pH Calculator
Calculate pH, pOH, H⁺ concentration, and OH⁻ concentration — enter any one value and get all related values instantly.
Enter any one value to calculate all others.
How Does the Formula Work?
The pH scale is one of the most important measurement systems in chemistry, biology, medicine, and environmental science. Introduced by Danish chemist Søren Peder Lauritz Sørensen in 1909, pH quantifies the acidity or alkalinity of an aqueous solution. The term pH stands for "power of hydrogen" — specifically, the negative base-10 logarithm of the hydrogen ion concentration. This calculator converts between pH, pOH, hydrogen ion concentration, and hydroxide ion concentration using the fundamental relationships that govern acid-base chemistry at 25 degrees Celsius.
[H⁺] = 10⁻ᵖᴴ
pOH = −log₁₀[OH⁻]
[OH⁻] = 10⁻ᵖᴼᴴ
pH + pOH = 14
[H⁺] × [OH⁻] = 1 × 10⁻¹⁴ (Kw at 25 °C)
The water autoionization constant (Kw) equals 1 times 10 to the negative 14 at 25 degrees Celsius. This means pure water at room temperature has equal concentrations of hydrogen and hydroxide ions — both at 10 to the negative 7 mol per liter, giving a pH of exactly 7. Any substance that increases the hydrogen ion concentration (lowers pH) is an acid, and any substance that decreases it (raises pH) is a base.
The Logarithmic Nature of pH
Because pH uses a logarithmic scale, each whole number change represents a tenfold difference in hydrogen ion concentration. A solution at pH 3 has ten times more hydrogen ions than one at pH 4, one hundred times more than pH 5, and one thousand times more than pH 6. This is why even small pH changes can have dramatic biological and chemical effects. Blood pH is maintained between 7.35 and 7.45 — a shift of just 0.3 units (to 7.05 or 7.75) can be life-threatening. Swimming pool water should stay between 7.2 and 7.8 — outside this range, chlorine becomes either too aggressive or ineffective.
Common pH Values in Daily Life
Understanding where everyday substances fall on the pH scale helps contextualize your calculations. Battery acid sits around pH 1, the most acidic substance most people encounter. Lemon juice and vinegar are pH 2 to 3. Coca-Cola and other soft drinks are pH 2.5 to 3.5 — acidic enough to dissolve a tooth enamel over time. Black coffee is pH 5. Milk is pH 6.5 to 6.8. Pure water is pH 7 (neutral). Human blood is pH 7.35 to 7.45. Baking soda dissolved in water is pH 8.3. Milk of magnesia is pH 10.5. Household ammonia is pH 11 to 12. Bleach (Clorox) is pH 12.5. Drain cleaner (lye/NaOH) is pH 13 to 14.
pH in Water Treatment
Municipal water treatment plants in the United States carefully control pH throughout the treatment process. The EPA's Secondary Drinking Water Standards recommend pH between 6.5 and 8.5. Chlorine disinfection is most effective at pH 7.0 to 7.5 — at higher pH, more of the chlorine converts to the less effective hypochlorite ion. Water that is too acidic (below 6.5) corrodes copper and lead pipes, leaching toxic metals into drinking water — this was a contributing factor in the Flint, Michigan water crisis. Water that is too alkaline (above 8.5) causes scale buildup in pipes and water heaters, reducing efficiency and lifespan. Home water test kits from brands like Taylor Technologies, Hach, and LaMotte (available at Home Depot and Amazon) measure pH alongside chlorine, hardness, and alkalinity.
pH in Swimming Pools and Hot Tubs
Pool maintenance revolves around pH control. The ideal swimming pool pH is 7.2 to 7.8, with 7.4 being optimal. Below 7.2, water becomes corrosive — it etches plaster, damages metal fixtures, and irritates eyes and skin. Above 7.8, chlorine loses effectiveness (at pH 8.0, only 22 percent of chlorine is in its active form versus 66 percent at pH 7.2), calcium precipitates causing cloudy water, and scale forms on surfaces. To lower pH, pool owners add muriatic acid (hydrochloric acid) or sodium bisulfate (dry acid). To raise pH, they add sodium carbonate (soda ash). Taylor test kits and digital testers from Hanna Instruments and Blue Connect are standard pool testing equipment at Leslie's Pool Supplies and Pinch A Penny.
pH in Agriculture and Soil Science
Soil pH determines nutrient availability for plants. Most vegetables thrive in slightly acidic to neutral soil (pH 6.0 to 7.0). Blueberries, azaleas, and rhododendrons prefer acidic soil (pH 4.5 to 5.5). Lavender and clematis prefer slightly alkaline soil (pH 7.0 to 8.0). When soil pH is too low, aluminum and manganese become toxic to plant roots. When too high, iron, zinc, and phosphorus become unavailable. Soil test kits from Luster Leaf (Rapitest) and MySoil (available at Home Depot and Lowe's) measure soil pH so gardeners can amend accordingly — sulfur or aluminum sulfate to lower pH, and lime (calcium carbonate) to raise it. The USDA Natural Resources Conservation Service publishes detailed soil pH maps for every county in the United States.
pH in Medicine and Biology
The human body maintains different pH levels in different systems. Blood pH is tightly regulated between 7.35 and 7.45 by the bicarbonate buffer system. Stomach acid (hydrochloric acid) has a pH of 1.5 to 3.5 — strong enough to dissolve metal and essential for protein digestion and killing ingested bacteria. Urine pH varies between 4.5 and 8.0 depending on diet and hydration — consistently acidic urine (below 5.5) increases kidney stone risk. Skin has a natural acid mantle at pH 4.5 to 5.5 that protects against bacteria — this is why pH-balanced skin care products from brands like CeraVe, La Roche-Posay, and Cetaphil are formulated at pH 5 to 6. Dental enamel begins to demineralize below pH 5.5, which is why acidic foods and drinks contribute to tooth decay.
Tips & Recommendations
pH 3 is ten times more acidic than pH 4 and one hundred times more than pH 5. Small pH changes have big chemical effects.
At 25 °C, pH and pOH always sum to 14. Know one, instantly get the other by subtraction.
pH 7 is neutral, but safety depends on the substance. Pure water is pH 7 and safe; other pH 7 solutions may not be.
Kw changes with temperature. At 37 °C (body temp), Kw = 2.4×10⁻¹⁴ so neutral pH is 6.8, not 7.0.
Frequently Asked Questions
What does pH measure?
pH measures the hydrogen ion concentration in a solution on a logarithmic scale from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). Each whole number change represents a tenfold difference in acidity.
What is the relationship between pH and pOH?
pH + pOH = 14 at 25 °C (77 °F). If you know pH, pOH is simply 14 minus pH. This relationship comes from the water autoionization constant Kw = 1×10⁻¹⁴.
What is a normal pH for drinking water?
The EPA recommends drinking water pH between 6.5 and 8.5. Most municipal water systems maintain pH between 7.0 and 8.0. Water that is too acidic can corrode pipes, while water that is too basic can cause scale buildup.
Why is pH a logarithmic scale?
Because hydrogen ion concentrations span many orders of magnitude — from 1 mol/L (pH 0) to 10⁻¹⁴ mol/L (pH 14). A logarithmic scale compresses this enormous range into a manageable 0–14 number line.
Can pH be negative or above 14?
Yes, technically. A solution with H⁺ concentration greater than 1 mol/L has a pH below 0 — for example, concentrated hydrochloric acid can have pH around −1. Similarly, very concentrated bases can exceed pH 14.
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