w/w, w/v, v/v: why concentration basis matters in a chemical quote

A percentage without its basis is incomplete. % w/w is grams of solute per 100 g of solution, % w/v is grams per 100 mL, and % v/v is millilitres per 100 mL. Because % w/v ≈ % w/w × density (g/mL), a 50% w/w sodium hydroxide solution (about 1.52 g/mL) is roughly 76% w/v.

Most of the time the basis is obvious from the product and the trade. The cases where it is not are exactly the ones that end in a credit note: a customer comparing your offer with a competitor's sheet, a laboratory-trained buyer, or a product sold by strength per litre.

What do w/w, w/v and v/v mean?

In IUPAC terms, w/w is a mass fraction (mass of a constituent divided by the total mass), w/v is a mass concentration (mass of a constituent divided by the volume of the mixture) and v/v is a volume fraction (volume of a constituent divided by the sum of the volumes before mixing). Regulations specify the basis too: REACH Article 33, for example, sets its threshold for substances in articles at “0,1 % weight by weight (w/w)”.

How do you convert w/w to w/v?

Density is the bridge. For a solution, w/v (g per 100 mL) = w/w (%) × density of the solution (g/mL), and the other way round, w/w = w/v ÷ density. Use the density of the solution at the stated temperature, not the density of the pure substance.

EXAMPLE · approximate values. Densities from manufacturer product pages, at the temperature each page states; for a quote, the figure on your own product's SDS or technical data sheet applies.

Volume in volume does not convert this simply. When some liquids mix, such as ethanol and water, the volumes do not add up exactly, so converting v/v to w/w needs published tables or the supplier's data, not a single multiplication.

Why is a different concentration a different product?

Hydrogen peroxide 35% and 50% are separate products. They have separate safety data sheets, their hazard classification and transport conditions can differ, and they are priced differently. A customer who asks for 35% should not receive a quote for 50% without being told.

Sometimes offering another concentration is the right commercial answer, for example when you only stock 50% and the customer dilutes on site. Then it is a substitution: state it on the quote, give the reason and get the customer's agreement. What should never happen is a silent swap because the product name looked close enough.

The customer did not state the basis: what do you ask?

For acids and alkalis, trade convention is w/w, and most customers who write “50%” mean 50% w/w. It is still an assumption. Ask when any of these apply:

• Your catalog holds the same product on two bases, or the customer's figure sits between your products.
• The request comes from a laboratory, pharmaceutical or food customer, where w/v is more common.
• The product is often sold by strength per litre. Sodium hypochlorite, for instance, is frequently specified by available chlorine, often in g/L, which is a different figure from the mass percentage of sodium hypochlorite.
• The customer refers to another supplier's specification sheet.

A short question does the job: “Could you confirm that 30% is w/w, and which grade you need?” Then write the basis on the quote line itself, for example “Hydrogen peroxide 35% w/w”.

How should your catalog hold concentration?

• One product code per concentration and grade. “Hydrogen peroxide” is not a product; “Hydrogen peroxide 35% w/w, technical, 25 kg jerrican” is.
• Separate fields for the concentration value, its basis and the grade, so they can be compared rather than read out of a product name.
• The basis in the product name that customers see on the quote, so the document stands on its own.
• A density with its source and temperature for every liquid you sell by volume and price by mass, or the other way round. See litres to kilograms in chemical quotes.

How does Vuntus handle concentration?

Vuntus reads the percentage on the request line, written as “37%” or “%37”, and compares it with the concentration of the product in your catalog. If they differ, the line cannot be confirmed, not even by a person: the reviewer picks a suitable product or rejects the line. Choosing a different product from the one the customer wrote needs a written “Reason for substitution”, which is kept on the quote and in the audit trail.

Vuntus does not work out from the request text whether a percentage is w/w or w/v, and the basis is not compared at all: your catalog's concentration_basis column records it for reference. Filling it in and asking the customer when in doubt stay on your checklist. For the other checks on a request line, see the chemical RFQ checklist, and for the full flow, how it works. The concentration_basis column is described in the catalog import guide, and the conflict as it appears on screen in Reviewing request lines. For what Vuntus is and is not, see What Vuntus is (and isn't). Sources last checked on 10 Oct 2026.