pH Calculator
Compute pH from hydrogen ion concentration or the reverse, in both directions. Accepts e-notation like 1e-7 and shows the substituted pH = −log₁₀[H⁺] working plus an acidic / neutral / basic classification based on 25 °C water.
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Key Formula & Conditions
Learning modelConverts hydrogen ion concentration [H⁺] (mol/L) and pH using pH = −log₁₀[H⁺] and [H⁺] = 10^(−pH), supporting scientific e-notation and step-by-step logarithmic working.
Try an example
Enter a concentration or a pH to compute the other value.
Done
pH Calculator
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Worked Examples & Verified Values
Strong acid [H⁺]=2.5e-3 M
Weak acid [H⁺]=1.404e-3 M
Blood pH 7.40 → [H⁺]
Common Pitfalls & Calculation Rules
- •Entering weak-acid initial molarity c directly as [H⁺]. Because weak acids only partially dissociate, compute equilibrium [H⁺] from Ka first.
- •Applying −log₁₀(c) to ultra-dilute strong acids (c ≤ 10⁻⁷ M) and getting pH > 7. Water autoionization (Kw = 10⁻¹⁴ at 25°C) must be included via the quadratic charge-balance equation.
- •Miscounting logarithmic significant figures. Only digits after the decimal point in a pH value count as significant figures matching the precision of [H⁺].
Verification scope and citation information
Use worked examples within their stated inputs, units and assumptions. Tests cover specific cases at a specific time; they do not certify every input, expert review or standards compliance. Check the references and tool-specific verification notes, and enter your own access date when citing.
@misc{tools_hub_ph_calculator_2026,
author = {{Tools Hub}},
title = {pH Calculator},
year = {2026},
url = {https://the-tools-hub.net/en/tools/ph-calculator/},
note = {Accessed: YYYY-MM-DD}
}How to use
- 1
Pick a direction
Choose Concentration → pH or pH → Concentration.
- 2
Enter the value
Type the hydrogen ion concentration (e.g. 1e-7) or a pH (e.g. 2.5).
- 3
Read the value and class
Alongside the result, the substituted working and an acidic / neutral / basic verdict are shown.
Features
- Both directions — concentration to pH and back — on one page
- Accepts e-notation like 1e-7; pH shown to two decimal places
- Acidic / neutral / basic classification (25 °C basis) with substituted working
- Non-positive concentrations reported as errors; inputs are never transmitted
Use cases
Check chemistry homework
Verify pH ↔ concentration conversions and the logarithm steps.
Tidy lab report values
Convert a measured pH into a concentration, or the reverse, for reports.
Classify a solution
Enter a pH to see whether the solution reads acidic, neutral or basic.
Details
pH is defined as pH = −log₁₀[H⁺], the negative common logarithm of the hydrogen ion concentration [H⁺] in mol/L. A concentration of 1 × 10⁻⁷ mol/L gives pH = 7. To go the other way, [H⁺] = 10⁻ᵖᴴ. The tool shows the substituted working in either direction.
The classification assumes water at 25 °C: pH < 7 is acidic, pH = 7 neutral, and pH > 7 basic. The neutral point drifts away from 7 at other temperatures (about 6.3 at 80 °C), so treat it as a 25 °C guideline. Relatedly, pH + pOH = 14 holds at 25 °C.
Concentrations accept e-notation such as 1e-7. Following common practice, pH is displayed to two decimal places, and values outside the usual 0–14 window (possible for concentrated acids and bases) are still computed with a note. A concentration of 0 or less has no logarithm and is rejected as an error. Inputs are never transmitted; everything is computed locally in your browser.
Molarity, Dilution, Buffer Preparation, Beer-Lambert & pH
From molarity and stock dilution volumes (C₁V₁=C₂V₂) to Henderson-Hasselbalch buffer recipes, Beer-Lambert absorbance/calibration (A=εcℓ), and solution pH.
FAQ
What is pH?
It is the negative common logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺] with [H⁺] in mol/L. It expresses how acidic or basic a solution is, typically on a 0–14 scale.
Why is pH 7 neutral?
Pure water at 25 °C has [H⁺] = 1 × 10⁻⁷ mol/L, whose logarithm gives pH = 7. At other temperatures the balance shifts — neutral is about 6.3 at 80 °C.
Can pH be below 0 or above 14?
Yes. Concentrated hydrochloric acid or sodium hydroxide can leave that window. The tool computes such values as-is and notes that they sit outside the usual range.
How are pH and pOH related?
At 25 °C, pH + pOH = 14. pOH is −log₁₀[OH⁻] for the hydroxide concentration: the more acidic the solution, the smaller the pH and the larger the pOH.
How much does [H⁺] change when pH changes by 1 unit?
Because pH uses a base-10 logarithm, decreasing pH by 1 multiplies [H⁺] by 10, while increasing pH by 1 divides [H⁺] by 10. A 2-unit shift represents a 100-fold concentration change.
How do I convert pOH to pH?
Use pH = pKw − pOH. At 25°C where Kw = 1.0×10⁻¹⁴ (pKw = 14.00), pH = 14.00 − pOH.
Related guide
GuideBuffer Solution Preparation & Henderson-Hasselbalch Guide: Hydrates, Titration & Temperature Shifts
Master conjugate acid-base buffer preparation using the Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])). Step-by-step calculations for monobasic/dibasic sodium phosphate hydrate mass discrepancies, Tris base 1 M HCl titration volumes, 4°C cold-room pKa shifts (ΔpKa/ΔT), Van Slyke buffer capacity β, and acid-resistance simulations.
GuideHow to Calculate pH: [H⁺], pOH, Weak-Acid 5% Rule & Ultra-Dilute Limits
Master pH and pOH calculations across all conditions: strong vs. weak acids (5% ionization rule vs. quadratic formula), water autoionization in 10⁻⁸ M ultra-dilute solutions, temperature-dependent pKw, and logarithmic significant figures.
Related Tools
Buffer Solution & Henderson-Hasselbalch Calculator
Calculate conjugate acid-base buffer recipes using the Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])). Solves mass in grams for diverse hydrates, 1 M HCl/NaOH titration volumes, Good buffer temperature corrections (4°C, 25°C, 37°C), and Van Slyke buffer capacity β.
Logarithm Calculator
Compute logarithms in any base, like log₂(8). One-tap bases for 2, 10 and e, with the change-of-base working (ln x ÷ ln b) and a check that bʸ returns x. Common and natural logarithm notes included.
Moles & Molarity
Calculate molarity from moles and solution volume, or convert mass to moles with a supplied molar mass. View results in mol/L and mmol/L.
Dilution Calculator
Find the required stock volume from stock concentration, target concentration and final solution volume using C₁V₁ = C₂V₂.
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Verified: Known values ([H⁺]=1e-7 → pH 7, pH 5 → 1.0×10⁻⁵), non-positive errors, the reverse direction and the English page are covered by browser tests
Did you know?
One pH unit means a tenfold change in hydrogen ion concentration: lemon juice at pH 3 is ten times as acidic as tomato juice at pH 4. Remembering "one unit = ten times" makes pH values readable at a glance.