pOH Calculator
Calculate pOH, pH, [OH-] and [H+] for any solution — strong bases, strong acids, weak bases, weak acids, and Henderson-Hasselbalch basic buffers — with full step-by-step working.
Pick a calculation, then enter the known values.
pOH value
Formula used: pOH = -log10[OH-]
9
pH value
1.000 x 10^-5 M
[OH-] concentration
1.000 x 10^-9 M
[H+] concentration
14
pOH + pH check
Interactive pOH Scale
See exactly where your solution sits on the 0–14 pOH scale, next to everyday substances.
Step-by-Step pOH Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: [OH-] = 0.00001 mol/L
Step 1: Start with the pOH formula
pOH is defined as the negative base-10 logarithm of hydroxide ion concentration in mol/L.
pOH = -log10[OH-]Step 2: Substitute the hydroxide ion concentration
pOH = -log10(1.000 x 10⁻⁵)Step 3: Result
pOH = 5
Calculated pOH:
5
pOH Calculator: Work Out pOH, pH and Hydroxide Concentration in Seconds
This pOH calculator is built for chemistry students, lab assistants, wastewater and water-treatment staff, cleaning-product formulators, and anyone who needs to know how basic (alkaline) a water-based solution really is. It covers seven common situations: pOH from hydroxide ion concentration, pOH from hydrogen ion concentration, strong base pOH, strong acid pOH, weak base pOH using Kb, weak acid pOH using Ka, and basic buffer pOH with the Henderson-Hasselbalch equation.
Every result shows pOH alongside the matching pH, the [OH-] and [H+] concentrations, a plain-language classification of how basic or acidic the solution is, and a complete written solution so you can follow exactly how the answer was reached. Pick the calculation that matches the information you already have, type in your numbers, and the result appears instantly with no sign-up and no software to install.
What Is pOH, and Why Does It Matter Alongside pH?
pOH measures how much hydroxide ion (OH-) is dissolved in a water-based solution. While pH gets most of the attention in school, pOH is just as important whenever the question is really about how basic something is, rather than how acidic it is. A strong lye solution, a bleach sample, or an ammonia-based cleaner is far more naturally described by its hydroxide concentration, which is exactly what pOH captures directly.
Because hydroxide ion concentrations in typical solutions are also tiny decimal numbers, pOH uses the same logarithmic trick as pH: instead of writing 0.00001 mol/L, you take the negative log and get a clean number, 5. A low pOH means a lot of hydroxide ions and a strongly basic solution. A high pOH means very few hydroxide ions, which actually points toward an acidic solution.
The pOH Formula: pOH = -log10[OH-]
The formula behind every direct pOH calculation is pOH = -log10[OH-], where [OH-] is the hydroxide ion concentration in moles per litre (mol/L). This is the mirror image of the pH formula, and it works the same way: a smaller, more negative exponent in the concentration (meaning more hydroxide ions present) gives a lower pOH, and fewer hydroxide ions push the pOH higher.
For instance, if [OH-] = 1 x 10^-4 mol/L, the pOH is -log10(0.0001), which equals 4. The formula reverses cleanly as well: once you know pOH, you can recover the hydroxide concentration with [OH-] = 10^(-pOH). This calculator performs both directions automatically depending on which value you enter.
How pOH and pH Connect Through Kw
In any aqueous solution at 25 degrees Celsius, hydrogen and hydroxide ion concentrations are linked by the water dissociation constant, Kw, where [H+] multiplied by [OH-] always equals 1.0 x 10^-14. Taking the negative logarithm of that relationship gives the shortcut every chemistry student eventually memorises: pH + pOH = 14.
This is genuinely useful, because it means you never need both a hydrogen-based and a hydroxide-based measurement separately. If you know pH, subtracting it from 14 gives pOH immediately, and the same works in reverse. This calculator uses that relationship automatically any time you provide a pH-based input instead of a direct hydroxide value, so you always see both numbers side by side.
How to Calculate the pOH of a Strong Base
Strong bases such as sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2) dissociate completely in water, so essentially all of the dissolved base breaks apart into ions right away. That makes the hydroxide concentration simple to find: [OH-] = n x C, where C is the molar concentration and n is the number of hydroxide ions each formula unit releases.
A base like NaOH releases one OH- per formula unit, so a 0.001 M NaOH solution gives [OH-] = 0.001 M and a pOH of 3. A base like Ca(OH)2 releases two OH- ions per formula unit, so the same molar concentration produces twice the hydroxide concentration and a correspondingly lower, more strongly basic pOH. Select the correct hydroxide count in the calculator and this is handled for you.
How to Calculate the pOH of a Strong Acid
Strong acids like hydrochloric acid (HCl) or sulfuric acid (H2SO4) also dissociate completely, but they release hydrogen ions rather than hydroxide ions, so the fastest path to pOH goes through pH first. The hydrogen ion concentration is [H+] = n x C, where n is the number of acidic hydrogens the acid can donate, giving pH = -log10[H+] as usual.
Once pH is known, pOH follows directly from pOH = 14 - pH. For example, a 0.01 M HCl solution gives [H+] = 0.01 M, a pH of 2, and therefore a pOH of 12, correctly showing that a strongly acidic solution has very little hydroxide ion present.
Weak Base pOH: Using Kb and the ICE Table
Weak bases, such as ammonia (NH3) or many amine compounds used in industry, only partially react with water, so the full-dissociation shortcut used for strong bases does not apply. Instead, chemists use the base dissociation constant, Kb, together with an ICE table (Initial, Change, Equilibrium) to work out exactly how much hydroxide is actually produced.
Setting Kb equal to x squared divided by (C minus x), where x represents the hydroxide produced, leads to the quadratic x^2 + Kb.x - Kb.C = 0. This calculator solves that quadratic exactly rather than using the shorter square-root approximation, which keeps the pOH accurate even for more concentrated weak base solutions or bases with a comparatively larger Kb.
Weak Acid pOH: Using Ka
For a weak acid such as acetic acid, the direct measurement chemists calculate first is [H+], using the acid dissociation constant Ka in the same style of ICE-table quadratic: x^2 + Ka.x - Ka.C = 0, where x is the hydrogen ion concentration produced. From there, pH follows from pH = -log10(x).
Once pH is known, converting to pOH is a single step: pOH = 14 - pH. This route matters because it is easy to assume a weak acid barely affects pOH at all, but even a modestly concentrated weak acid solution can shift pOH by several units compared with pure water, which is exactly why this mode solves the full equilibrium rather than skipping straight to an estimate.
Henderson-Hasselbalch for Basic Buffers
A basic buffer pairs a weak base with its conjugate acid, for example ammonia together with ammonium chloride, and resists large pOH swings when small amounts of acid or base are added. The quickest way to find its pOH is the basic form of the Henderson-Hasselbalch equation: pOH = pKb + log10([BH+]/[B]), where [BH+] is the conjugate acid concentration and [B] is the remaining weak base concentration.
When the conjugate acid and weak base concentrations are equal, the ratio becomes 1, log10(1) is 0, and the buffer's pOH simply equals its pKb. This is why basic buffers are usually built so the desired working pOH sits close to the pKb of the base in use, giving the strongest possible resistance to change in both directions.
The pOH Scale in Everyday Life
Looking at everyday substances through their pOH rather than their pH highlights how much hydroxide they actually contain. Drain cleaner, at a pH near 14, has a pOH close to 0, meaning it is packed with hydroxide ions. Bleach sits around pOH 1.4, ammonia around pOH 2.5, and baking soda solution around pOH 5, all firmly on the basic side.
Moving toward the middle, seawater is close to pOH 5.9, pure water sits at exactly pOH 7, and human blood, tightly controlled for health reasons, sits close to pOH 6.6. On the acidic side, milk is near pOH 7.3, black coffee near pOH 9, vinegar near pOH 11.5, and battery acid close to pOH 13.5, meaning it has almost no hydroxide ion present at all.
Common Mistakes to Avoid When Calculating pOH
A frequent mistake is treating pOH like a second, unrelated measurement instead of remembering it is directly tied to pH through pH + pOH = 14 at 25 degrees Celsius. Another common slip is applying the strong-base shortcut ([OH-] = C) to a weak base, which overstates how basic the solution really is, since a weak base only partially reacts with water.
It is also easy to lose track of the negative sign in pOH = -log10[OH-], turning a correctly basic result into a confusing negative pOH by mistake, or to enter a concentration in the wrong unit. The pOH formula only gives a correct answer when concentration is in mol/L, so always convert grams per litre, percent by mass, or parts per million into molarity first.
Why pOH Calculations Matter in Real Work
Outside the classroom, pOH is central to industries built around bases and hydroxides rather than acids. Soap and detergent manufacturers monitor pOH during saponification, where a strong base reacts with fats to make soap. Wastewater treatment operators track pOH to know exactly how much acid is needed to safely neutralize alkaline industrial discharge before it reaches a river or a public sewer system.
In construction, lime and cement slurries are strongly basic, and pOH monitoring helps confirm a mix is behaving as expected during curing. Agricultural lime treatments used to correct overly acidic soil are dosed based on target pH and pOH calculations, and household cleaning product formulators check pOH to balance cleaning power against safety for skin and surfaces. In every case, the same logarithmic hydroxide relationship this calculator applies is doing the underlying work.
pOH Calculator: Quick Reference Summary
Use pOH = -log10[OH-] whenever hydroxide ion concentration is known directly. Use pH = -log10[H+] together with pOH = 14 - pH whenever only hydrogen ion concentration or pH is available. For strong bases and acids, multiply concentration by the number of ionizable OH- or H+ ions per formula unit before applying the log formula. For weak bases and acids, solve the ICE-table quadratic using Kb or Ka rather than assuming complete ionization. For basic buffers, go straight to pOH = pKb + log10([BH+]/[B]).
This free pOH calculator is intended to support learning, lab planning, and everyday chemistry questions involving alkalinity. Always keep concentrations in mol/L and remember that pH + pOH = 14 applies specifically at 25 degrees Celsius. For safety-critical, regulated, clinical, or industrial work, confirm results with validated lab instruments and your organisation's approved procedures.
Frequently Asked Questions
What is the formula for pOH?
pOH equals the negative base-10 logarithm of hydroxide ion concentration: pOH = -log10[OH-], where [OH-] is measured in moles per litre.
How do I convert pH to pOH?
Subtract pH from 14: pOH = 14 - pH. This works at 25 degrees Celsius, where pH + pOH always equals 14.
How do I calculate pOH from molarity?
For a strong base, multiply the molar concentration by the number of hydroxide ions it releases to get [OH-], then apply pOH = -log10[OH-]. For a weak base, use its Kb value in the ICE-table quadratic instead of assuming full dissociation.
What pOH is considered neutral?
A pOH of exactly 7 is neutral at 25 degrees Celsius, matching a neutral pH of 7. Below 7 is basic (alkaline) and above 7 is acidic.
What is the Henderson-Hasselbalch equation for a basic buffer?
pOH = pKb + log10([BH+]/[B]), where [BH+] is the conjugate acid concentration and [B] is the weak base concentration.
Why does a lower pOH mean a more basic solution?
A lower pOH corresponds to a higher hydroxide ion concentration, since pOH = -log10[OH-]. More hydroxide ions in solution mean stronger basicity, so the pOH number goes down as basicity goes up.
Can this calculator find the pOH of a weak acid?
Yes. Choose the weak acid mode, enter the concentration and Ka value, and the calculator solves the equilibrium quadratic for pH first, then converts it to pOH automatically.
Is pOH used as often as pH in real laboratories?
pH is more commonly reported, but pOH is used directly whenever a solution's basicity or hydroxide content is the main concern, such as in soap making, wastewater neutralization, or lime and cement work.