pH Calculator
Convert between [H⁺], pH, pOH, and [OH⁻] — pick a mode, enter the one value you know, and get the full acid–base picture at 25°C.
Example: with Conversion [H⁺] concentration → pH · Known value (mol/L) 0.001 → pH: 3.00 (acidic).
Computed by the calculator below using its default values. Change any input to see your own numbers.
🧪 Check any solution with lab pH strips
Check it outpH = −log₁₀[H⁺], and back again
pH compresses hydrogen-ion concentrations spanning many powers of ten into a 0–14 scale: pH = −log₁₀[H⁺]. For a strong acid like HCl, the molarity is the H⁺ concentration, so 0.001 M gives pH = −log(0.001) = 3.00. Reverse it with [H⁺] = 10⁻ᵖᴴ. Bases route through pOH: at 25°C, pH + pOH = 14, so 0.01 M NaOH has pOH 2 and pH 12.
For a weak acid, only a fraction dissociates, so use its Ka first: [H⁺] ≈ √(Ka × C). A 0.10 M acetic acid solution (Ka = 1.8 × 10⁻⁵) gives [H⁺] = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ mol/L — enter that in the concentration mode to get pH 2.87, far milder than the 1.00 a strong acid would give.
How it’s calculated
pH = −log₁₀[H⁺] with [H⁺] in mol/L; the inverse is [H⁺] = 10⁻ᵖᴴ. Hydroxide inputs convert via pOH = −log₁₀[OH⁻], and the scale ties together through the water ion product at 25°C: Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14. Solutions are labeled acidic below pH 7, neutral at 7, and basic above 7.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Plugging a weak acid's molarity straight into −log₁₀ — weak acids only partly dissociate, so compute [H⁺] from Ka first.
- Assuming pH + pOH = 14 at every temperature — that value is for 25°C; hot water is neutral near pH 6.6, not 7.
- Entering a percent or g/L concentration — the formula needs molarity (mol/L).
Frequently asked questions
How do I calculate pH from molarity?
For a strong monoprotic acid, pH = −log₁₀(molarity): 0.01 M HCl has pH 2.00. For a strong base, find pOH first: 0.01 M NaOH has pOH 2.00, so pH = 14 − 2 = 12.00.
How do I find the H+ concentration from pH?
Raise 10 to the negative pH: [H⁺] = 10⁻ᵖᴴ. A solution of pH 4.5 has [H⁺] = 10⁻⁴·⁵ ≈ 3.16 × 10⁻⁵ mol/L.
What is the pH of pure water?
7.00 at 25°C, because [H⁺] = 1.0 × 10⁻⁷ mol/L. It is temperature-dependent: near 50°C neutral water sits around pH 6.6, yet it is still neutral because [H⁺] = [OH⁻].
How do I calculate pH from Ka?
For a weak acid of concentration C, approximate [H⁺] = √(Ka × C), then take −log₁₀. For 0.10 M acetic acid (Ka 1.8 × 10⁻⁵) that gives 1.34 × 10⁻³ mol/L and pH 2.87.
Can pH be negative or above 14?
Yes. The 0–14 range just covers common solutions: 10 M HCl is about pH −1, and saturated NaOH exceeds 14. The formula itself has no limits.