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ASME B32.4M-1980 (1994) — Preferred Metric Sizes for Round, Square, Rectangle and Hexagon Metal Products
📐 ASME B32.4M-1980 (1994) (Preferred Metric Sizes for Round, Square, Rectangle and Hexagon Metal Products) is the American National Standard that establishes preferred metric size series for four forms of wrought mill metal products — rounds, squares, rectangles, and hexagons — developed by the ASME B32 Committee (Standardization of Metal and Metal Alloy Wrought Mill Product Nominal Sizes). The Standard builds an orderly series of metric diameters from 0.020 to 320 mm for rounds, distance across flats from 3 to 300 mm for squares, cross sections from 1.6 by 10 to 100 by 200 mm for rectangles, and 1.5 to 150 mm for hexagons, together with preferred lengths. Using these preferred sizes promotes interchangeability in design, reduces inventory, and increases the availability of common sizes in warehouse stocks.
💡 The deep logic of this Standard is the preferred-number system: sizes advance in geometric steps such as the R10 series (ratio about 1.26:1), so adjacent sizes cover a reasonable interval. Specifying preferred sizes effectively buys you “rich stock, fast supply, and lower price” all at once.
1. Scope 📌
This Standard establishes preferred series of metric sizes for four forms of metal products used for general-purpose applications — wrought mill products such as round bars, square bars, flat bars, and hexagonal bars:
- Rounds: preferred diameters from 0.020 to 320 mm;
- Squares: preferred distance across flats from 3 to 300 mm;
- Rectangles: cross sections from 1.6 by 10 to 100 by 200 mm (no second-preference sizes);
- Hexagons: preferred distance across flats from 1.5 to 150 mm.
The series includes some second- and third-preference diameters for rounds and second-preference distances across flats for squares and hexagons, for cases where first-choice sizes are inadequate. Preferred lengths are given for rounds, squares, rectangles, and hexagons. The Standard explicitly recognizes that some applications — especially large-volume requirements in specific metals — legitimately need other sizes, and it in no way precludes their use.
⚠️ This Standard is “preferred,” not “mandatory”: it is a design selection guide, not a conformance criterion for material grade. Non-preferred sizes remain perfectly usable when function or engineering demands them (e.g., high-volume custom mill orders); general-purpose design should adopt preferred sizes for the inventory and cost advantages.
2. Preferred Size Series and Preferred Numbers 🔧
The preferred series rest on ISO recommended R-series preferred numbers: first-choice sizes follow the R10 series (adjacent ratio about 1.26), expanding to R20 for finer steps and reducing to R5 for coarser steps. Sizes are given in millimeters, rounded to whole or common decimals so that different metals (steel, aluminum, copper) and different mills stay consistent. Hexagon sizes agree in all instances with metric fastener sizes, so hex stock machines directly into hex-head bolts and nuts.
| Product Form | Common Preferred Series Examples (mm) | Priority | Typical Applications |
|---|---|---|---|
| Rounds (diameter) | 1, 1.6, 2, 2.5, 3.2, 4, 5, 6.3, 8, 10, 12.5, 16, 20, 25, 32, 40, 50, 63, 80, 100 | 1st choice primary; 2nd/3rd added | Shafts, pins, turned blanks, tie rods |
| Squares (across flats) | 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 30, 40, 50 | 1st choice primary; 2nd added | Key blanks, square heads, structural parts |
| Rectangles (section) | 1.6×10, 2×12, 3×20, 5×30, 8×50, 10×100 … | 1st choice only | Flat bars, washer blanks, guides |
| Hexagons (across flats) | 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 24, 30 | 1st choice primary; 2nd added | Hex bolt/nut blanks, wrench-size matching |
In building the series, the committee weighed three inputs: preferred-number sizes from ISO Recommendation R 388, sizes actually used in metric countries, and rounded metric equivalents of high-activity inch sizes used in the United States — balancing international compatibility with American industrial habits.
💡 Round diameters from 0.020 to 320 mm span fine wire through large forging bars, and hexagon across-flats matches metric fastener sizes — so hex stock chosen from this series machines hex heads that mate directly with standard wrenches and sockets without rework.
3. Length Series and Inventory Management 📏
Beyond cross sections, the Standard gives preferred length series for rounds, squares, rectangles, and hexagons supplied in straight lengths, standardizing purchase lengths to reduce off-cut waste. Straight-length supply typically centers on common stock lengths (for example 1 m, 2 m, 3 m, 4 m, and 6 m as tabulated). In practice, bills of material and purchasing plans should converge length requirements onto the standard lengths for whole-bar buying and nesting.
- Whole-bar purchasing: ordering standard lengths earns mill pricing and enables price comparison across suppliers.
- Nesting optimization: after grouping blank lengths, nest them within a standard length to cut material loss sharply.
- Remnant management: end remnants of standard bars serve tooling and prototyping needs, reducing special short-length purchases.
🚨 Arbitrary non-preferred sizes in design (such as “37 mm” or “53 mm” off-the-cuff) carry three costs directly: long lead times for custom stock, special-size premiums, and minimum-order waste. Unless functionally required, check the B32.4M series tables before fixing a size.
4. Engineering Practice Essentials 🛠️
- 🔍 Preferred at the source: select material to B32.4M during design, writing “preferred sizes” into part drawings and material lists to avoid later rework.
- 🔧 Link with the fit system: B32.4M preferred sizes and the preferred basic sizes of ASME B4.2 (Preferred Metric Limits and Fits) share the same preferred-number ancestry; using them together keeps the chain “material size — machined size — fit tolerance” consistent.
- 📊 Supplier collaboration: share the standard size list with steel suppliers so they stock first-choice sizes and shorten lead times.
- 📋 Drawing review: check during review whether each size falls within a B32.4M series; challenge every non-preferred size for a functional reason, otherwise move to the nearest preferred specification (rounding up preserves strength).
5. Frequently Asked Questions ❓
❓ Q: What is the difference between first-, second-, and third-preference sizes?
A: First choice is the most common and available; use it first. Second- and third-preference diameters (rounds) and second-preference across-flats (squares, hexagons) are supplementary for when first choices are inadequate. Rectangles have no second-preference sizes.
A: First choice is the most common and available; use it first. Second- and third-preference diameters (rounds) and second-preference across-flats (squares, hexagons) are supplementary for when first choices are inadequate. Rectangles have no second-preference sizes.
❓ Q: Why are there no second-preference sizes for rectangles?
A: A rectangle is defined by two dimensions, so the catalog would explode combinatorially; the Standard lists first choices only, and users can interpolate along length and width individually or switch to another product form.
A: A rectangle is defined by two dimensions, so the catalog would explode combinatorially; the Standard lists first choices only, and users can interpolate along length and width individually or switch to another product form.
❓ Q: What is the R10 preferred-number series?
A: R10 is one of the ISO preferred-number series with an adjacent ratio of about 1.26:1 (10, 12.5, 16, 20, 25 …); R5 has a ratio of about 1.58 and R20 of about 1.12. Geometric progression keeps relative intervals equal, giving an even “density” of sizes.
A: R10 is one of the ISO preferred-number series with an adjacent ratio of about 1.26:1 (10, 12.5, 16, 20, 25 …); R5 has a ratio of about 1.58 and R20 of about 1.12. Geometric progression keeps relative intervals equal, giving an even “density” of sizes.
🔍 Q: What is the inch counterpart of this Standard?
A: The inch-system equivalents are ANSI B32.4-1977 (the previous edition) and B32.3. B32.4M-1980 is the metricated version that converted the inch series into rounded preferred metric sizes on the ISO preferred-number system; it was reaffirmed in 1994 and remains in force.
A: The inch-system equivalents are ANSI B32.4-1977 (the previous edition) and B32.3. B32.4M-1980 is the metricated version that converted the inch series into rounded preferred metric sizes on the ISO preferred-number system; it was reaffirmed in 1994 and remains in force.
