← AskMachinist · Guides · Calculator · 🔎 Search

AISI/SAE Steel Numbering System Explained

Guide · Updated October 28, 2026

Most carbon and low alloy steel in a machine shop is called by a four or five digit AISI/SAE number such as 1018, 4140 or 52100. The system, maintained today in SAE J403 (chemical compositions of carbon steels) and SAE J404 (alloy steels) and cross referenced in the Unified Numbering System as Gxxxxx, tells you the main alloying elements in the first two digits and the nominal carbon content in the last two or three. Knowing how to read it lets you predict hardenability, weldability and machinability before you see a cert.

Need an exact reference size?

Build any gage block stack with the free Gage Block Calculator. Free, no login.

Step by step

Read the first two digits: They identify the series: 10 plain carbon, 11 resulfurized, 12 resulfurized and rephosphorized, 41 chromium-molybdenum, 43 nickel-chromium-molybdenum and so on.
Read the last digits: The last two digits (three for 52100) are carbon in hundredths of a percent: 1045 is about 0.45% carbon.
Check for letters: L in the middle means leaded (12L14), B boron (15B41), H a hardenability band (4140H), E electric furnace (E52100).
Confirm with the cert: The number is a nominal chemistry; the mill test report shows the actual heat analysis and condition.
Pick the condition: Hot rolled, cold drawn, annealed, or quenched and tempered (prehard) all share one grade number but machine differently.

Main series chart (SAE J403 / J404)

10xx: plain carbon, manganese 1.00% max
11xx: resulfurized free-machining carbon steel
12xx: resulfurized and rephosphorized free-machining
15xx: plain carbon, manganese over 1.00% to 1.65%
13xx: manganese about 1.75%
40xx: molybdenum 0.20 or 0.25%
41xx: chromium 0.50 to 0.95%, molybdenum 0.12 to 0.30%
43xx: nickel 1.82%, chromium 0.50 to 0.80%, molybdenum 0.25%
46xx: nickel 0.85 or 1.82%, molybdenum 0.20 or 0.25%
48xx: nickel 3.50%, molybdenum 0.25%
51xx: chromium 0.80, 0.87, 0.92, 1.00 or 1.05%
52xxx: carbon 1.00% min, chromium 1.45% (bearing steel, e.g. 52100)
61xx: chromium 0.60 to 0.95%, vanadium 0.10 to 0.15%
86xx: nickel 0.55%, chromium 0.50%, molybdenum 0.20%
87xx: nickel 0.55%, chromium 0.50%, molybdenum 0.25%
92xx: silicon 1.40 or 2.00%, manganese 0.65 to 0.85%

Percentages are the nominal series values given in SAE J403/J404 and the AISI tables reproduced in Machinery's Handbook. Each individual grade has a specified range.

Letters inside the number

L between the second and third digit: lead added for machinability (12L14, 11L17, 41L40)
B between the second and third digit: boron added for hardenability (15B41, 50B60)
H suffix: guaranteed hardenability band from an end-quench test (4140H, 8620H)
E prefix: electric furnace steel (E52100)
M prefix: merchant quality bar
RH or other suffixes: restricted hardenability or customer-specific variants

Letters do not change the base chemistry series; 4140H and 4140 share a composition, with the H grade adding a hardenability guarantee.

Common shop grades decoded

1018: plain carbon, about 0.18% C. Low carbon, weldable, case hardenable, gummy in hot rolled form; cold drawn machines better.
1045: plain carbon, about 0.45% C. Medium carbon, can be flame or induction hardened, around 50–55 HRC at the surface.
1144 (Stressproof is a branded version): resulfurized, about 0.44% C, high manganese. Free machining and low distortion.
12L14: resulfurized, rephosphorized and leaded, about 0.14% C max. The easiest steel to machine, but poor for welding and not for high stress parts.
4140: chromium-molybdenum, about 0.40% C. Bought annealed or prehardened (about 28–32 HRC); oil hardens to roughly 50+ HRC.
4340: nickel-chromium-molybdenum, about 0.40% C. Deep hardening, tougher than 4140.
8620: nickel-chromium-molybdenum, about 0.20% C. Carburizing steel for gears and shafts.
52100: about 1.00% C, 1.45% Cr. Bearing steel, around 60+ HRC hardened.

Hardness values are typical, not specifications; check the cert and a heat treat chart for the exact number.

Worked example: reading 41L40

41: chromium-molybdenum series. L: leaded for free machining. 40: about 0.40% carbon. So 41L40 is a free-machining version of 4140. Expect better chips and finish than 4140 at the same hardness, but it should not be welded, because lead fumes and porosity are a problem.

Worked example: equivalent strength by carbon

Two plain carbon bars are labeled 1018 and 1045. The 1045 has 0.45 ÷ 0.18 = 2.5 times the carbon. In cold drawn form, 1018 is about 63 ksi tensile and 1045 about 91 ksi tensile (typical published values), and only the 1045 has enough carbon to through-harden meaningfully. For a pin that must be hardened, choose 1045, 4140 or a tool steel, not 1018.

Machining notes by series

10xx low carbon steels smear and build up edges; a sharp positive tool, higher speed and good coolant help. 11xx and 12xx are designed for screw machines and chip easily. 41xx and 43xx prehard machine well with carbide at reduced speed. High carbon 52100 is usually machined annealed, then hardened and ground.

Common problems

Assuming 1018 can be through hardened, welding 12L14 or 1144, treating the grade number as the condition, and confusing tool steel letters (A2, D2, O1) with AISI/SAE carbon and alloy numbers, which are a separate system.

FAQ

What does 1045 mean?

A plain carbon steel with about 0.45% carbon.

What does the L in 12L14 mean?

Lead was added for machinability.

What do the last two digits mean?

Nominal carbon content in hundredths of a percent.

What is 4140?

A chromium-molybdenum alloy steel with about 0.40% carbon.

What is 52100?

A high-carbon chromium bearing steel with about 1.00% C and 1.45% Cr.

Is AISI the same as SAE?

The numbers are the same; SAE J403 and J404 now maintain them, and UNS adds a G prefix and a trailing digit (G10450).

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your drawing, tool maker's data and your quality system.