🧪 pH Calculator
Convert between pH and hydrogen ion concentration [H+]. Get pH, pOH, [OH-] and the acid-base classification in one click — fully offline.
Enter [H+] in mol/L (scientific notation OK) or a pH between 0 and 14.
What is this tool?
The pH scale measures how acidic or basic a solution is, running from 0 (extremely acidic) to 14 (extremely basic), with 7 as neutral. Technically, pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H+], where [H+] is expressed in moles per litre (mol/L). A solution with [H+] of 1×10⁻⁷ mol/L therefore has a pH of exactly 7, while one with 1×10⁻³ mol/L has a pH of 3.
This tool works in both directions. Enter a hydrogen ion concentration and it calculates pH, pOH and the hydroxide ion concentration [OH-]; enter a pH value and it works backwards to find [H+] and [OH-]. Every result includes an acid-base classification — strongly acidic, acidic, neutral, basic or strongly basic — so you can interpret the number immediately without memorising the scale.
Everything is computed locally in your browser. Nothing is uploaded, no account is needed, and the tool works on any device. It is handy for chemistry homework, lab prep, aquarium and pool care, gardening soil checks, and any situation where you need a quick pH conversion without a spreadsheet or a scientific calculator.
For related calculations, try the log calculator, the scientific calculator and the molecular weight calculator to explore more useful tools.
How it works
The calculator applies two core relationships from acid-base chemistry. The first is the definition of pH itself: pH = −log₁₀[H+]. The second is the water ion product: at 25 °C, [H+] × [OH-] = 1×10⁻¹⁴, which also means pH + pOH = 14. From these two equations the tool derives every output.
When you enter [H+] (for example 2.5×10⁻⁴ mol/L), it computes pH = −log₁₀(2.5×10⁻⁴) ≈ 3.60, then pOH = 14 − 3.60 = 10.40, and [OH-] = 1×10⁻¹⁴ ÷ 2.5×10⁻⁴ ≈ 4×10⁻¹¹ mol/L. When you enter a pH instead, the same relationships are solved in reverse: [H+] = 10⁻ᵖᴴ and [OH-] = 10^(pH−14).
A useful derived metric is the hydroxide concentration, which is often forgotten but directly useful in titration and wastewater work. The classification threshold follows the standard convention: pH < 3 is strongly acidic, 3–7 acidic, exactly 7 neutral, 7–11 basic, and > 11 strongly basic. Note that the tool assumes room temperature (25 °C), where the neutral point is 7; at other temperatures the neutral pH shifts slightly.pH Scale Visual
pH scale from 0 to 14. See where common substances fall.
Common Substance pH Reference Table
Typical pH values for everyday substances at 25 °C. Individual samples vary depending on brand and freshness.
| Substance | Approximate pH | Classification |
|---|---|---|
| Battery acid | 0–1 | Strongly acidic |
| Stomach acid | 1–2 | Strongly acidic |
| Lemon juice | 2–3 | Acidic |
| Vinegar | 2.5–3.5 | Acidic |
| Coffee | 4.5–5.5 | Acidic |
| Milk | 6.5–6.8 | Weakly acidic |
| Pure water | 7.0 | Neutral |
| Blood | 7.35–7.45 | Basic |
| Seawater | 7.8–8.3 | Basic |
| Baking soda solution | 8–9 | Basic |
| Soap | 9–10 | Basic |
| Ammonia cleaner | 11–12 | Strongly basic |
| Lye (drain cleaner) | 13–14 | Strongly basic |
pH Scale Classification Table
Standard pH classification bands used by most chemistry curricula.
| pH Range | Classification | Example |
|---|---|---|
| 0–3 | Strongly acidic | Battery acid |
| 3–7 | Acidic | Lemon juice, coffee |
| 7 | Neutral | Pure water |
| 7–11 | Basic | Seawater, soap |
| 11–14 | Strongly basic | Ammonia, lye |
How to use
- Choose the direction: convert [H+] to pH, or pH back to [H+].
- Type the value in the input box (scientific notation like 2.5e-4 is accepted).
- Click Calculate.
- Read the pH, pOH, [OH-] and classification results.
- Adjust the input and recalculate as needed.
Frequently Asked Questions
What does pH stand for?
pH stands for "potential of hydrogen" (or "power of hydrogen") and is a logarithmic measure of hydrogen ion activity in a solution. It was introduced by Danish chemist Søren Sørensen in 1909.
Why is pH on a 0-14 scale?
The scale comes from the water ion product: at 25 °C, [H+] × [OH-] = 1×10⁻¹⁴. Taking negative logarithms gives pH + pOH = 14, so the practical range for aqueous solutions runs from 0 to 14. Extremely concentrated acids or bases can fall outside this range.
Can I enter scientific notation?
Yes. Type values like 1e-7, 2.5e-4 or 3.16e-11 directly — the calculator parses standard scientific notation, which is the natural way to express hydrogen ion concentrations.
Why is my result classified as strongly acidic?
The classification uses the common convention: pH below 3 is strongly acidic, 3 to 7 acidic, exactly 7 neutral, 7 to 11 basic, and above 11 strongly basic. This matches most general chemistry textbooks.
Does temperature affect the result?
The calculation assumes 25 °C, where pH + pOH = 14 and neutral is 7. At higher temperatures the neutral pH drops slightly (about 6.8 at 50 °C), so results are most accurate for room-temperature solutions.
What is the difference between pH and pOH?
pH measures hydrogen ion concentration; pOH measures hydroxide ion concentration. They always add up to 14 at 25 °C, so knowing one instantly gives you the other.
Tips & Advice
Use scientific notation for very small concentrations — entering 1e-7 is the same as 1×10⁻⁷ mol/L, and it is far less error-prone than typing 0.0000001. Remember that each whole pH number represents a tenfold change: a solution at pH 4 has ten times the hydrogen ions of one at pH 5. For pool or aquarium water, always measure temperature too, because the neutral point drifts below 7 when water gets warmer than 25 °C. When diluting acids, add acid to water, never the reverse, and use the [OH-] output to sanity-check your titration calculations. The classification band (strongly acidic vs acidic) follows the common 3/7/11 convention, but some textbooks use slightly different cut-offs — check which one your course expects.
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Sources & References
- IUPAC. Compendium of Chemical Terminology (Gold Book) - pH definition. goldbook.iupac.org.
- US EPA. pH - Water Quality Monitoring. epa.gov.
Last reviewed: August 2026.
Limitations
This tool provides pH conversions for informational and educational purposes. It does not measure the pH of a real sample — only converts between concentration and pH values you supply.
What this tool does not account for:
- Temperature effects: the calculation assumes 25 °C, where pH + pOH = 14. Real solutions at other temperatures have slightly different neutral points.
- Activity coefficients: strong acids and bases at high concentration deviate from ideal behaviour, so the true pH may differ from the calculated value.
- Measurement errors: laboratory pH meters require calibration and give more reliable results than calculated values for real samples.
Always verify critical pH results with a calibrated meter or a qualified professional.