and a more acidic solution; higher pH means less H+ and a more basic (alkaline) solution. pH 7 is neutral at 25 C.
pOH is the negative base-10 logarithm of the hydroxide ion concentration. At 25 C, pH + pOH = 14 because the water ion product Kw is 1.0 x 10^-14. Knowing one of pH or pOH gives you the other.
Use scientific notation in the [H+] or [OH-] field, like 1e-4 for 0.0001 or 3.2e-9 for 3.2 x 10^-9. The calculator parses both decimal and scientific notation.
Yes, in concentrated strong acid or strong base solutions, but the standard 0 to 14 scale assumes dilute aqueous behavior at 25 C. Outside that range the activity coefficient stops being close to 1 and the simple formula loses accuracy.
Only at 25 C. The relationship comes from Kw = [H+][OH-] = 10^-14 at that temperature. Kw rises with temperature, so neutral pH drops below 7 in hot water and pH + pOH drops below 14.
Not in this version. Weak-acid calculations need Ka, the initial concentration, and an ICE table. This page converts among pH, pOH, [H+], and [OH-] given one of them.
Related calculators
Education
Final Grade Calculator
Calculate your final course grade from your current grade and your final exam score.
Education
Weighted Grade Calculator
Combine assignments, quizzes, and exams by weight to get a precise weighted course average.
Education
Final Exam Calculator
Find out exactly what you need to score on your final exam to hit your target course grade.