pH Calculator
Find pH from hydrogen ion concentration or vice versa. Converts between pH, pOH, [H+], and [OH-] values. Identifies whether a solution is acidic, neutral, or basic.
pH is the standard measure of how acidic or basic a water-based solution is. The scale runs from roughly 0 (extremely acidic) to 14 (extremely basic), with pH 7 being neutral at 25 °C. Because pH is a logarithmic scale, each whole-number change represents a 10× change in hydrogen ion concentration — a pH 4 solution has 1,000× more H⁺ than pH 7.
This calculator converts between pH, pOH, [H⁺], and [OH⁻]. Enter any one value and the others are derived from the standard water-autoionization equilibrium at 25 °C. The calculator also identifies whether the solution is acidic, neutral, or basic.
pH matters everywhere water chemistry does: blood pH (tightly regulated near 7.4), soil pH (affects plant nutrient uptake), pool and aquarium chemistry, food preservation, industrial process control, and laboratory work. Small pH shifts can have outsized biological consequences — human blood pH outside the 7.35–7.45 range causes serious physiological problems.
Inputs
Results
pH
7.00
pOH
7.00
Type
Neutral
pH Calculator Results
| Parameter | Value |
|---|---|
| pH | 7.0000 |
| pOH | 7.0000 |
| [H+] Concentration | 1.0000e-7 M |
| [OH-] Concentration | 1.0000e-7 M |
| Solution Type | Neutral |
| Strength | Near neutral |
| pH + pOH | 14.00 (should = 14) |
| [H+] × [OH-] | 1.0000e-14 (should = 10⁻¹⁴) |
| Formulas | pH = -log[H+], pOH = 14 - pH |
Formula
How to use this calculator
- Choose which value you have: pH, [H⁺] concentration, or pOH.
- Enter the value. For [H⁺] in M (mol/L), use scientific notation if needed (1 × 10⁻⁷ = 0.0000001 M for pure water).
- Read the converted values. The calculator returns all of: pH, pOH, [H⁺], [OH⁻], and the acid/base classification.
- For temperature dependence: Kw rises with temperature, so pure water at higher temperatures has pH slightly below 7. The 7-is-neutral rule applies only at 25 °C.
Worked examples
Stomach acid vs blood
Stomach acid: pH ≈ 1.5 [H⁺] = 10⁻¹·⁵ ≈ 0.032 M Human blood: pH ≈ 7.4 [H⁺] = 10⁻⁷·⁴ ≈ 4.0 × 10⁻⁸ M Stomach acid is about 800,000× more concentrated in H⁺ than blood. The stomach's mucosal lining and tight pH regulation are why this is survivable; ulcers happen when those defenses fail.
Pool chemistry
Ideal pool pH: 7.2–7.8. If pH drops to 6.8, [H⁺] is about 4× higher than the upper bound of ideal — chlorine becomes more effective at killing bacteria but also more corrosive to equipment and irritating to eyes. If pH climbs to 8.2, chlorine becomes much less effective. A pool with sodium carbonate added raises pH; muriatic acid or sodium bisulfate lowers it. Test strips give a one-decimal estimate; digital meters give two-decimal precision.
When to use this calculator
Use this calculator for any chemistry, biology, or environmental science work involving acidity: - Lab homework (acid-base titrations, buffer prep) - Aquarium and pool chemistry - Soil pH adjustment for gardening - Food and beverage formulation (fermentation, preservation) - Wastewater treatment
For weak acids and bases, equilibrium calculations are more involved — pH depends on the dissociation constant (Ka or Kb) and concentration, not just the formal concentration. Use the equilibrium-constant or dilution calculators for those cases.
For non-aqueous solvents, the pH scale doesn't apply directly. Apparent pH measurements in alcohols or organic solvents need solvent-specific calibration.
Common mistakes to avoid
- Treating pH as a linear scale. A pH change from 7 to 6 is 10× more H⁺, not "a little more acidic."
- Assuming pH 7 = neutral at all temperatures. Kw increases with temperature; neutral pH at body temperature (37 °C) is about 6.8.
- Confusing concentration with pH. A "1 M" solution of HCl has pH around 0; a 1 M solution of acetic acid has pH around 2.4 because acetic acid only partially dissociates.
- Reporting pH with too many decimal places. Test strips give ±0.5; pH meters give ±0.01 when properly calibrated. Three-decimal pH is rarely meaningful outside a research lab.
- Ignoring temperature. Calibrate pH meters at the temperature you're measuring; temperature drift can shift readings by 0.05–0.1 pH units per 5 °C.
Frequently Asked Questions
Sources & further reading
- Measurement of pH — U.S. National Institute of Standards and Technology