Molarity Calculator
Molarity (M) is the number of moles of solute per litre of final solution. Enter any three of mass, molecular weight, volume and target concentration and this calculator solves the fourth, then shows every intermediate value in mol/L, mmol/L, total moles and grams.
Molarity is the concentration unit you reach for most often at the bench — buffering a lysis step, making a 1 M Tris stock, or dosing a reagent into a reaction all start here. It is one of the core tools in our laboratory calculator set, and unlike a single-answer widget it solves in four directions, so you can work backwards from the concentration you want to the mass you need to weigh out.
The whole calculation rests on one relationship: amount of substance divided by final solution volume. Everything else is unit bookkeeping. If you want the concept rather than the number, what is molarity covers the definition, the difference between M and mM, and why temperature quietly changes the answer.
Molarity Calculator
How to use the molarity calculator
Pick what you are solving for in the Solve for dropdown. The default mode calculates molarity from a mass you weighed; the other three modes rearrange the same equation to return mass, volume or amount of substance.
Enter the molecular weight of your solute in g/mol. For a small molecule use its formula weight (glucose is 180.16, NaCl is 58.44); for a protein use the value from its sequence or datasheet. If you do not know it yet, get it from a molar mass calculation first.
Enter the final solution volume, not the water you added. Dissolve the solute in roughly 80% of the target volume, then bring it up to the mark — this distinction is the single most common source of error and is explained in how to calculate molarity.
Press Calculate. The primary card shows the answer in the unit you solved for; the secondary cards restate it as mol/L, mmol/L, total moles and the mass required, so you can sanity-check the result without re-entering anything.
Molarity formula
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| M | Molarity (molar concentration) | mol/L |
| n | Amount of substance | mol |
| m | Mass of solute | g |
| MW | Molecular weight (molar mass) | g/mol |
| V | Final solution volume | L |
How molarity is calculated
- Convert the mass to moles: n = m / MW.
- Convert the volume to litres (1 mL = 0.001 L, 1 µL = 1×10⁻⁶ L).
- Divide moles by volume in litres: M = n / V.
- To go the other way, multiply: mass (g) = M × MW × V(L), or amount (mol) = M × V(L), or volume (L) = n / M.
Worked laboratory examples
Example 1 — Weigh out 100 mL of 1 M NaCl
Given: Target 1 M · MW NaCl = 58.44 g/mol · final volume 100 mL = 0.1 L
- mass = M × MW × V
- mass = 1 mol/L × 58.44 g/mol × 0.1 L
- mass = 5.844 g
Result: 5.844 g NaCl, dissolved then brought to 100 mL
Example 2 — Concentration of 5 g glucose in 250 mL
Given: mass = 5 g · MW glucose = 180.16 g/mol · final volume 250 mL = 0.25 L
- n = 5 / 180.16 = 0.02775 mol
- M = 0.02775 / 0.25 = 0.1110 mol/L
- 0.1110 M = 111.0 mM
Result: 0.111 M (111 mM) glucose
Example 3 — Mass for 50 mL of a 10 mM reagent
Given: Target 10 mM = 0.010 M · MW = 180.16 g/mol · final volume 50 mL = 0.050 L
- mass = 0.010 × 180.16 × 0.050
- mass = 0.09008 g
- mass = 90.1 mg
Result: 90.1 mg of reagent
Want the reasoning behind the numbers? Read What Is Molarity?, How to Calculate Molarity and Molarity vs Molality.
How to interpret the result
A result of 0.111 M means every litre of that solution holds 0.111 moles of solute — for glucose, about 20 g/L. The secondary cards translate the same answer into mmol/L (the unit most protocols use) and into the total moles present in your specific volume, which is what you actually need when scaling a recipe up or down.
If the number looks absurd — a 40 M aqueous solution, for instance — treat it as a red flag, not a result. Most solutes stop dissolving well below that, and the calculator will warn you above 100 M. Almost always the cause is a unit slip: mass entered in mg instead of g, or volume in mL treated as L.
Assumptions
- The solute is fully dissolved and does not change the final volume beyond what you measured to the mark.
- Molecular weight is the average (formula) weight for the species as weighed, including any waters of hydration.
- Volume is the final solution volume at the temperature of use.
Limitations
- Molarity shifts with temperature because volume expands or contracts; for temperature-critical work, molality (mol per kg of solvent) is the stable choice.
- Solute purity is not modelled — a 95% pure solid needs about 5% more mass, which you must add yourself.
- The calculator does not flag solubility limits; a mathematically valid concentration may still be impossible to dissolve.
Common mistakes
- Using the volume of water added instead of the final solution volume. Always dissolve first, then bring to volume.
- Forgetting to convert mL to L (or µL to L) before dividing — this throws the answer off by 1000× or more.
- Using an anhydrous molecular weight for a hydrated salt (or the reverse). Check the bottle: CuSO₄ and CuSO₄·5H₂O differ by 90 g/mol.
- Reading "0.5 M" as "0.5 mol per mL". M is always per litre.
FAQ
What is the formula for molarity?
How do I calculate molarity from grams?
Is mM the same as mmol/L?
Should I use final volume or solvent volume?
How do I convert M to mM?
How many grams of NaCl for 100 mL of 1 M?
Scientific Sources
- Promega Biomath — Molarity Calculator · Tier 2 source
- NEB NEBioCalculator · Tier 2 source
- NIST Chemistry WebBook · Tier 1 source
External links open in a new tab and carry rel="nofollow". We cite primary references (NIST, NIH/NCBI, peer-reviewed papers) and established manufacturer technical documentation (NEB, Thermo Fisher, Promega, IDT). Calculated results are estimates; your protocol and manufacturer guidance take precedence.
Last reviewed: 2026-09 · Method version: 1.0