pH is a logarithmic scale that measures the hydrogen ion concentration [H+] in a solution. It is defined as pH = -log₁₀([H+]). Values below 7 are acidic, 7 is neutral, and values above 7 are basic (alkaline) at 25 °C.

What is pOH and how does it relate to pH?

pOH measures the hydroxide ion concentration: pOH = -log₁₀([OH-]). At 25 °C, pH + pOH = 14 (the pKw of water), so pOH = 14 - pH. An increase in pH corresponds to a decrease in pOH.

What units should I use for [H+] and [OH-]?

Concentrations must be entered in mol/L (molarity, M). For example, pure water has [H+] = 1 × 10⁻⁷ mol/L, which gives pH 7. Use scientific notation (e.g., 1e-7) or decimal form.

Is the pH + pOH = 14 rule always valid?

This relationship holds at 25 °C (298 K) where Kw = 1 × 10⁻¹⁴. At higher temperatures Kw increases, so the neutral point and the sum change slightly. This calculator uses the 25 °C standard.

What happens if I enter a negative pH?

Negative pH values are physically possible for very concentrated strong acids (above 1 mol/L). This calculator accepts them, though they are outside typical lab conditions.

pH Calculator

Name: pH Calculator
Availability: InStock
Author: Nham Vu

Enter any one value — pH, pOH, [H+], or [OH-] — and instantly convert to all four related acid-base quantities.

Enter Any One Value

Fill in one field — all others are calculated instantly.

pOH

[H⁺] concentration (mol/L)

[OH^-] concentration (mol/L)

Enter a value to see results

Summary

Enter any one value — pH, pOH, [H+], or [OH-] — and instantly convert to all four related acid-base quantities.

How it works

Choose your known quantity: pH, pOH, [H+] concentration, or [OH-] concentration.
Type the value into the corresponding input field.
The calculator instantly derives and displays all other three values.
A color-coded scale shows where your solution falls on the acid-neutral-base spectrum.
Use the Reset button to clear all fields and start a new calculation.

Use cases

Convert a measured [H+] concentration into a pH reading for a lab report.
Quickly find [OH-] when only pH is given in a textbook problem.
Verify pOH calculations for chemistry homework or exam prep.
Identify whether a solution is acidic, neutral, or basic at a glance.
Check buffer solution properties by entering the target pH.
Explore how small changes in [H+] shift the logarithmic pH scale.
Support titration analysis by converting endpoint concentrations.
Cross-check values from a pH meter against theoretical predictions.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu