pH Calculator
This pH calculator finds the pH of a solution from its hydrogen ion concentration, or works backward from a pH to the concentration. Pick what you know, type the value, and you'll get the pH, the pOH, both ion concentrations, and whether it's acidic, neutral, or basic. Everything updates as you type, so it's quick for class or the lab. It's free and works with nothing to install, so your entries are never sent off.
- pH and pOH
- Both ion concentrations
- Acidic or basic
- Either direction
- pH scale chart
Last updated July 27, 2026 pH = minus log H+ Written by Muhammad Younus
Enter a hydrogen ion concentration above zero, or a pH value.
Enter the hydrogen ion concentration in moles per liter, like 0.001 or 1e-3. The pOH and hydroxide assume water at 25 degrees Celsius, so you don't have to find the pOH yourself, since it's just 14 minus the pH.
To calculate pH, take the negative base-10 logarithm of the hydrogen ion concentration: pH = -log₁₀[H+]. To work backward from a pH, raise 10 to the negative pH: [H+] = 10-pH. For the basic side, use pOH = 14 - pH and [OH-] = 10-pOH. Example: a pH of 3 means [H+] = 0.001 mol/L, a pOH of 11, and [OH-] = 1e-11 mol/L, so the solution's acidic.
How do you calculate pH?
pH measures how acidic or basic a solution is, and it's the negative base-10 logarithm of the hydrogen ion concentration. In plain terms, pH equals minus log of the H plus concentration in moles per liter. So if a solution has a concentration of 0.001 moles per liter, which is 1 times 10 to the minus 3, the log is minus 3 and the pH is 3, an acid. To run it the other way, you raise 10 to the negative pH, so a pH of 3 gives back 0.001. The scale usually sits between 0 and 14, with 7 neutral, lower numbers acidic, and higher numbers basic. Each whole step is a tenfold change, so a pH of 4 holds ten times the hydrogen ions of a pH of 5. Water also ties pH to pOH, the same idea built on the hydroxide concentration, and at room temperature the two always add to 14. That means one measurement gives you everything: pH, pOH, and both ion concentrations. This tool runs the logarithm or the power for you, so you'll have the answer instantly and you don't need a log table, and it fills in the partner values and labels the solution the moment you type a number. That's the whole pH scale in one tap.
Working out pH, step by step
Here's the routine for a 0.001 mol/L solution, and it's just three steps:
- 1
Pick what you knowYou have the concentration, so choose that mode.
- 2
Take the logMinus log of 0.001 is 3, so the pH is 3, an acid.
- 3
Read the partnerspOH is 14 minus 3, which is 11, and [OH-] is 1e-11.
The pH scale
Here's where everyday liquids land on the scale. You'll see how far apart an acid and a base sit, with neutral water right in the middle, and that's the dividing line.
| pH | Example | Type |
|---|---|---|
| 2 | Lemon juice | Acidic |
| 5 | Black coffee | Acidic |
| 7 | Pure water | Neutral |
| 8 | Baking soda | Basic |
| 13 | Bleach | Basic |
Finding [H+], [OH-], and pOH from a pH
The tool runs in both directions, so a pH on its own gives you every partner value. To get the hydrogen ion concentration, raise 10 to the negative pH, so a pH of 3 returns 0.001 mol/L. For the hydroxide side, subtract the pH from 14 for the pOH, then raise 10 to the negative pOH, so that same pH of 3 gives a pOH of 11 and an [OH-] of 1e-11 mol/L. If you're starting from moles and a volume instead of a concentration, find the moles or molarity first, then read the pH here.
| pH | [H+] mol/L | pOH | [OH-] mol/L | Type |
|---|---|---|---|---|
| 1 | 0.1 | 13 | 1e-13 | Acidic |
| 3 | 0.001 | 11 | 1e-11 | Acidic |
| 7 | 1e-7 | 7 | 1e-7 | Neutral |
| 11 | 1e-11 | 3 | 0.001 | Basic |
| 13 | 1e-13 | 1 | 0.1 | Basic |
Working out a change in pH. A change in pH is just the final pH minus the starting one. Moving from a pH of 3 to a pH of 5 is a rise of 2, and since each unit is a tenfold step, that's a hundredfold drop in hydrogen ions. Even a single-unit shift already means ten times more or less acid, which is why small pH moves matter so much in a buffer or a titration.
Frequently asked questions
You choose what you know, then type one value and it fills in the rest. From a hydrogen ion concentration it takes the negative base-10 logarithm for the pH, and from a pH it raises 10 to the negative pH for the concentration. It also works out the pOH and the hydroxide concentration, since they're linked, and labels the solution acidic, neutral, or basic.
pH is the negative base-10 logarithm of the hydrogen ion concentration, written pH equals minus log of the H plus concentration in moles per liter. So a concentration of 0.001, which is 1 times 10 to the minus 3, gives a pH of 3. To go backward, the concentration is 10 to the negative pH. Because pH and pOH always add to 14 in water at room temperature, once you have one you have all four values.
You reverse the pH formula: the hydrogen ion concentration is 10 raised to the negative pH, written [H+] equals 10 to the power of minus pH, in moles per liter. So a pH of 3 gives 10 to the minus 3, which is 0.001 mol/L, and a pH of 5 gives 0.00001 mol/L. Switch this tool to Know the pH, type the pH, and it prints the [H+] for you, along with the pOH and the hydroxide concentration.
For a strong monoprotic acid like hydrochloric acid, the molarity is the hydrogen ion concentration, so the pH is just minus the log of the molarity. A 0.01 mol/L solution gives a pH of 2, and 0.001 mol/L gives a pH of 3. If you only know the moles and the volume, work out the molarity first, then drop it into the concentration box here.
Subtract the pOH from 14, because pH and pOH always add to 14 in water at 25 degrees Celsius. A pOH of 4 means a pH of 10, and a pOH of 11 means a pH of 3. That's handy when a base problem hands you the hydroxide side first, since you can find the pOH and flip it to the pH in one step.
The pH scale runs from 0 to 14 and measures how acidic or basic a water-based solution is. A pH of 7 is neutral, like pure water. Below 7 is acidic, and the lower it goes the stronger the acid, so lemon juice sits near 2. Above 7 is basic, also called alkaline, with bleach up near 13. Each whole step is a tenfold change in hydrogen ion concentration, so a pH of 4 is ten times as acidic as a pH of 5.
pOH is the partner of pH, the negative logarithm of the hydroxide ion concentration instead of the hydrogen one. In water at 25 degrees Celsius, pH and pOH always add up to 14, so a solution with a pH of 3 has a pOH of 11. It's handy when a problem gives you the hydroxide concentration of a base, since you can find pOH first and then subtract from 14 for the pH. This tool shows both at once.
Because the scale is logarithmic, not linear. The pH is built on a base-10 logarithm, so dropping the pH by one whole number multiplies the hydrogen ion concentration by ten. That's why a pH of 3 is ten times as acidic as 4 and a hundred times as acidic as 5. It lets a single small number describe concentrations that span many orders of magnitude, which is exactly why chemists find the scale so useful.
A weak acid only partly ionizes, so you need its acid constant Ka, not just the concentration. A common estimate is that the pH is about minus the log of the square root of Ka times the concentration. For a 0.1 mol/L acetic acid solution, with a Ka near 1.8 times 10 to the minus 5, that lands close to pH 2.9. This calculator assumes a strong acid or a known hydrogen ion concentration, so for a weak acid you'd first estimate the [H+] and then enter that.
Related tools
More chemistry and math tools.
Molarity, dilution, pH, and more.
Molarity CalculatorMoles per liter of a solution.
Dilution CalculatorC1V1 = C2V2 made easy.
Studying acids and bases?
Find the pH above, or work out the molarity of a solution.