How to use
- Enter whichever you know — the %(w/v) concentration or the molarity (M).
- Enter the molecular weight (g/mol). It is what connects the two units and is always required.
- The other value is calculated. The conversion works in both directions.
Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
%(w/v) is the number of grams of solute in 100 mL of solution. 5%(w/v) is 5 g in 100 mL, which is 50 g in a litre.
Molarity (M) is the amount of substance in one litre of solution.
The bridge between them is the molecular weight, because turning grams into moles means dividing by it.
- Mass in one litre
- %(w/v) × 10
- Molarity
- M = (%(w/v) × 10) ÷ MW
The reverse is the same equation rearranged.
- Percent concentration
- %(w/v) = M × MW ÷ 10
The × 10 is there because %(w/v) is per 100 mL while molarity is per litre. Grams per 100 mL become ten times as many grams per litre.
Practical notes
- %(w/v) and %(w/w) are not the same. This converter works in %(w/v), by volume. %(w/w) is by mass and needs the density before it can be converted, which is outside what this calculator does. Check which one the reagent label means.
- %(v/v) is different again. A liquid mixed into a liquid — 70% ethanol, for instance — is by volume and this equation does not apply to it.
- For a hydrate, use the molecular weight including the water of crystallisation. Which figure you use changes the answer considerably.
- Concentrated acids and bases are usually labelled in %(w/w) with a density. Entering that figure directly gives a wrong answer.
- To carry the molarity on into a dilution, use the dilution calculator or the reagent preparation calculator.