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Molarity calculator

Solve for the mass, volume or molar concentration of a peptide solution from its molecular weight — in-vitro laboratory use only.

For research use only. Not for human or veterinary use. Sold strictly for in-vitro laboratory research; not for diagnosis or treatment.

Molecular weight

From the certificate of analysis or specification. Both panels below use it.

Mass, volume or concentration

Choose the quantity to solve for and enter the other two.

Solve for

Mass required
1.42 mg
Amount of substance
1 µmol

Mass to moles

The amount of substance in a given mass.

Amount of substance
3.522 µmol

How the calculator works

Molarity is moles of solute per litre of solution. The calculator links four quantities through mass = concentration × volume × molecular weight. Enter the molecular weight in g/mol, choose the quantity to solve for, and enter the other two.

The mass-to-moles panel divides a mass by the molecular weight and shows the amount of substance in the most readable unit, from mol down to nmol.

Units and conversions

  • 1 M = 1000 mM, 1 mM = 1000 µM and 1 µM = 1000 nM.
  • 1 g = 1000 mg and 1 mg = 1000 µg.
  • 1 L = 1000 mL and 1 mL = 1000 µL.
  • 1 mM equals 1 µmol per mL, a quick hand check.

Worked examples

Each example uses a generic peptide with a molecular weight of 1419.5 g/mol.

  1. Mass for a target concentration: 1 mL of a 1 mM solution needs 0.001 mol/L × 0.001 L × 1419.5 g/mol = 1.4195 mg.
  2. Volume for a target concentration: 5 mg made up to 1 mM needs 0.005 g ÷ (1419.5 g/mol × 0.001 mol/L) = 3.522 mL.
  3. Concentration of a known solution: 5 mg in 2 mL is 0.005 g ÷ (1419.5 g/mol × 0.002 L) = 1.761 mM.
  4. Moles in a 5 mg vial: 0.005 g ÷ 1419.5 g/mol = 3.522 µmol.
  5. Working dilution: 1 mL at 10 µM from a 1 mM stock takes 10 µM × 1 mL ÷ 1000 µM = 0.01 mL (10 µL) of stock, the pipetting threshold the peptide calculator flags.

Which molecular weight to enter

Use the molecular weight stated for the material you hold. A salt form, such as an acetate salt, weighs more per mole than the free peptide, so the two give different molarities for the same mass. Solvent choice and handling are left to the receiving laboratory's own procedures.

Frequently asked questions

Divide the mass in grams by the molecular weight in g/mol to get moles, then divide by the volume in litres. 5 mg of a 1419.5 g/mol peptide in 2 mL is 0.005 ÷ 1419.5 ÷ 0.002 = 0.001761 M, or 1.761 mM.

Multiply the concentration (mol/L) by the volume (L) and the molecular weight (g/mol). For 1 mL of a 1 mM solution of a 1419.5 g/mol peptide: 0.001 × 0.001 × 1419.5 = 0.0014195 g, or 1.4195 mg.

Each step is a factor of 1000: 1 mM is 1000 µM, and 1 µM is 1000 nM. Stocks are often made in the millimolar range and diluted to micromolar or nanomolar working concentrations.

Use the value on the certificate of analysis or specification for the material you hold. For peptides it is usually given in g/mol, which is numerically the same as daltons (Da).

Yes. Divide the mass concentration in g/L by the molecular weight. 1 mg/mL is 1 g/L, so for a 1419.5 g/mol peptide it is 1 ÷ 1419.5 = 0.0007045 M, or 704.5 µM.