Molality Calculator

Per kilogram of solvent — not per litre of solution.

Per kilogram of solvent — not per litre of solution. Molality is moles per kilogram of solvent; molarity is moles per litre of solution.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Moles of solute

mol

Mass of solvent

Molality (mol/kg)

2 mol in 0.5 kg of solvent

Molality4 m

Moles of solute2

Solvent mass0.5 kg

Molality is per kilogram of solvent; molarity is per litre of solution. The distinction matters whenever temperature changes: volume expands with heat and mass does not, so a solution's molarity drifts as it warms while its molality is fixed. That is why colligative property calculations — boiling point, freezing point, osmotic pressure — are all defined on molality.

How the Molality Calculator works

Molality is moles per kilogram of solvent; molarity is moles per litre of solution. The distinction matters whenever temperature changes, because volume expands with heat and mass does not — which is why every colligative property is defined on molality.

Also known as: molality vs molarity · moles per kilogram calculator · molal concentration calculator · how to calculate molality

Where to go next

The Molality question rarely arrives on its own. These are the ones that usually come with it:

Molarity Calculator — Moles per litre of solution — not of solvent.
Normality Calculator — Equivalents per litre, and why the factor depends on the reaction.
Freezing Point Depression Calculator — Why road salt works, quantified.
Osmolarity Calculator — Counts particles, so 0.154 M saline is 308 mOsm/L.

Sources

Frequently asked questions

What is the difference between molality and molarity?

Molality is per kilogram of solvent, molarity per litre of solution. Molality is temperature-independent because mass does not expand; molarity drifts as a solution warms.

How do I calculate molality?

Divide moles of solute by kilograms of solvent. Two moles in 500 g of water is 4 molal.

Why do freezing point calculations use molality?

Because they are performed across a temperature range by definition, and a concentration that changes with temperature would make the equation circular.

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