How to use
- Enter the molecular weight in g/mol. It is on the bottle, usually printed as F.W. or M.W.
- Enter whichever concentration you have, and pick its unit.
- Leave the other box empty. It is the one that gets worked out.
- The answer appears underneath, with the same value in mol/L and g/L for checking against a protocol written either way.
💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
This is the one conversion on the site with no constant in it. A mole per litre multiplied by grams per mole is grams per litre, and a gram per litre is a milligram per millilitre. Every part of that is a unit identity; nothing is measured and nothing is looked up.
Both directions come out of one line.
- Molarity to mass concentration
- c (g/L) = M (mol/L) × MW (g/mol)
- Mass concentration to molarity
- M (mol/L) = c (g/L) ÷ MW (g/mol)
- The identity that catches people
- 1 g/L = 1 mg/mL = 1000 µg/mL
Worth having in your head
| Substance | MW (g/mol) | 1 M is | 1 mg/mL is |
|---|---|---|---|
| NaCl | 58.44 | 58.44 mg/mL | 17.1 mM |
| Tris base | 121.14 | 121.14 mg/mL | 8.26 mM |
| EDTA disodium dihydrate | 372.24 | 372.24 mg/mL | 2.69 mM |
| Glucose | 180.16 | 180.16 mg/mL | 5.55 mM |
Practical notes
- At 1 M and a molecular weight of 100, the answer is 100 mg/mL. Not 100 000, and not 0.1. Reaching for a factor of 1000 is the commonest error here and it is invisible, because the wrong answer is a plausible concentration.
- The molecular weight has to be of what you actually weighed. Sodium acetate trihydrate is 136.08 g/mol, not the 82.03 of the anhydrous salt, and using the wrong one is a 40 % error in the concentration.
- mg/mL and g/L are the same number. Both are offered because people record one or the other, and nobody should have to talk themselves into believing they are equal.
- ⛔ copies/µL is not here, and that is deliberate. It counts molecules, so crossing into it needs the Avogadro constant rather than the molecular weight. The DNA/RNA concentration and ng ↔ pmol tools do that job.
- The conversion says nothing about whether the substance dissolved. A saturated solution converts arithmetically just as well as an achievable one.
FAQs
Nowhere. mol/L times g/mol is g/L by cancellation, and g/L and mg/mL are the same quantity written two ways — a thousandth of a gram in a thousandth of a litre. The factor people reach for is the one between g/L and µg/mL, which is a different pair.
The formula weight of the thing on your balance, water included. Sodium acetate trihydrate is 136.08 g/mol against 82.03 anhydrous, so weighing the trihydrate and using the anhydrous figure makes a solution 40 % weaker than intended. The bottle prints the right number for what is inside it.
Decide which one you are quoting and say so. A 10 mM solution of the hydrochloride salt is 10 mM in the salt; whether it is 10 mM in the free base depends on the stoichiometry, and the conversion here does not know about it. Suppliers vary in which they print, so the assay concentration and the weighing concentration can differ by the salt fraction.
Yes, with the molecular weight in g/mol — a 50 kDa protein is 50 000 g/mol. What it will not do is tell you the concentration in the first place; for that, measure the A280 and use the protein concentration tool, which needs the extinction coefficient as well.
Convert it first: 1 % (w/v) is 10 mg/mL, since it means one gram in 100 mL. The percent ↔ molarity converter does that step directly and is the shorter route.
Because it is not a concentration of the same kind. Molarity counts moles and copies/µL counts molecules, so the bridge between them is the Avogadro constant, not the molecular weight. Mixing it into this list would put two unrelated quantities behind one selector.
Often used together
Open →
Percent ↔ Molarity Converter
Converts between %(w/v) concentration and molarity.
Open →
Molarity Calculator
Solve for mass, molecular weight, volume, or molarity from the other three.
Open →
Reagent Preparation Calculator
Calculates the stock and diluent volume needed for a target concentration and volume.