Molarity calculator
Mass to weigh, volume to make, or concentration from molecular weight.
Mass to weigh, volume to make, or concentration from molecular weight.
Molarity is the concentration of a solution expressed as moles of solute per litre of solution, written M or mol/L. A 1 M solution holds one mole, which is the molecular weight in grams, in every litre. Because a mole is a fixed number of molecules, molarity lets you compare and combine reagents by molecule count rather than by weight, which is why buffers, salts and substrates are almost always specified this way.
Rearrange it to solve for whichever quantity you are missing:
Worked example. To make 500 mL of 1 M Tris (121.14 g/mol): 1 mol/L × 0.5 L × 121.14 g/mol = 60.57 g. The other way round, 5.844 g of NaCl (58.44 g/mol) dissolved to 1 L is 5.844 ÷ 58.44 = 0.1 mol in 1 L, so 0.1 M, or 100 mM.
To dilute a stock you have already made, use the dilution calculator, and for a series of steps the serial dilution planner.
| Reagent | MW (g/mol) |
|---|---|
| NaCl | 58.44 |
| Tris base | 121.14 |
| EDTA disodium dihydrate | 372.24 |
| Glucose | 180.16 |
| DTT | 154.25 |
| IPTG | 238.30 |
Always take the value from the bottle label or the supplier's datasheet, since salts, hydrates and free acids of the same compound have different weights.
Moles = mass ÷ molecular weight, and mass = moles × molecular weight. The calculator shows the amount in moles under every result, so it also works as a grams-to-moles converter: enter the mass, any volume, and the molecular weight, and read the moles line.
Divide the mass by the molecular weight to get moles, then divide by the volume in litres. 5.844 g of NaCl (58.44 g/mol) is 0.1 mol; in 1 L that is 0.1 M, in 500 mL it is 0.2 M.
Mass = molarity × volume in litres × molecular weight. For 1 M Tris (121.14 g/mol) in 500 mL: 1 × 0.5 × 121.14 = 60.57 g. Dissolve in less than the final volume, adjust pH if needed, then make up to 500 mL.
Molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent. Molarity is what almost every biology protocol means by "M", and it changes slightly with temperature because volume does. Molality does not.
The weight of the form in the bottle, water included. EDTA disodium dihydrate is 372.24 g/mol, not the 292.24 of anhydrous EDTA. Using the anhydrous value would make the solution about 20% too weak.
Final volume of the solution. Dissolve the solute in less liquid than you need, then top up to the final volume in a graduated cylinder or volumetric flask. Adding the full volume of solvent to the powder gives a slightly larger, weaker solution.