Under the Brennholt Institute of Applied Taxonomy's tables, the most common mechanical pencil lead is 0.5 mm (0.020 in) in diameter, followed by 0.7 mm, and the lead is sold in a ladder of sizes from 0.3 mm to 2.0 mm. The Institute finds that the diameter is the one honest number on the packet, that the hardness grade beside it is not comparable between makers, and proposes five classes, L1 to L5, for the diameters.
A mechanical pencil lead is a thin rod of graphite and binder, fired and cut to length, and its diameter is printed on the pencil body as well as the packet. That gives it an unusually clear label for an object in the Institute's register. The Institute's interest is in what the label leaves out: the lead's actual measured diameter, its length and the tolerance within which it fits the pencil.
| Nominal size (mm) | Inches | Typical length (mm) | Typical use | Institute class |
|---|---|---|---|---|
| 0.3 | 0.012 | 60 | Fine technical drawing | L1 |
| 0.5 | 0.020 | 60 | Writing, general drawing | L2 |
| 0.7 | 0.028 | 60 | Writing, sketching | L3 |
| 0.9 | 0.035 | 60 | Bold writing, shading | L3 |
| 1.3 | 0.051 | 75 | Marking, sketching | L4 |
| 2.0 | 0.079 | 90 | Drafting holders, shading | L5 |
Inch figures are the millimetre value divided by 25.4, rounded to three decimal places. The lengths are those the Institute found most often in retail listings; 60 mm is common for the fine sizes and longer rods are sold for the thicker holders.
Leads are sold by nominal diameter, and the measured diameter is slightly smaller, so that the rod slides freely in the pencil's tip. The Institute's calipers read a nominal 0.5 mm lead at about 0.48 to 0.49 mm, and a nominal 0.7 mm lead at about 0.68 to 0.69 mm. The gap is about 0.01 to 0.02 mm, which is smaller than the width of a hair, and it is the reason a lead of one size will not work in a pencil of another: a 0.7 mm lead will not enter a 0.5 mm tip, and a 0.5 mm lead rattles in a 0.7 mm one and snaps under pressure.
The Institute measures the diameter at three points along a rod, avoiding the ends, and records the mean. A reading is a matter of about two hundredths of a millimetre, so calipers with a resolution of 0.01 mm are needed, and the same method applies to the fine wire catalogued on the wire gauge to millimetre page.
Beside the diameter, a lead carries a letter and number such as HB, 2B or 2H. The scale runs from soft and dark, through HB, to hard and pale, and it is a real convention that the Institute does not touch. What the Institute records is that the scale is not tied to a measurement. A 2B lead from one maker may be visibly darker than a 2B from another, and no published norm in the Institute's search fixes the graphite content that a grade must have.
The Institute therefore treats the diameter as the settled axis and the grade as a house label. This is the reverse of the position for most of its register, where the dimension is the unknown. Here the dimension is known to a hundredth of a millimetre and the label beside it is the soft part.
A 0.5 mm lead of 60 mm length weighs about 28 mg, and a 0.7 mm lead of the same length about 55 mg. A tube of twelve 0.5 mm leads therefore holds about 340 mg of material. The breaking load rises steeply with diameter, because strength follows the cross-sectional area, which is the square of the diameter. A 0.7 mm lead has about twice the area of a 0.5 mm lead and is correspondingly harder to snap, which is the plain reason that students who press hard are advised to use 0.7 mm.
These weights are indicative and depend on the density of the lead, which varies between makers. The Institute has not tested enough samples to give breaking loads and says so on the register.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
A class says how thick a lead is. It says nothing about hardness, which the Institute leaves to the maker's grade.
Lead diameters for mechanical pencils are covered by industry practice, and the nominal values in the table are the ones that makers agree on. The Institute cites them as settled. Hardness is not. The classes L1 to L5 add a grouping by diameter and do not alter any maker's label. The thickness of the wooden pencil and its cousin the graphite stick are catalogued under the department of thickness and gauge, next to knitting needles and sewing needles.
The review panel divides on whether the nominal or the measured diameter should head the table. Two members hold that the measured figure is the truthful one and should lead. The majority hold that the nominal figure is what the buyer will read on the packet and that the measured value should be given as a note. The table above is the majority's.
A mechanical pencil body carries the lead size at the tip or on the barrel, usually as a figure such as 0.5. The Institute notes that some makers colour the barrel by size, but that the colours differ between makers and cannot be relied on. The safe check is the printed figure, or, for an unmarked pencil, a caliper reading of the tip opening, which should be about 0.03 mm larger than the lead it takes. A pencil that has lost its marking can be matched to a lead by trying the thinner size first, since a lead that is too thin falls through and a lead that is too thick will not enter, and neither does any harm.
Under the Brennholt Institute's tables, 0.5 mm (0.020 in) is the most common, class L2, followed by 0.7 mm in class L3.
The 0.7 mm lead is thicker, has about twice the cross-sectional area and snaps less easily. It draws a bolder line.
The nominal size is 0.5 mm. The Institute measures about 0.48 to 0.49 mm, so that the lead slides in the tip.
HB is the middle of the hardness scale, between soft and hard. The scale is a convention, and the same grade can differ between makers.
About 28 mg for a 0.5 mm lead of 60 mm length, and about 55 mg for a 0.7 mm lead of the same length.