How to Calculate Molarity for Real Lab Solutions

The quick version

Molarity (M) is moles of solute per liter of solution. Since you almost never weigh out moles directly, the calculator works from mass and molar mass instead: M = mass ÷ molar mass ÷ volume. The four scenarios below cover the directions you actually need in practice — from mass to molarity, from molarity to mass, from molarity to volume, and between percent weight/volume and molarity.

Example 1: Checking a saline concentration

Physiological saline is labeled "0.9% w/v NaCl." What molarity is that? Sodium chloride has a molar mass of 58.44 g/mol.

Open the % w/v ↔ Molarity panel and enter:

• % w/v: 0.9
• Molar mass: 58.44

That converts to 154.0 mM— which is why you'll often see saline described as roughly 150 mM sodium chloride in physiology texts.

You can check the same number a different way using the Mass → Molarity panel: dissolving 5.844 g of NaCl (molar mass 58.44) into a final volume of 1 L gives exactly 100 mM — since 5.844 g is 10% of the 58.44 g that would make a full 1 M solution.

Example 2: Weighing out a Tris buffer stock

Say your protocol calls for 500 mL of 100 mM Tris buffer, and Tris base has a molar mass of 121.14 g/mol. How much powder do you weigh?

Open the Molarity → Mass panel and enter:

• Target molarity: 100 mM
• Final volume: 500 mL
• Molar mass: 121.14

The result is 6.057 g. Weigh that out, dissolve it, and bring the solution up to a final volume of 500 mL (not 500 mL of water added to the powder — the powder itself takes up some volume).

Example 3: Finding the volume for a target concentration

Suppose you have exactly 2 g of glucose (molar mass 180.16 g/mol) and want to know what final volume gives a 10 mM solution.

Open the Molarity → Volume panel and enter:

• Solute mass: 2 g
• Target molarity: 10 mM
• Molar mass: 180.16

That gives a final volume of 1,110.1 mL. This is the scenario to use when the amount of solute is fixed (say, a single vial of reagent) and the concentration is what you need to hit.

Step-by-step: using the Molarity Calculator

1. Open the Molarity Calculator.
2. Pick the scenario that matches what you already know: mass and volume (solve for molarity), molarity and volume (solve for mass), mass and molarity (solve for volume), or a percent w/v value.
3. Enter the molar mass of your solute — check the label or a reference source, and confirm whether it's the anhydrous or hydrate form (see below).
4. Enter your known values, matching the units shown next to each field.
5. Select Calculate. Each panel also gives a plain-language instruction, like how much diluent volume to bring the solution up to.
6. Use "Copy for lab book" to save the calculation with your notebook entry.

Common mistakes to avoid

• Using the wrong molar mass for a hydrate. Many common reagents (like CaCl₂·2H₂O or MgSO₄·7H₂O) are sold as hydrates with extra water built into the molar mass. Using the anhydrous molar mass when you actually have the hydrate form will give you the wrong concentration.
• Mixing up mass and molar concentration. A 1% solution is not the same as a 1 M solution — percent weight/volume depends on molar mass, which is exactly why the % w/v ↔ Molarity panel exists.
• Reading the wrong unit off the dropdown. The calculator supports g/mg/µg and M/mM/µM/nM side by side — double check the unit selector next to each field, especially when copying values from a different protocol.

Frequently asked questions

What's the difference between molarity and molality?

Molarity is moles of solute per liter of solution. Molality is moles of solute per kilogram of solvent. This calculator works in molarity, which is what almost all lab protocols specify.

Why does the calculator ask for a final volume instead of solvent volume?

Because solute takes up some space, "dissolve X in 500 mL of water" and "bring X up to a final volume of 500 mL" give slightly different concentrations. The calculator always means the final, total volume of the solution.

Can I use this for a solution that's already diluted?

This calculator handles making a solution from scratch. If you're starting from an existing stock solution instead of dry powder, use the Solution Dilution Calculator instead.