pH Calculator
Calculate pH, pOH, [H+] and [OH-] for any solution — strong acids, strong bases, weak acids, weak bases, and Henderson-Hasselbalch buffers — with full step-by-step working.
Pick a calculation, then enter the known values.
pH value
Formula used: pH = -log10[H+]
10
pOH value
1.000 x 10^-4 M
[H+] concentration
1.000 x 10^-10 M
[OH-] concentration
14
pH + pOH check
Interactive pH Scale
See exactly where your solution sits on the universal 0–14 pH scale, next to everyday substances.
Step-by-Step pH Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: [H+] = 0.0001 mol/L
Step 1: Start with the pH formula
pH is defined as the negative base-10 logarithm of hydrogen ion concentration in mol/L.
pH = -log10[H+]Step 2: Substitute the hydrogen ion concentration
pH = -log10(1.000 x 10⁻⁴)Step 3: Result
pH = 4
Calculated pH:
4
pH Calculator: Work Out pH, pOH and Ion Concentration in Seconds
This pH calculator is built for chemistry students, lab technicians, pharmacy staff, aquarium and pool owners, food scientists, and anyone who needs a fast, correct pH value. It covers seven common situations in one tool: pH from hydrogen ion concentration, pH from hydroxide ion concentration, strong acid pH, strong base pH, weak acid pH using Ka, weak base pH using Kb, and buffer pH with the Henderson-Hasselbalch equation.
Every result comes with the pOH, the [H+] and [OH-] concentrations, a plain-language reading of whether the solution is acidic, neutral or basic, and a full written solution so you can see exactly how the number was reached, not just the final answer. Pick the calculation that matches what you know, type in your numbers, and the tool does the rest instantly, right in your browser.
What Is pH, in Plain Language?
pH tells you how acidic or basic a water-based solution is. It is short for 'potential of hydrogen', and it is really just a convenient way of writing down the concentration of hydrogen ions (H+) in a solution without dealing with tiny decimal numbers. Pure water at room temperature has a hydrogen ion concentration of 0.0000001 mol per litre, which is awkward to write and compare, so chemists take the negative log of that number instead and get a clean, simple value of 7.
The pH scale usually runs from 0 to 14. A pH below 7 means there are more hydrogen ions than in pure water, so the solution is acidic. A pH above 7 means there are fewer hydrogen ions and more hydroxide ions, so the solution is basic, also called alkaline. A pH of exactly 7 is neutral, meaning hydrogen and hydroxide ions are perfectly balanced.
The pH Formula: pH = -log10[H+]
The core formula behind every pH calculation is pH = -log10[H+], where [H+] is the hydrogen ion concentration measured in moles per litre (mol/L or M). Because concentrations are usually very small numbers written in scientific notation, taking a negative logarithm turns them into an easy, human-friendly number, usually somewhere between 0 and 14.
For example, if a solution has [H+] = 1 x 10^-3 mol/L, the pH is -log10(0.001), which equals 3. A smaller exponent (a larger negative power, meaning more hydrogen ions) gives a lower pH, and a bigger exponent (fewer hydrogen ions) gives a higher pH. The formula also works in reverse: if you already know the pH, you can get the hydrogen ion concentration back with [H+] = 10^(-pH).
pOH and Why pH + pOH Always Equals 14
pOH works exactly the same way as pH but measures hydroxide ion concentration instead: pOH = -log10[OH-]. In any water-based solution at 25 degrees Celsius, the product of hydrogen and hydroxide ion concentrations is a constant called Kw, equal to 1.0 x 10^-14. Taking the negative log of both sides of that relationship gives a very handy shortcut: pH + pOH = 14.
This means that once you know one value, you instantly know the other. If a solution has a pOH of 5, its pH must be 9, which places it firmly on the basic side of the scale. The calculator uses this relationship automatically whenever you enter a hydroxide concentration, so you always see both pH and pOH together, along with the matching [H+] and [OH-] values.
How to Calculate the pH of a Strong Acid
Strong acids such as hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4) dissociate completely in water. That means essentially every molecule breaks apart into ions, so the hydrogen ion concentration is simply the acid's molar concentration multiplied by the number of acidic hydrogens it releases, written as [H+] = n x C.
A monoprotic acid like HCl releases one H+ per molecule, so a 0.01 M HCl solution gives [H+] = 0.01 M and a pH of 2. A diprotic acid like H2SO4 releases two H+ ions per molecule in its first strong dissociation step, so a 0.01 M H2SO4 solution is treated as [H+] = 0.02 M for this calculation, giving a slightly lower, more acidic pH. Choose the correct number of acidic hydrogens in the calculator and it applies this rule for you.
How to Calculate the pH of a Strong Base
Strong bases like sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2) also dissociate completely, releasing hydroxide ions directly into solution. The hydroxide concentration is [OH-] = n x C, where n is the number of hydroxide ions each formula unit contributes. NaOH gives one OH- per unit, while Ca(OH)2 gives two.
Once [OH-] is known, the calculator finds pOH with pOH = -log10[OH-], then converts that into pH using pH = 14 - pOH. For example, a 0.001 M NaOH solution gives [OH-] = 0.001 M, a pOH of 3, and therefore a pH of 11, which correctly lands on the basic end of the scale.
Weak Acid pH: Using Ka and the ICE Table
Weak acids, such as acetic acid (found in vinegar) or citric acid, only partly dissociate in water. Instead of assuming full ionization like a strong acid, chemists use the acid dissociation constant, Ka, together with an ICE table (Initial, Change, Equilibrium) to find how much of the acid actually breaks apart.
Setting Ka equal to x squared divided by (C minus x), where x is the amount that ionizes, leads to the quadratic equation x^2 + Ka.x - Ka.C = 0. Solving this quadratic exactly, rather than relying on the shortcut approximation x = square root of (Ka multiplied by C), keeps the answer accurate even for more concentrated or more strongly dissociating weak acids. This calculator always solves the full quadratic, then reports both the resulting pH and the percentage of the acid that actually ionized.
Weak Base pH: Using Kb
Weak bases work the same way in reverse. A weak base such as ammonia (NH3) reacts partially with water to form its conjugate acid and hydroxide ions, and the extent of that reaction is described by the base dissociation constant, Kb. The same ICE table logic applies, giving the quadratic x^2 + Kb.x - Kb.C = 0, where x is the resulting hydroxide ion concentration.
Once [OH-] is found, the calculator converts it to pOH and then to pH using the same pH + pOH = 14 relationship used for strong bases. A typical 0.1 M ammonia solution, with a Kb close to 1.8 x 10^-5, comes out to a pOH near 2.87 and a pH close to 11.13, which matches what you would find in a standard chemistry textbook.
Henderson-Hasselbalch Equation for Buffer pH
A buffer solution contains a weak acid together with its conjugate base (or a weak base with its conjugate acid), and it resists large swings in pH when small amounts of acid or base are added. The fastest way to find the pH of a buffer is the Henderson-Hasselbalch equation: pH = pKa + log10([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the remaining weak acid.
This equation shows something useful on its own: when the conjugate base and weak acid concentrations are equal, the ratio is 1, log10(1) is 0, and the buffer's pH simply equals its pKa. That is exactly why buffers are usually chosen so that the desired working pH sits close to the pKa of the acid being used, giving the buffer the strongest possible resistance to pH change in both directions.
The pH Scale in Everyday Life
pH shows up everywhere outside the laboratory. Lemon juice sits around pH 2.2, vinegar around 2.5, black coffee near 5, and rainwater is naturally slightly acidic at about 5.6 because of dissolved carbon dioxide. Milk is close to neutral at around 6.7, pure water is exactly 7, and human blood is tightly held between 7.35 and 7.4, since even small shifts outside that narrow range can seriously affect health.
On the basic end, seawater sits around 8.1, baking soda solution near 9, household ammonia around 11.5, and bleach close to 12.6. Understanding where a substance falls on this scale matters for things like keeping fish tank water safe for aquatic life, adjusting garden soil for the plants you are growing, balancing swimming pool chemistry, and monitoring blood, urine, or saliva pH in medical and biology settings.
Common Mistakes to Avoid When Calculating pH
The most frequent mistake is forgetting the negative sign in pH = -log10[H+], which flips an acidic result into a nonsensical negative pH or an impossible answer. Another common slip is mixing up pH and pOH, or forgetting that they only add up to 14 when the temperature is 25 degrees Celsius; at other temperatures, Kw changes and that relationship shifts slightly.
For weak acids and bases, a frequent error is using the strong-acid shortcut ([H+] = C) instead of properly applying Ka or Kb, which can overestimate acidity or basicity by a wide margin. It is also easy to enter a concentration in the wrong unit, for example milligrams per litre instead of moles per litre; pH formulas only work correctly with molar (mol/L) concentrations, so always convert other units first.
Why pH Calculations Matter in Real Work
Outside the classroom, pH calculations support real decisions. Agronomists test and adjust soil pH so crops can actually absorb nutrients, since many nutrients become unavailable to plants when soil is too acidic or too alkaline. Water treatment plants monitor and correct pH before releasing water back into rivers or supplying it to homes. Swimming pool operators keep pH between about 7.2 and 7.8 to protect swimmers' eyes and skin while keeping chlorine working effectively.
In medicine, blood gas analysis relies on precise pH readings to catch conditions like acidosis or alkalosis early. In food and drink production, pH affects taste, safety, and shelf life, which is why brewers, winemakers, and food manufacturers measure it at multiple stages. Whatever the setting, the underlying maths is the same logarithmic relationship this calculator applies, just pointed at a different real-world sample.
pH Calculator: Quick Reference Summary
Use pH = -log10[H+] when you know hydrogen ion concentration directly. Use pOH = -log10[OH-] together with pH = 14 - pOH when you know hydroxide ion concentration. For strong acids and bases, multiply concentration by the number of ionizable H+ or OH- ions per formula unit before applying the log formula. For weak acids and bases, solve the ICE-table quadratic using Ka or Kb rather than assuming full dissociation. For buffers, go straight to pH = pKa + log10([A-]/[HA]).
This free pH calculator is meant to support learning, lab planning, and everyday chemistry questions. Always double-check units, keep concentrations in mol/L, and remember the pH + pOH = 14 relationship 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 pH?
pH equals the negative base-10 logarithm of hydrogen ion concentration: pH = -log10[H+], where [H+] is measured in moles per litre.
How do I calculate pH from molarity?
For a strong acid, multiply the molar concentration by the number of acidic hydrogens to get [H+], then apply pH = -log10[H+]. For a weak acid, use its Ka value in the ICE-table quadratic instead of assuming full dissociation.
What is the relationship between pH and pOH?
At 25 degrees Celsius, pH and pOH always add up to 14, because [H+] multiplied by [OH-] equals the constant Kw = 1.0 x 10^-14.
What pH is considered neutral?
A pH of exactly 7 is neutral at 25 degrees Celsius. Below 7 is acidic and above 7 is basic (alkaline).
How do you find pH from pOH?
Subtract the pOH value from 14: pH = 14 - pOH. For example, a pOH of 4 gives a pH of 10.
What is the Henderson-Hasselbalch equation used for?
It quickly finds the pH of a buffer solution from the pKa of the weak acid and the ratio of conjugate base to weak acid concentration: pH = pKa + log10([A-]/[HA]).
Why is pH negative for very strong acids?
A calculated pH below 0 can happen with highly concentrated strong acids where [H+] is greater than 1 mol/L, since the logarithm of a number greater than 1 is positive, making -log10[H+] negative.
Can this calculator find the pH of a weak base like ammonia?
Yes. Choose the weak base mode, enter the concentration and Kb value, and the calculator solves the equilibrium quadratic to give pOH and pH.