How to Calculate Molarity

To calculate molarity, divide the moles of solute by the final solution volume in litres. In practice that means three moves: turn your weighed mass into moles, turn your volume into litres, then divide. The formula is M = m / (MW × V), and every example below follows the same three steps. If you would rather not do the arithmetic, the molarity calculator runs all of it and shows the intermediate values.

What is the molarity formula?

Start from the definition — moles per litre:

M = n / V

Substitute n = m / MW (mass divided by molecular weight) and you get the form you actually use at the bench:

M = m / (MW × V)

• M = molarity (mol/L)
• m = mass of solute (g)
• MW = molecular weight (g/mol)
• V = final solution volume (L)

Step 1: How do I convert mass to moles?

Divide the mass you weighed by the molecular weight of the solute:

n = m / MW

You need the correct molecular weight first. For a small molecule, use its formula weight (NaCl = 58.44, glucose = 180.16). For a hydrate, include the water molecules. If you do not have the value, work it out from the chemical formula with a molar mass calculation before continuing.

Step 2: How do I convert volume to litres?

Molarity is per litre, so the volume must be in litres:

• 1 mL = 0.001 L
• 1 µL = 0.000001 L (1 × 10⁻⁶ L)
• 250 mL = 0.250 L

Use the final solution volume — the volume after the solute is dissolved and brought to the mark — not the volume of solvent you poured in. This single distinction is the most common source of error in concentration work.

Step 3: How do I finish the calculation?

Divide the moles from Step 1 by the litres from Step 2:

M = n / V

That is the molarity. Multiply by 1000 if you want millimolar (mM), which is the unit most protocols use for working solutions.

Worked example 1: glucose

What is the molarity of 5 g of glucose (MW 180.16 g/mol) in a final volume of 250 mL?

1. n = 5 / 180.16 = 0.02775 mol
2. V = 250 mL = 0.250 L
3. M = 0.02775 / 0.250 = 0.111 M (111 mM)

Worked example 2: sodium chloride

What is the molarity of 2.922 g of NaCl (MW 58.44 g/mol) in 500 mL?

1. n = 2.922 / 58.44 = 0.0500 mol
2. V = 0.500 L
3. M = 0.0500 / 0.500 = 0.100 M (100 mM)

Worked example 3: a protein or reagent

A reagent has MW 25,000 g/mol (25 kDa) and you dissolve 0.05 g in 20 mL:

1. n = 0.05 / 25,000 = 2.0 × 10⁻⁶ mol
2. V = 0.020 L
3. M = 2.0 × 10⁻⁶ / 0.020 = 1.0 × 10⁻⁴ M = 100 µM

Large molecules give small molarities from seemingly reasonable masses, which is why the calculator also reports µM and nM.

How do I calculate mass from a target molarity?

This is the reverse problem — you know the concentration you want and need the mass to weigh. Rearrange the formula:

m = M × MW × V

To make 50 mL of a 10 mM reagent (MW 180.16 g/mol):

• M = 0.010 mol/L, V = 0.050 L
• m = 0.010 × 180.16 × 0.050 = 0.0901 g = 90.1 mg

The molarity calculator does this in mass mode and reports the answer in g, mg or µg automatically.

How do I calculate volume or moles instead?

The same equation rearranges two more ways:

• Volume: V = n / M — how much solution a given number of moles makes at a target concentration.
• Amount: n = M × V — how many moles are present in a known volume of a known concentration.

All four modes (molarity, mass, volume, amount) share one engine in the calculator, so the conversions stay consistent whichever direction you work.

What unit errors should I watch for?

• mL left as mL. Dividing by 250 instead of 0.250 understates molarity by 1000×.
• mg entered as g. A 90.1 mg mass typed as 90.1 g overstates it by 1000×.
• mM confused with M. 10 mM is 0.010 M, not 10 M.
• Anhydrous vs hydrated MW. Using the wrong form shifts every downstream calculation.

If a result looks physically impossible — above about 100 M for anything aqueous — assume a unit slip and re-check the inputs. The calculator flags this explicitly rather than returning a silent nonsense number.

Where does this fit in the wider workflow?

Once you can move between mass, moles and concentration, dilutions and stock preparation become straightforward. The concept behind these steps — including M vs mM and the temperature caveat — is covered in what is molarity, and the rest of the laboratory calculators build on the same foundation.