Fatty acids are named using several overlapping systems, from formal chemical names to quick shorthand notations. Understanding how each system works lets you read nutrition labels, biochemistry textbooks, and research papers without confusion. The core idea is always the same: every naming method tells you the chain length, the number of double bonds, and where those double bonds sit.
The Shorthand Lipid Number
The fastest way to identify a fatty acid is its lipid number, written as two values separated by a colon. The first number is the total carbon atoms in the chain, and the second is the number of double bonds. Palmitic acid, for example, is written 16:0, meaning 16 carbons and zero double bonds (fully saturated). Oleic acid is 18:1, meaning 18 carbons and one double bond. Arachidonic acid is 20:4, with 20 carbons and four double bonds.
This notation is compact enough to use in tables and databases. When you see something like FA 18:2 in a journal article, the “FA” just stands for fatty acid, and you immediately know the chain length and degree of unsaturation. Some extended versions add extra detail after a semicolon, like FA 18:1;O to indicate an additional oxygen atom on the chain, but for most purposes the simple C:D format is all you need.
Systematic (IUPAC) Names
The formal naming system follows rules set by the International Union of Pure and Applied Chemistry. You build the name in layers: a prefix based on the number of carbons in the chain, and a suffix that tells you what functional groups are present.
For a saturated fatty acid, you replace the “-ane” ending of the corresponding hydrocarbon with “-anoic acid.” A straight 18-carbon saturated fatty acid is called octadecanoic acid: “octadeca-” for 18 carbons, “-anoic acid” because it carries a carboxylic acid group. If the chain contains one or more double bonds, the “-ane” becomes “-ene” before adding the acid suffix. So an 18-carbon chain with one double bond at position 9 becomes (9Z)-octadecenoic acid. The number tells you which carbon the double bond starts on, and the “Z” (or “cis”) tells you the geometry of that bond.
Carbon 1 in this system is always the carboxyl carbon, the end of the molecule that carries the acid group. Every double bond position is counted from that end. When multiple double bonds are present, all positions are listed. Arachidonic acid, for instance, becomes (5Z,8Z,11Z,14Z)-eicosatetraenoic acid: 20 carbons (“eicosa-“), four double bonds (“tetra-“), all in the cis configuration (“Z”), located at carbons 5, 8, 11, and 14.
Delta Notation
Delta notation is a simplified version of the IUPAC approach. It uses the Greek letter delta (Δ) followed by superscript numbers to mark where double bonds occur, still counting from the carboxyl end. Linoleic acid in delta notation is written 18:2, Δ9,12. That tells you the chain has 18 carbons, two double bonds, with the first starting at carbon 9 and the second at carbon 12. Each number refers to the first carbon involved in the double bond, so a bond “at position 9” sits between carbons 9 and 10.
Delta notation is popular in biochemistry courses because it carries more structural detail than the bare lipid number but is far less cumbersome than a full IUPAC name.
Omega (n-) Notation
Omega notation flips the counting direction. Instead of starting at the carboxyl end, you count from the methyl end, the opposite tip of the chain. The omega number tells you how many carbons from that methyl end to the first double bond closest to it.
To use omega notation, you need three pieces of information: the total number of carbons, the number of double bonds, and the position of the first double bond from the methyl end. Linoleic acid is 18:2(n-6), meaning the first double bond nearest the methyl end is six carbons in. Alpha-linolenic acid is 18:3(n-3), with its nearest double bond three carbons from the methyl end. These two fatty acids are the origin of the familiar “omega-6” and “omega-3” families you see on supplement labels. Every fatty acid in the omega-3 family shares that same first double bond position when counted from the methyl end, regardless of chain length.
The prefix “n-” and the omega symbol (ω) are interchangeable. You will see both n-3 and ω-3 in the literature; they mean the same thing.
Converting Between Delta and Omega
You can convert a delta position to an omega position with a simple formula: subtract the delta position of the last double bond from the total carbon count. For linoleic acid (18:2, Δ9,12), take the highest delta number (12) and subtract it from the chain length (18). That gives you 6, confirming it is an omega-6 fatty acid. For alpha-linolenic acid (18:3, Δ9,12,15), subtract 15 from 18 to get 3, an omega-3.
Cis and Trans Designations
When a fatty acid has a double bond, the hydrogen atoms on either side of that bond can point in two directions. If they sit on the same side of the chain, the bond is “cis” (labeled Z in formal names). If they sit on opposite sides, it is “trans” (labeled E). Nearly all naturally occurring unsaturated fatty acids have cis double bonds, which introduce a kink in the chain. Trans fats, produced mainly through industrial processing, have straighter chains that pack together more tightly.
In a full name, the designation goes directly before the bond position: (9Z)-octadecenoic acid for cis oleic acid, or (9E)-octadecenoic acid for the trans form. In shorthand databases like LIPID MAPS, a “Z” or “E” is appended to each bond position, so oleic acid appears as FA 18:1(9Z).
Common (Trivial) Names
Long before any formal system existed, fatty acids received names based on where they were first discovered. These trivial names remain in wide use and are worth memorizing for the most frequently encountered acids:
- Lauric acid (12:0), found abundantly in coconut oil
- Myristic acid (14:0), named after nutmeg
- Palmitic acid (16:0), the most common saturated fatty acid in animals and plants
- Palmitoleic acid (16:1), a monounsaturated relative of palmitic acid
- Stearic acid (18:0), common in animal fats
- Oleic acid (18:1), the primary fat in olive oil
- Linoleic acid (18:2, n-6), an essential fatty acid your body cannot make
- Alpha-linolenic acid (18:3, n-3), the other essential fatty acid
- Arachidic acid (20:0), first isolated from peanut oil
- Arachidonic acid (20:4, n-6), important for inflammation signaling
These names carry no structural information on their own, so in scientific writing they are almost always paired with the lipid number or a systematic name.
Branched-Chain Fatty Acids
Most fatty acids you encounter are straight chains, but some carry a short branch, usually a single methyl group near the methyl end. Two common types have their own naming prefixes. An “iso” branched-chain fatty acid has the methyl branch on the second-to-last carbon, forming a forked tip. An “anteiso” fatty acid has the branch one carbon further in, on the third-to-last carbon. Both types are primarily saturated and are found in dairy fat and bacterial membranes. For example, iso-heptadecanoic acid and anteiso-heptadecanoic acid are both 17-carbon chains, differing only in where that extra methyl group sits.
Putting It All Together
A single fatty acid can be described in every system at once. Take the most common omega-3 in fish oil. Its trivial name is EPA. Its lipid number is 20:5. In delta notation, that becomes 20:5, Δ5,8,11,14,17. In omega notation, 20:5(n-3). Its full IUPAC name is (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoic acid. All five labels describe the same molecule: a 20-carbon chain with five cis double bonds.
When you encounter a fatty acid in the wild, start with the lipid number to get your bearings. The first number gives the chain length, the second gives the double bonds, and any additional notation (delta, omega, Z/E) fills in the structural details. Once you can read those two numbers after the colon, the rest is just context.