Softonic

pH Calculator

Step 1 / 3 pH
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pH is defined from hydrogen-ion activity; this calculator uses the entered molar concentration as a practical approximation. Choose one of three conversions: pH from [H⁺], pH from [OH⁻], or [H⁺] from pH. Every mode also displays pOH and both ion concentrations, using pH + pOH = 14 at 25 °C.

How to calculate pH

  1. 1

    Pick the conversion

    Choose pH from [H⁺], pH from [OH⁻], or [H⁺] from pH.

  2. 2

    Enter one value

    Enter ion concentration in mol/L or enter the pH value. Concentrations must be positive.

  3. 3

    See the result

    pH to 2 decimal places, plus pOH and [H+] / [OH-] equivalents.

  4. 4

    Check the interpretation

    A broad acidity or alkalinity description appears beside the numeric results.

Core pH equations

Quantity Formula
pH from [H+] pH = -log10([H+])
[H+] from pH [H+] = 10^-pH
pH + pOH at 25 degC pH + pOH = 14.00
pOH from [OH-] pOH = -log10([OH-])
[OH-] from pOH [OH-] = 10^-pOH

The calculator treats the entered concentration as the ion concentration itself. It does not derive [H⁺] or [OH⁻] from an acid/base formula, total concentration, Ka, or Kb.

pH reference points at 25 degC

Substance Approx pH
1 M HCl 0.00
Stomach acid 1.5-3.5
Lemon juice 2.0-2.6
Coffee 4.5-5.5
Pure water 7.00
Blood 7.35-7.45
Seawater ~8.1
Household ammonia 11-12
1 M NaOH 14.00

Limits of these conversions

  • Temperature other than 25 degC. Kw changes with temperature, so “neutral” pH drifts: at 50 degC, neutral is about 6.63, not 7.00. This calculator assumes 25 degC.
  • Acid/base dissociation. The calculator has no Ka, Kb, formula or total-concentration input, so it does not solve strong, weak or polyprotic acid/base equilibria.
  • Very dilute solutions. It applies the entered ion concentration directly and does not solve water autoionisation or a full charge balance.
  • Activity vs concentration. In high ionic strength, a_H+ differs from [H+]. For exact lab work use an activity coefficient.

Why the log scale matters

A pH change from 5 to 4 is a 10x increase in [H+]. From 5 to 3 is 100x. Ocean acidification moving from pH 8.2 to 8.0 sounds small but is a ~58% increase in hydrogen ion concentration. Always think exponential when comparing pH values.

Frequently Asked Questions

No. Enter a known [H⁺] or [OH⁻] concentration, or a known pH. The calculator does not solve acid/base dissociation equilibria.

No - the calculator assumes 25 degC (Kw = 1.0e-14). Body temperature pH calculations or high-temperature chemistry need a Kw adjustment, which is not applied here.

No. The formulas can produce values below 0 or above 14 for sufficiently large entered ion concentrations. The familiar 0–14 range is a common reference, not a hard mathematical limit.

The values are submitted to the site component to calculate and display the result. Do not use this educational converter as a substitute for calibrated laboratory measurement.

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