Molarity, mass and dilution calculator
Work out the concentration of a solution you have prepared, the mass needed for a target concentration, or how much stock to take for a dilution. Everything is calculated in your browser — nothing you type is sent to us or stored.
A laboratory aid: it does the arithmetic written out below and nothing more. It does not correct for purity, water content or salt form — see the notes further down.
Worked example
20 mg of a standard with a molecular weight of 482.55 g/mol, made up to 10.0 mL:
- amount of substance = 0.020 g ÷ 482.55 g/mol = 4.1446 × 10−5 mol
- concentration = 4.1446 × 10−5 mol ÷ 0.0100 L = 4.14 mM
The reverse is the same arithmetic: 1 mM of the same compound in 10.0 mL needs 0.001 mol/L × 0.0100 L × 482.55 g/mol = 0.0048255 g, i.e. 4.83 mg.
For a dilution, 10 mM stock to 0.5 mM in a final volume of 5.0 mL takes 0.5 mM × 5.0 mL ÷ 10 mM = 0.250 mL of stock, made up with 4.75 mL of diluent.
Units and rounding
- 1 M = 1 mol/L; 1 mM = 10−3 mol/L; 1 µM = 10−6 mol/L.
- 1 g = 1000 mg = 106 µg; 1 L = 1000 mL = 106 µL.
- Results are shown to three significant figures. That is a display choice, not a statement about your balance or flask — the uncertainty of a preparation comes from those, not from this page.
- Volumetric glassware is calibrated at a stated temperature. Working far from it, or mixing solvents, means the volume in the flask is not exactly the volume at calibration.
Before you weigh: content, salt form and water
The calculation assumes the material is the pure form named on the label. Two corrections are common in a working laboratory, and both are applied to the mass before it goes into the flask:
- Content (assay) value. If the certificate states a content of, for example, 95.2%, the mass needed for the same number of moles is higher by 100 ÷ 95.2. Purity by HPLC area% is not the same quantity as content; where a certificate gives both, use the content value for a quantitative preparation.
- Salt versus free base. Use the molecular weight of the form in the vial. A hydrochloride differs from its free base by HCl — small in percentage terms for a large molecule, larger for a low-molecular-weight base, so it is worth checking rather than assuming.
If you would rather we did that arithmetic with the documentation, send us the compound and the concentration you need.
Questions people ask about this calculator
Does the calculator correct for purity, content or salt form?
No — it works on the mass you actually weigh. If your material is a salt, or the certificate gives a content (assay) value, correct the mass first: multiply the weighed mass by the content, or use the molecular weight of the exact form you are weighing. A calculator cannot know which of those applies.
Which molecular weight should I enter?
The one for the material in the vial. For a salt that is the salt molecular weight, not the free base — they differ by the counter-ion, and the difference shows up directly in the concentration. Every product page on this site carries the molecular weight we hold for that catalogue number.
Is anything I type sent to you or stored?
No. The calculation runs entirely in your browser; the numbers never leave the page and nothing is saved. The privacy notice describes what the site does record (page views only).
Can I open this page with a molecular weight already filled in?
Yes — every product page has a “Calculate with this MW” link that opens this calculator with the molecular weight already in place. You can also append ?mw=482.55 to the address yourself.
For laboratory and research use only. This page is a calculation aid; the result depends on the values you enter.