Molarity calculator

How the molarity calculator works

The calculator links four quantities with one equation: mass = concentration × volume × molecular weight. Enter any three and it solves the fourth. Before calculating, every value is converted to base units (mol/L, L, g and g/mol), so you can mix µM with mL or ng with µL without converting by hand.

The formula and its rearrangements

• Mass: m = C × V × MW
• Concentration: C = m ÷ (V × MW)
• Volume: V = m ÷ (C × MW)
• Molecular weight: MW = m ÷ (C × V)

Molarity (M) is moles of solute per litre of solution. A mole of a peptide weighs its molecular weight in grams, which is why the MW links a weighed mass to a molar concentration.

Units

• Concentration: M = mol/L; 1 mM = 10⁻³ M; 1 µM = 10⁻⁶ M; 1 nM = 10⁻⁹ M
• Volume: 1 mL = 10⁻³ L; 1 µL = 10⁻⁶ L
• Mass: 1 mg = 10⁻³ g; 1 µg (mcg) = 10⁻⁶ g; 1 ng = 10⁻⁹ g
• Molecular weight: g/mol, numerically equal to daltons (Da)

Worked example: BPC-157 (MW 1,419.55 g/mol)

1. Mass for a working solution. 1 mL of a 1 µM solution needs 10⁻⁶ mol/L × 0.001 L × 1,419.55 g/mol = 1.42 × 10⁻⁶ g, i.e. about 1.42 µg. These are the calculator's default values.
2. Molarity of a stock. 5 mg dissolved to a final volume of 2 mL gives 0.005 g ÷ (0.002 L × 1,419.55 g/mol) = 0.00176 M, i.e. about 1.76 mM.
3. Dilution from that stock. For 1 mL at 10 µM, use C₁V₁ = C₂V₂: V₁ = 10 µM × 1,000 µL ÷ 1,761 µM ≈ 5.7 µL of stock, made up to 1 mL with buffer.

From mg/mL to molarity

Because 1 mg/mL equals 1 g/L, divide the mass concentration by the molecular weight: 1 mg/mL of BPC-157 is 1 ÷ 1,419.55 = 7.04 × 10⁻⁴ M, or about 704 µM. For mass-only conversions (mg to µg, mL to µL) use the unit converter.

What affects accuracy

• Use the molecular weight of the exact material, from the product page or the certificate of analysis. Salt forms such as a TFA salt or acetate salt weigh more per mole of peptide than the free base.
• Lyophilised powder also contains counter-ions and residual water. If the CoA states a net peptide content, multiply the weighed mass by that fraction before calculating; see how to read a peptide CoA.
• V is the final volume of the solution, not the volume of solvent added.

The calculator is a planning aid for in-vitro laboratory work with research-use-only materials; check critical values independently.