Molarity Calculator — Mass • Volume • Concentration

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 (mM, µM, nM; L, mL, µL; g, mg, µg) 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.