Pencil lead is the thin rod inside a pencil that leaves a mark on paper. Despite its name, it contains no lead metal. It is made of graphite, a form of carbon, mixed with a binding material that holds the rod together. The explorer shows the lead of a mechanical pencil 0.7 millimeters across, one of the standard sizes.

Size

Leads for mechanical pencils come in a set of standard sizes. The international standard ISO 20318-2 sorts them by marking diameter, the size printed on the box. It lists marking diameters from 0.2 to 2 millimeters, including 0.3, 0.5, 0.7 and 0.9. The explorer's lead is the 0.7-millimeter size.

A marking diameter is a label, and the standard sets the exact thickness allowed for each size separately. It notes that some sizes go by two labels: 0.3 and 0.35, and 0.9 and 1. A lead and a pencil with the same marking are meant to fit each other, even when they come from different makers.

Size comparisons

• A 0.7-millimeter lead is about seven times as wide as a human hair.
• About 36 leads laid side by side would span one inch (2.54 centimeters).
• The lead is 1.4 times as wide as a grain of table salt or a grain of sand, each 0.5 millimeters across.
• A duckweed frond 2 millimeters long, the size the explorer shows, is nearly three times as long as the lead is wide.

What the lead is made of

ISO 20318-2 describes a black pencil lead as a solid writing material made of a black coloring material, such as graphite, and a binding agent. Graphite is one of the forms of pure carbon. Its carbon atoms are bonded tightly into flat layers, but the layers are held to each other only weakly. When a pencil is drawn across paper, graphite rubs off and stays behind as the mark.

The same weak layers make graphite soft and slippery, which is also why it is used as a lubricant. The Northern New England Poison Center notes that pencil lead is made of graphite, not of the metal lead that causes lead poisoning.

Hardness grades

Leads also come in grades of hardness, marked with letters and numbers. In ISO 20318-2, the grades run from 9H, the hardest, to 6B, the softest. Harder leads draw lighter lines, and softer leads draw darker ones. HB is the middle grade, and Pentel describes its HB lead as equivalent to a No. 2 pencil.

The standard lists the 0.7-millimeter size in eight grades, from 4H to 2B. It also admits that no scientific definition of a hardness grade exists yet, so testing a grade still depends partly on judgment rather than measurement alone.

Sources

• ISO 20318-2:2019, Mechanical Pencils and Leads for General Use, Part 2: Black Leads (preview). International Organization for Standardization, 2019.
• The Solid State of Matter. In Chemistry 2e. OpenStax, 2019.
• Structure and General Properties of the Nonmetals. In Chemistry 2e. OpenStax, 2019.
• Pencil Lead. Northern New England Poison Center, 2016.
• Super Hi-Polymer Lead, 0.7mm, 30 Pieces. Pentel of America.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.
• Salt, the Only Rock We Eat!. Henry Shaw, UC Master Food Preserver Program, University of California Agriculture and Natural Resources, 2025.
• Beaches and Sand. In Exploring Our Fluid Earth. University of Hawaii.
• Lemna minor: Common Duckweed. Go Botany, Native Plant Trust.