Molarity calculator: mass, moles, volume and concentration
Give a molecular weight and any two quantities — the rest follow. Mix units freely: kDa with nanograms with microlitres is fine.
How much do I weigh out? most used
Target concentration × final volume × molecular weight.
Mass ↔ moles ↔ volume ↔ molarity
Choose what you want to find, fill the rest, and every related quantity is reported underneath.
Mass concentration ↔ molar concentration mg/mL ↔ mM
The workhorse conversion. Enter a molecular weight, then type in either box — the other updates.
| Unit | Molar | Mass |
|---|
Working through an example
Say you need 500 mL of 50 mM Tris. Tris base has a formula weight of 121.14 g/mol, so:
- 0.050 mol/L × 0.500 L = 0.025 mol of Tris
- 0.025 mol × 121.14 g/mol = 3.029 g
Dissolve that in about 400 mL of water, titrate to the pH you want with HCl, then make up to 500 mL. Note the order — adjusting pH before the final volume matters, because adding acid adds volume.
Choosing the right units
Concentration units in biochemistry cluster around the scale of the thing being measured. Small molecules and buffer components live in millimolar; protein stocks in micromolar; ligands, inhibitors and hormones in nanomolar or picomolar; and single-molecule work reaches femtomolar. The conversion is always a factor of 1000 per step.
| Unit | In molar | Typical use |
|---|---|---|
| M (molar) | 1 | Concentrated salt and buffer stocks |
| mM (millimolar) | 10⁻³ | Working buffers, substrates, metabolites |
| µM (micromolar) | 10⁻⁶ | Purified protein, Km values, cofactors |
| nM (nanomolar) | 10⁻⁹ | Ligand binding, Kd, primers in a PCR |
| pM (picomolar) | 10⁻¹² | High-affinity antibodies, hormones |
| fM (femtomolar) | 10⁻¹⁵ | Single-molecule and digital-PCR work |
| aM (attomolar) | 10⁻¹⁸ | Ultra-sensitive detection limits |
Molecular weight, formula weight and Dalton
For a small molecule the formula weight printed on the bottle is what you want — it already includes waters of hydration, so MgCl₂·6H₂O is 203.30 g/mol, not 95.21. Using the anhydrous value for a hydrated salt is one of the most common weigh-out errors.
For proteins the mass is normally quoted in kilodaltons. One dalton is numerically identical to one g/mol, so a 66.5 kDa protein has MW 66 500 g/mol. The calculators above accept Da, kDa and MDa directly.
Frequently asked questions
How do I convert grams to moles?
Divide the mass in grams by the molecular weight in g/mol: n = m / M. For example, 5 g of glucose (180.16 g/mol) is 5 / 180.16 = 0.02775 mol, or 27.75 mmol.
How do I calculate how much powder to weigh out?
Mass = molarity × volume × molecular weight. For 50 mL of 100 mM Tris (121.14 g/mol): 0.1 mol/L × 0.05 L × 121.14 g/mol = 0.6057 g.
How do I convert mg/mL to mM?
Divide the mass concentration in g/L (numerically the same as mg/mL) by the molecular weight, then multiply by 1000. A 2 mg/mL solution of a 50 kDa protein is 2 / 50 000 = 4 × 10⁻⁵ M = 40 µM.
What is the difference between molarity and molality?
Molarity (M) is moles of solute per litre of solution and changes with temperature because volume does. Molality (mol/kg) is moles of solute per kilogram of solvent and is temperature-independent, so it is preferred for colligative-property work.