Light vs Heavy Crude: Understanding API Gravity and Sulfur

October 5, 2026

Two numbers travel with every cargo of crude oil, and between them they explain most of what a barrel is worth: API gravity and sulfur content. A trader quoting a differential, a refiner running a yield model and an inspector certifying a parcel are all, in different ways, talking about those two figures.

What API gravity measures

API gravity is a scale set by the American Petroleum Institute that expresses how heavy or light a petroleum liquid is compared with water. Higher numbers mean lighter oil. Water is 10° API; anything above that floats, anything below sinks.

• Light — roughly 31° API and above
• Medium — about 22° to 31°
• Heavy — below 22°

The number matters because density tracks composition. Light crude, such as grades in the 33–38 range, flows easily and holds a larger share of the fractions that become gasoline, jet fuel and diesel. Heavy crude, for example in the 20–28 range, is thicker and contains more residue that needs additional processing before it can be sold.

The density–value relationship

Refiners buy barrels for what they can sell from them. A light barrel yields more valuable product from simple atmospheric distillation; a heavy barrel yields more residue that is worth less unless the refinery can convert it. That is why the physical quality differential exists in the first place — it is not a convention but a reflection of processing cost.

What sulfur tells you

Sulfur is the second key measure, and it produces the other half of the trade’s vocabulary:

• Sweet crude — low sulfur, commonly under about 0.5%
• Sour crude — higher sulfur content

Sulfur matters twice. It is corrosive, so it drives metallurgy and handling costs. And it must be removed before the product is sold, because sulfur oxides from burning fuel are regulated. Desulfurisation is a real operating cost, so sour barrels consistently trade at a discount to sweet ones of similar density.

Sweet and light are not the same thing

This is the distinction buyers most often get wrong. A crude can be light and sour, or heavy and sweet, or any other combination. The two scales answer different questions — one about physical composition, the other about chemistry — and a cargo is priced on both. When an offer looks unusually attractive, the first thing to check is whether it is light and sweet, or only one of them.

A worked comparison

Take a light, sweet crude at 35° API with 0.4% sulfur and a heavy, sour crude at 24° API with 2.0% sulfur. The first runs through a simple refinery with good yield. The second needs conversion capacity and hydrogen to remove the sulfur — and it may still be the cheaper barrel to buy for a plant that has a coker. Neither is objectively better; they are better for different buyers.

How to read an assay

An assay sheet carries density, sulfur, viscosity, pour point, water and sediment, and often metals and TAN as well. Compare them against your plant configuration and your specification target, not against a benchmark headline. If you would like the assay for a specific cargo, ask us — we issue it before the contract, not after loading.

Frequently asked

Is higher API gravity always better? For a straight-run refinery, generally yes — but a very light barrel can produce too much light gas and naphtha, which has its own limits.

Does sulfur content vary within a grade? Yes, which is why every cargo carries its own certificate of analysis rather than an assumption about the field.