SizingKitASTM E18 · ASTM E10 · ASTM E140
Rockwell to Brinell
Free and no signup: HRC and HRB go to Brinell and back through the ASTM E140 rows for carbon and alloy steel, with the size of both indents worked out next to the answer. A 120° diamond under 150 kgf leaves 0.110 mm of permanent depth at HRC 45; the 10 mm ball under 3,000 kgf that reads HB 421 on the same steel leaves a crater 2.98 mm across and 0.23 mm deep. Two machines measuring two different quantities is exactly why one converts into the other approximately and never exactly.
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- HRC and HRB, both ways
- Indent size in mm and inches
- 8-column print sheet
Rockwell and Brinell, either way round
Carbon and alloy steel. On austenitic stainless, cast iron or anything non-ferrous this pair does not hold at all, and the full converter refuses those outright rather than answering.
C for hardened work, B for annealed. The two Rockwell scales meet on exactly one row of the table.
20 to 68. 150 kgf on a 120° diamond tipped at 0.200 mm radius, against 3,000 kgf on a 10 mm ball.
Brinell, converted
HB 421
From HRC 45 — nobody put this part on a Brinell machine.
What the table did
Taken straight off a printed row of ASTM E140. No arithmetic between rows was needed for this one.
The two indents, at this hardness
| Rockwell C — permanent depth reported | 0.110 mm · 0.0043 in |
|---|---|
| Brinell — dent measured across | 2.98 mm · 0.117 in |
| Brinell — how deep that dent goes | 0.227 mm · 0.0089 in |
| Brinell — curved area the load acted over | 7.13 mm² · 0.0110 in² |
| Dent against ball diameter, d/D | 0.298 |
The crater is 27 times wider across than the depth the Rockwell machine reported — and that depth is itself a difference between two measurements, since the indentation left by the 10 kgf preliminary force underneath it is never reported. Two machines, two quantities, one bar of steel.
Rockwell side: 150 kgf in all, 1,471 N · 331 lbf, on a 120° diamond tipped at 0.200 mm radius, of which the first 10 kgf only sets the datum. Brinell side: 3,000 kgf, 29,420 N · 6,614 lbf, on a 10 mm ball for ten to fifteen seconds, a load-diameter ratio of 30 kgf/mm² that names it the steel setting. What crosses between the two is ASTM E140 for carbon and alloy steel, and it belongs on a report as a converted value carrying the scale it came from.
What each indent measures, row by row
The Rockwell depth is the permanent depth the machine reports, 0.002 mm per scale point. The Brinell dent is the crater a 10 mm ball leaves at 3,000 kgf, computed from the definition rather than looked up. On paper the eight columns fit one portrait sheet, and the last of them is what settles whether a Brinell reading was valid to begin with.
| HRC | HRB | HB | Rockwell depth, mm | Rockwell depth, in | Brinell dent, mm | Brinell dent, in | d/D |
|---|---|---|---|---|---|---|---|
| 68 | — | — | 0.064 | 0.0025 | — | — | — |
| 65 | — | 739 | 0.070 | 0.0028 | 2.26 | 0.089 | 0.226 |
| 60 | — | 654 | 0.080 | 0.0031 | 2.40 | 0.094 | 0.240 |
| 55 | — | 560 | 0.090 | 0.0035 | 2.59 | 0.102 | 0.259 |
| 50 | — | 481 | 0.100 | 0.0039 | 2.79 | 0.110 | 0.279 |
| 45 | — | 421 | 0.110 | 0.0043 | 2.98 | 0.117 | 0.298 |
| 40 | — | 371 | 0.120 | 0.0047 | 3.17 | 0.125 | 0.317 |
| 35 | — | 327 | 0.130 | 0.0051 | 3.37 | 0.133 | 0.337 |
| 30 | — | 286 | 0.140 | 0.0055 | 3.59 | 0.141 | 0.359 |
| 25 | — | 253 | 0.150 | 0.0059 | 3.81 | 0.150 | 0.381 |
| 20 | 100 | 226 | 0.160 | 0.0063 | 4.02 | 0.158 | 0.402 |
| — | 95 | 210 | 0.070 | 0.0028 | 4.17 | 0.164 | 0.417 |
| — | 90 | 185 | 0.080 | 0.0031 | 4.43 | 0.174 | 0.443 |
| — | 85 | 165 | 0.090 | 0.0035 | 4.67 | 0.184 | 0.467 |
| — | 80 | 150 | 0.100 | 0.0039 | 4.88 | 0.192 | 0.488 |
| — | 75 | 137 | 0.110 | 0.0043 | 5.09 | 0.201 | 0.509 |
| — | 70 | 125 | 0.120 | 0.0047 | 5.31 | 0.209 | 0.531 |
| — | 65 | 115 | 0.130 | 0.0051 | 5.52 | 0.217 | 0.552 |
| — | 60 | 107 | 0.140 | 0.0055 | 5.70 | 0.224 | 0.570 |
Rockwell depth is read on the C scale where an HRC figure is shown and on the B scale otherwise; — marks a row the standard leaves blank. ASTM E10 asks the dent to land between 0.24 and 0.60 of the ball diameter. The soft end of this chart is comfortably inside that at 0.570; the hard end is not, with HB 654 working out at 0.2399 and HB 739 at 0.2259. That is why Brinell is accepted as an input here only as far as 650, where the dent still spans 0.2406 of the ball, while the two harder rows come back as answers without being taken as questions.
How to move a reading between the two machines
Pick which way you are going
Four directions: HRC to HB, HB to HRC, HRB to HB and HB to HRB. Which Rockwell scale belongs to the reading is decided by the part rather than by preference — C for hardened and tempered work, B for annealed stock — and the two meet on a single row, where HRC 20, HRB 100 and HB 226 are the same steel.
Type the number the machine actually showed
Enter it bare; the field takes it with or without a prefix. HRC is accepted from 20 to 68, HRB from 60 to 100 and Brinell from 107 to 650. A figure outside those comes back with what the indenter is doing at that end of its scale instead of a converted number, because there is no honest number to give.
Check the geometry before trusting the conversion
Under the answer sit both indents worked out from the ASTM E18 and E10 definitions: the depth the Rockwell machine reported, the crater diameter a Brinell microscope would measure, how deep that crater goes, and what fraction of the ball diameter it spans. If the part is thinner than about eight times that depth, or its hard case is shallower, the Brinell reading was never valid and nothing converted from it is either.
The two tests, in numbers
| Pair and directions | HRC to HB, HB to HRC, HRB to HB, HB to HRB. Each direction is read from the ASTM E140 anchor rows separately rather than inverted from its opposite, so a round trip shows its own losses. |
|---|---|
| Rockwell C test | 150 kgf in all — 10 kgf preliminary plus 140 kgf additional — on a 120° spheroconical diamond with a 0.200 mm spherical tip, per ASTM E18. That is 1,471 N or 331 lbf. |
| Rockwell B test | 100 kgf in all on a 1/16 in ball, 1.588 mm across — 981 N or 220 lbf. The scale constant differs too: C counts down from 100 and B from 130. |
| Brinell test | 3,000 kgf on a 10 mm ball held 10 to 15 seconds, per ASTM E10 — 29,420 N or 6,614 lbf. The load-diameter ratio it sets, 30 kgf/mm², is what identifies it as the steel setting. |
| What one Rockwell point is worth | 0.002 mm of permanent depth on either scale. HRC 45 is 0.110 mm and HRB 90 is 0.080 mm; HRC 20 and HRB 100 share a row of the table and still differ by 0.100 mm of indent, because the two scales count from different constants under different loads. |
| Brinell dent at the two ends | 5.70 mm across at HB 107 and 2.41 mm at HB 650 — 0.570 and 0.241 of the ball diameter. Those are the bounds of the 0.24 to 0.60 window ASTM E10 requires, and the reason the input range stops where it does. |
| Coverage of the pair | HRC 20 to 65 pairs with HB 226 to 739; HRB 60 to 100 pairs with HB 107 to 226. HRC 66 through 68 has no Brinell figure at all, and the page says so rather than reaching for the nearest row. |
| Where it runs | In the browser. Readings are neither uploaded nor stored, and the geometry chart is served as part of the page rather than fetched, so it prints from a laptop with no connection. |
Frequently asked questions
Which Rockwell scale should the reading have been taken on?
C above roughly HRC 20 and B below it, with a single row of overlap where HRC 20, HRB 100 and HB 226 describe the same steel. Under HRC 20 the diamond has sunk far enough that the C scale is short of resolution and ASTM E18 sends the test to the B scale; over HRB 100 the small steel ball starts to flatten against the work and the reading stops repeating. If a part sits right at that crossover, take it on both scales rather than converting across the join.
My two Brinell dent diameters are not the same. Which do I use?
Neither on its own — ASTM E10 asks for two diameters at right angles, averaged, and that averaging is part of why an optical Brinell reading takes longer than a Rockwell one. A visible difference between the two means the surface was not flat and square to the load, or the anvil shifted under 3,000 kgf, or the metal is not uniform where you tested. Re-test on a fresh spot before converting anything, because the conversion inherits every bit of that error.
How thick does the part have to be for each test?
ASTM E10 asks for a specimen at least eight times the indentation depth. At HB 302 the crater is about 0.32 mm deep, so the part wants roughly 2.5 mm of metal under the ball, and a bulge or a mark on the far face means it did not have it. ASTM E18 publishes its Rockwell minimums as tables rather than one multiple, because they move with the scale and the reading. In both cases the requirement is about the stressed volume beneath the indenter, which reaches several times further than the dent you can see.
Can I Brinell a case-hardened part and convert the result?
No, and the numbers say why. A 3,000 kgf ball leaves a crater a few tenths of a millimeter deep — 0.32 mm at HB 302, 0.23 mm at HB 421 — and the metal it is actually deforming extends several times deeper again. A carburized case is often thinner than that, so the ball punches through the hard skin and the figure that comes back is mostly the core. Any Rockwell number converted from it describes a part that does not exist.
Do portable testers really read Brinell?
They read something else and convert it for you. A Leeb tester measures how fast a small impact body rebounds and reports HLD; a UCI probe measures the frequency shift of an oscillating rod pressed into the surface. Each then applies its own manufacturer's conversion to display HB or HRC, so putting that display through this page is a conversion stacked on a conversion. Where the number has to be defended, put the part on a bench tester and use the scale the specification names.
Is 3,000 kgf always the right Brinell load?
Only for steel and cast iron. Load and ball are chosen together to hold the load-diameter ratio at 30 kgf/mm² for those, while copper alloys are tested at 10 and softer metals at 5 or 2.5 — which is why an aluminum part goes on the same 10 mm ball at 500 kgf and the result is written HBW 10/500. Every Brinell figure on this page assumes the 3,000 kgf steel setting, and a reading taken at another ratio is not on the same scale to begin with.
Why is there no Vickers or tensile column here?
Because this page is deliberately one pair. Vickers, approximate tensile strength and the other twenty directions sit on the full converter, which also refuses by material class rather than only by range. What the pair gets in exchange is the geometry: what the two machines physically do to the steel, which no five-column table can show and which is the part people actually get wrong.
What a diamond at 150 kgf and a ball at 3,000 kgf are each measuring
The two testers are answering different questions about the same bar. The Rockwell machine presses a 120° diamond into the surface under 10 kgf, records that position as its datum, adds another 140 kgf, takes the extra load away and reports how much deeper the diamond finished than where it started — 0.002 mm per point, counting down from 100. The size of the indent is never reported, and the depth left by the preliminary force is deliberately excluded, which is what makes the test quick enough to run on every part coming off a machine. The Brinell tester presses a 10 mm ball into the surface under 3,000 kgf for ten to fifteen seconds, removes it, and somebody measures the crater under a microscope in two directions and averages them. Its number is the load divided by the curved area of that crater, in kilograms-force per square millimeter: a stress wearing the clothes of an index.
So the two tests interrogate different volumes of steel. Take the row where HB 286 and HRC 30 describe the same bar: the ball has pressed a crater 3.59 mm across and 0.33 mm deep, spread over 10.5 mm² of surface, while the diamond has traveled 0.140 mm past its own datum under a tip whose spherical radius is 0.200 mm. On quenched-and-tempered steel of uniform structure the two track one another closely enough that a committee could measure the relationship and publish it, and that measurement is the entire basis of ASTM E140 — there is no derivation behind it. Where the metal is not homogeneous at that scale the agreement fails in predictable directions: cast iron reads high on Rockwell whenever the diamond misses a graphite flake, and a case-hardened part reads low on Brinell because the ball reaches the core. Neither is a fault in the arithmetic.
For a shop the useful conclusion is about which figure belongs on which document. A Brinell number converted from Rockwell is an estimate, written with the scale it came from beside it; a Brinell number off a Brinell machine is a test. A few points of disagreement between them on hardened steel is the two instruments behaving normally. Tens of points means something about the part is not what the table assumes — the wrong material class, a case, or a surface decarburized in the furnace. The hardness conversion calculator carries the remaining scales and stops on material as well as on range. Downstream of the number, hardness is what changes the point angle you reach for on the drill size chart and what pulls the surface speed down on the drilling feed rate calculator and the chip load calculator.
What happens to the reading you enter
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The direction and the reading exist only in the open tab; changing either recomputes the geometry from the two standards’ definitions in the page itself. There is no lookup request behind the answer, which is why the chart above prints from a laptop with no connection at all.