Ionic Strength Calculator
The ionic strength of a solution from the concentration and charge of each ion — the quantity activity coefficients and Debye lengths depend on.
Ionic strength is half the sum of each ion’s concentration times the square of its charge, I = ½ Σ cᵢzᵢ².
How the ionic strength calculator works
Ionic strength is half the sum of each ion’s concentration times the square of its charge, I = ½ Σ cᵢzᵢ². Charge is squared, so divalent ions count four times as much per mole: 0.1 M NaCl has I = 0.1, while 0.1 M CaCl₂ has I = 0.3. Enter each ion separately — a salt dissociates into two rows.
Formula: I = ½ Σ cᵢ · zᵢ²
Worked examples
| Inputs | Ionic strength | Note |
|---|---|---|
| 0.1 M CaCl₂ | 0.3 mol/L | 0.3 |
| 0.1 M NaCl | 0.1 mol/L | 0.1 |
| 0.05 M MgSO₄ | 0.2 mol/L | 0.2 |
FAQFrequently asked questions
Why is the charge squared?
The electrostatic screening an ion provides scales with the square of its charge: a doubly charged ion has four times the effect on activity coefficients of a singly charged one at the same concentration.
What is ionic strength used for?
Activity coefficients (Debye–Hückel, Davies), the Debye length, salting-in and salting-out of proteins, and buffer recipes that must match physiological conditions (about 0.15 M).
Do I enter the salt or the ions?
The ions — one row each. 0.1 M CaCl₂ is 0.1 M Ca²⁺ and 0.2 M Cl⁻. The charge-balance row tells you if you have forgotten one.
Should I use molarity or molality?
Strictly molality for thermodynamic work, but in dilute aqueous solutions the two are within a percent, and molarity is what most people have.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.