NaCl standard
For 1.000 L of 1.000 M NaCl, use 1.000 × 1.000 × 58.44 = 58.44 g NaCl, then bring to final volume.
Mass from molarity
Molarity Calculator — Mass • Volume • Concentration
Inputs
Enter any three values below and leave one blank to solve it.
Result: Enter known values, leave the unknown blank, and click Calculate.
Equation
mass = M × V × MW
Unit bases
g · mol/L · L · g/mol
Tips: Ctrl/Cmd + Enter calculates · Esc clears.
How it works
Molarity (M) is moles per litre (mol/L). The core relationship is mass (g) = M (mol/L) × volume (L) × molecular weight (g/mol), which lets you solve for any unknown when the other three are known.
Core formulas
M = n / V
n = mass / MW
M = mass / (MW × volume)
mass = M × volume × MW
volume = mass / (M × MW)
C1 × V1 = C2 × V2
Common examples
- 1.0 M NaCl (1 L): 1.0 × 1.0 × 58.44 → 58.44 g.
- 10 mM Tris (250 mL): 0.01 × 0.25 × 121.14 ≈ 0.303 g.
- Find volume for 5 mg caffeine at 2 mM: V = mass / (M × MW) = 0.005 / (0.002 × 194.19) ≈ 12.87 mL.
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What Is Molarity and How Do You Use It in Real Labs?
Molarity (M) is the most common way to express solution concentration in chemistry and the life sciences. It measures the number of moles of solute per litre of solution (mol/L). Whether you work in a university lab, a clinical diagnostics unit, an environmental testing facility, or an R&D lab in the UK, US, EU, or elsewhere, molarity provides a clear, portable way to scale protocols and reproduce results across teams and geographies.
The core relationship is simple: mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol). With any three of those values you can calculate the fourth—ideal for preparing buffers, standards, reagents and assay working solutions. This calculator accepts common unit prefixes and larger/smaller units (M, mol/m3, mM, µM, nM; m3, L, mL, cm3, µL, nL; kg, g, mg, µg, ng, pg) so you can work in the units your method specifies without manual conversions.
Why molarity (and not percent)?
Percent solutions can be ambiguous (w/w, w/v, or v/v) and may change with temperature or density. Molarity ties directly to stoichiometry, which makes it easier to predict reaction yields, enzyme kinetics, and titrations. When your protocol calls for a precise number of molecules per volume—think PCR mixes, ELISAs, HPLC standards—molarity is the right tool.
Practical tips for solution preparation
- Use the correct molecular weight: Check the bottle label or Certificate of Analysis for hydrates (e.g., Na2HPO4·2H2O differs from the anhydrous salt). Small MW mistakes lead to large concentration errors.
- Account for final volume: Dissolve the solute in less than the target volume, then bring to volume in a volumetric flask or calibrated cylinder.
- Mind temperature and density: Volumes expand/contract with temperature; prepare at the temperature specified in your method, especially for analytical work.
- Document units consistently: Record both the numeric value and the unit (e.g., 10.0 mM, 250.0 mL). Mixing mL and µL is a classic source of 10× errors.
- Label clearly: Include concentration, date, initials, and any stabilisers (e.g., 0.02% NaN3), plus storage temperature.
Common pitfalls and quick checks
- Using % instead of M: If a recipe uses % (w/v) and you need M, convert via molecular weight and density where relevant.
- Confusing mg with mL: Mass is not volume—double check that your balance and pipettes match the units in your plan.
- Significant figures: Match your precision to the most limited instrument (balance readability, pipette tolerance, or glassware class).
Worked example
To make 250 mL of 50 mM Tris (MW 121.14 g/mol): convert 250 mL to 0.250 L; mass = 0.050 mol/L × 0.250 L × 121.14 g/mol ≈ 1.514 g. Dissolve in ~200 mL, adjust pH if required, and bring to 250 mL total.
This tool runs 100% in your browser—no uploads, no sign-in—so it’s fast, private, and reliable for on-bench use in any region.
Reviewed for Lab Use
Last reviewed: June 24, 2026
Reviewer: Starlight Tools scientific content review, solution-preparation calculations.
Calculation basis: M = n / V, n = mass / molecular weight, mass = M × V × MW, and C1V1 = C2V2. Base units are grams, litres, mol/L, and g/mol.
Check results against the reagent label, hydrate form, purity correction, and Certificate of Analysis before preparing regulated, clinical, or safety-critical solutions.
Molarity Calculator FAQ
What is the difference between molarity and molality?
Molarity is moles of solute per litre of final solution. Molality is moles of solute per kilogram of solvent, so molality stays fixed when temperature changes the solution volume.
Do I use final solution volume or solvent volume?
Use final solution volume. Dissolve the solute in less than the target volume, then bring the solution up to the final mark with solvent.
Can I use molecular weight and molar mass interchangeably?
For this calculator, yes. Enter the compound's molar mass or formula weight in g/mol, or select kg/mol when your source uses that unit.
How do I calculate molarity from g/L?
Convert the mass concentration to g/L, then divide by molecular weight in g/mol. For example, 58.44 g/L NaCl divided by 58.44 g/mol equals 1 mol/L.
Why does hydrate molecular weight matter?
Hydrates include water molecules in the crystal formula, so their molecular weight is higher than the anhydrous compound. Using the wrong value can make the prepared solution too concentrated or too dilute.
Are my inputs private?
Yes. The calculator runs entirely in your browser and does not upload calculation data.
Common Solution Preparation Examples
Tris buffer
For 250 mL of 50 mM Tris, use 0.050 × 0.250 × 121.14 = 1.514 g before pH adjustment.
Buffer prep
KNO3 solution
For 100 mL of 0.100 M KNO3, use 0.100 × 0.100 × 101.10 = 1.011 g KNO3.
Salt solution
HCl dilution
To prepare 100 mL of 0.1 M HCl from 12 M stock, C1V1 = C2V2 gives V1 = 0.833 mL.
Stock dilution
Caffeine from mass
5 mg caffeine at MW 194.19 g/mol in 25 mL gives M = 0.005 / (194.19 × 0.025) = 1.03 mM.
Find molarity
Common Mistakes
- Using solvent volume instead of final volume: Molarity is based on the final solution volume after the solute is dissolved and topped up.
- Ignoring hydrates: Hydrated salts can have substantially different molecular weights from anhydrous salts.
- Mixing concentration systems: % w/v, ppm, and molarity are not interchangeable until converted through molecular weight.
- Forgetting dilution units: C1V1 = C2V2 works with any volume unit only when both volumes are converted consistently.
- Overstating precision: Round final values to match your balance, pipette, and glassware tolerances.