About pH Calculator
Calculate pH, pOH, hydrogen ion concentration, and hydroxide ion concentration. Convert between all four values instantly with common substance presets.
How to use
- Select an input mode: enter a pH value, pOH value, hydrogen ion concentration [H+], or hydroxide ion concentration [OH-]. The calculator accepts any of the four as the starting point and computes the remaining three using the relationships pH + pOH = 14 and Kw = [H+][OH-] = 1×10⁻¹⁴ at 25°C.
- Type your value into the input field. For ion concentrations, use scientific notation (e.g., 1e-7 for pure water, 1e-3 for vinegar, 1e-12 for ammonia). For pH or pOH, enter a number typically between 0 and 14 — though the tool will accept negative pH (concentrated strong acid) or values above 14 (concentrated strong base).
- Alternatively, select a common substance from the preset dropdown — battery acid (pH 0), lemon juice (pH 2), coffee (pH 5), pure water (pH 7), blood (pH 7.4), baking soda (pH 8.3), bleach (pH 12.6), or drain cleaner (pH 14) — to auto-fill a known pH value and see all four corresponding values populated.
- Watch the visual scale indicator: a colored bar shows where your solution lands on the acid-base spectrum from red (strongly acidic) through green (neutral) to violet (strongly basic). The classification label (strongly acidic, weakly acidic, neutral, weakly basic, strongly basic) updates automatically.
- For example, entering [H+] = 3.2e-4 M tells you instantly that pH ≈ 3.49, pOH ≈ 10.51, and [OH-] ≈ 3.13×10⁻¹¹ M — saving the manual log calculation. This is the typical workflow for chemistry homework or buffer-solution prep.
- Use scientific notation for very small or very large concentrations: 1e-9 displays cleanly instead of 0.000000001. The tool also shows results in both decimal and scientific notation so you can copy whichever format your lab notebook requires.
- Remember the temperature caveat: pH 7 is neutral only at 25°C because Kw shifts with temperature. At 50°C, neutral pH drops to about 6.63 because water dissociates more readily — relevant for hot-water aquariums, brewing, and high-temperature chemical processes.
Frequently asked questions
What is pH and what does the pH scale measure?
pH stands for 'potential of hydrogen' and measures the hydrogen ion activity in a solution. The scale runs from 0 (most acidic) to 14 (most basic/alkaline), with 7 being neutral. The scale is logarithmic: each unit represents a tenfold change in H+ concentration. pH 4 is 10 times more acidic than pH 5 and 100 times more acidic than pH 6. The formula is pH = -log10([H+]). Most natural water systems have pH between 6.5 and 8.5, human blood is tightly regulated at 7.35-7.45, and stomach acid ranges from pH 1.5 to 3.5.
How do you convert between pH, pOH, [H+], and [OH-]?
All four values are mathematically linked at 25 degrees C. From pH: pOH = 14 - pH, [H+] = 10^(-pH), [OH-] = 10^(-pOH). From [H+]: pH = -log10([H+]), then derive pOH and [OH-]. From [OH-]: pOH = -log10([OH-]), pH = 14 - pOH, [H+] = 10^(-pH). The key relationship is Kw = [H+] x [OH-] = 1 x 10^-14 at 25 degrees C. This calculator performs all conversions instantly from any starting value.
Why is pH important in everyday life?
pH affects water quality (drinking water should be pH 6.5-8.5), pool maintenance (target pH 7.2-7.6 to prevent eye irritation and equipment corrosion), gardening (most plants prefer soil pH 6.0-7.0; blueberries need 4.5-5.5), cooking (bread dough acidity affects yeast activity), skincare (healthy skin is pH 4.5-5.5), and aquariums (freshwater fish need pH 6.5-7.5). Industrial processes like brewing, dyeing, and pharmaceutical manufacturing require precise pH control.
What is the difference between a strong acid and a weak acid?
Strong acids (hydrochloric acid, sulfuric acid, nitric acid) fully dissociate in water: every molecule releases its H+ ions. Their pH can be calculated directly from concentration. Weak acids (acetic acid, citric acid, carbonic acid) only partially dissociate. A 0.1 M solution of HCl has pH = 1, but a 0.1 M solution of acetic acid has pH = 2.87 because only about 1.3% of its molecules release H+ ions. The dissociation constant Ka quantifies this: strong acids have very large Ka values, weak acids have small ones.
Can pH go below 0 or above 14?
Yes, though it is uncommon. Concentrated strong acids can have negative pH values: 10 M hydrochloric acid has a theoretical pH of -1. Concentrated strong bases can exceed pH 14: a 10 M sodium hydroxide solution has a theoretical pOH of -1 (pH = 15). The 0-14 range applies to dilute aqueous solutions at 25 degrees C where concentrations stay at or below 1 M. At other temperatures, the neutral pH also changes because Kw varies with temperature.
Part of ToolFluency’s library of free online tools for Science. No account needed, no data leaves your device.