pOH Calculator
Find pOH from hydroxide concentration, or convert straight from pH using pH + pOH = 14. A live 3D beaker and charts show how OH⁻ density tracks pOH.
Reviewed by the ToolNestr Editorial Team — July 2026
Two ideas that trip students up
1. Low pOH means basic, not acidic
The blue beaker is a low pOH (basic — lots of OH⁻), and the red beaker is a high pOH (acidic — little OH⁻). It is the opposite of the pH color intuition many people expect.
2. pH and pOH always add to 14
Each marker sits on the line pH + pOH = 14 for a common substance. Move along the line in either direction and the two values trade off exactly.
pOH graphs
How it works
The core idea in one line: pOH measures how much hydroxide is dissolved in a solution — the mirror image of pH, linked to it by pH + pOH = 14 at 25°C.
pOH = −log₁₀[OH⁻]
pOH from hydroxide concentration
[OH⁻] = 10−pOH
hydroxide concentration from pOH
pH + pOH = 14
at 25°C, from Kw = [H⁺][OH⁻] = 1.0×10⁻¹⁴
Because pOH = −log₁₀[OH⁻], each one-unit drop in pOH means a tenfold increase in hydroxide concentration. And because pH + pOH = 14 at room temperature, you never need to measure both — knowing one gives you the other instantly.
Worked example 1 — from [OH⁻]
Given: A solution has [OH⁻] = 1.0 × 10⁻³ mol/L. Find the pOH.
A pOH of 3.00 is well below 7, so this is a strongly basic solution.
Worked example 2 — from pH
Given: A solution has pH = 9.5. Find the pOH and [OH⁻].
pOH, pH, [OH⁻] and [H⁺] for common substances
Approximate values at 25°C — real samples vary with concentration, temperature and source.
| Substance | pH | pOH | [OH⁻] (mol/L) | [H⁺] (mol/L) |
|---|---|---|---|---|
| Battery acid | ≈ 0.5 | ≈ 13.5 | ≈ 3.2 × 10⁻¹⁴ | ≈ 0.32 |
| Lemon juice | ≈ 2 | ≈ 12 | ≈ 1.0 × 10⁻¹² | ≈ 1.0 × 10⁻² |
| Pure water | 7 | 7 | 1.0 × 10⁻⁷ | 1.0 × 10⁻⁷ |
| Baking soda solution | ≈ 9 | ≈ 5 | ≈ 1.0 × 10⁻⁵ | ≈ 1.0 × 10⁻⁹ |
| Household ammonia | ≈ 11.5 | ≈ 2.5 | ≈ 3.2 × 10⁻³ | ≈ 3.2 × 10⁻¹² |
| Drain cleaner | ≈ 13.5 | ≈ 0.5 | ≈ 0.32 | ≈ 3.2 × 10⁻¹⁴ |
pOH = 14 − pH at 25°C. [OH⁻] and [H⁺] are back-calculated from pOH and pH respectively; [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴.
Where pOH actually matters
💧 Water treatment
Operators track both pH and pOH when dosing lime or caustic soda to raise alkalinity, since the hydroxide concentration directly drives coagulation and corrosion-control chemistry.
🌱 Agriculture & soil chemistry
Soil scientists use pOH alongside pH to understand hydroxide availability in alkaline soils, which affects micronutrient solubility (iron, manganese, zinc) and how well crops can absorb them.
💊 Buffer solutions in biology & medicine
Physiological buffers (like blood, pH ≈ 7.4) are described just as validly by their pOH ≈ 6.6. Pharmaceutical formulation uses both to keep drugs stable and compatible with tissue.
🏭 Industrial process control
Manufacturing processes that use strong bases — pulp bleaching, soap making, metal etching — monitor pOH because it is the more sensitive number when a solution is deep in basic territory.
Common misconceptions
"pH and pOH always sum to exactly 14."
Only at 25°C. Kw = [H⁺][OH⁻] is temperature-dependent, so −log(Kw) is not always 14. At 0°C the sum is about 14.94; at 60°C it is about 13.02. The "14" is a room-temperature convenience, not a universal constant.
"A solution can't have negative pOH or a pOH above 14."
It can — for very concentrated strong bases, [OH⁻] can exceed 1 mol/L, giving a negative pOH; for very concentrated strong acids, [OH⁻] can be smaller than 10⁻¹⁴ M, giving a pOH above 14. Values outside 0–14 are unusual but not impossible.
"Low pOH means acidic."
The opposite — a low pOH means a high [OH⁻], which is basic (alkaline). Low pH is what signals acidic; low pOH signals basic.
"pOH is a separate, independent measurement from pH."
At a given temperature they are two views of the same equilibrium, linked by Kw. Measuring one and knowing the temperature's Kw always lets you compute the other — you never need to measure both directly.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — acid-base equilibria, Kw and the pH/pOH scales (free, peer-reviewed). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 16, Acid–Base Equilibria.
- • Zumdahl & Zumdahl, Chemistry — the ion product of water and the pH/pOH relationship.
pOH = −log₁₀[OH⁻]; pH + pOH = 14 at 25°C only, since Kw is temperature-dependent. Results are rounded for display.
How to use this calculator
Pick the mode
"From [OH⁻]" solves pOH ⇄ [OH⁻]; "From pH" converts using pH + pOH = 14.
Enter one value
Type the concentration or the pH/pOH; the paired quantities solve live.
See it in the beaker
Use the slider to watch OH⁻ ion density track the pOH value.
Related tools
Frequently asked questions
What is pOH?
pOH is a measure of the hydroxide ion concentration of a solution: pOH = −log₁₀[OH⁻]. It works exactly like pH, but for [OH⁻] instead of [H⁺]. A low pOH means a high [OH⁻] — a strongly basic solution.
How does pOH relate to pH?
At 25°C, pH + pOH = 14. This comes from the ion product of water, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, and taking −log₁₀ of both sides: −log[H⁺] + −log[OH⁻] = −log(Kw) = 14. So pOH = 14 − pH and pH = 14 − pOH.
What is Kw and why does it matter here?
Kw is the ion product (autoionization constant) of water: Kw = [H⁺][OH⁻]. At 25°C, Kw = 1.0 × 10⁻¹⁴, which is where the "14" in pH + pOH = 14 comes from. Every aqueous equilibrium at that temperature must satisfy this product.
Why is it 14 specifically at 25°C?
Kw is temperature-dependent because water's autoionization is an equilibrium reaction with its own enthalpy change. At 25°C, Kw happens to equal 1.0 × 10⁻¹⁴, so −log(Kw) = 14. At 0°C, Kw is smaller (about 1.14 × 10⁻¹⁵, so pH+pOH ≈ 14.94); at 60°C, Kw is larger (about 9.6 × 10⁻¹⁴, so pH+pOH ≈ 13.02).
What are some common pOH values?
Pure water at 25°C is pOH 7 (neutral). Household ammonia is around pOH 2–2.5 (strongly basic). Baking soda solution is around pOH 5 (mildly basic). Drain cleaner (strong base) can be near pOH 0.5. Battery acid (strong acid) is near pOH 13.5.