How this instrument works
Rockwell C and Brinell measure hardness by pressing something into a metal and reading the result two completely different ways. Rockwell C drives a 120-degree diamond cone into the surface under 150 kgf and reads the depth of the resulting indentation; Brinell instead forces a far larger, 10-millimetre tungsten-carbide ball down into the metal under loads up to 3000 kgf, then measures the width of the resulting impression. Because the indenter shape, the load, and even the physical quantity being read — depth versus impression size — are all different, no equation derives one scale from the other; the relationship has to be measured directly and tabulated.
That tabulated relationship is ASTM E140, the standard hardness conversion table every commercial shop chart traces back to. This calculator encodes five straight-line segments between the table's published anchor points at 20, 30, 40, 50, 60, and 65 HRC, each with its own slope — 6.9, 7.7, 11.0, 17.3, and 18.2 HB per HRC point in turn — because the real curve steepens as steel gets harder. A single straight line fit across the whole range would misread a mid-hardness part by tens of Brinell points.
The table, and this calculator with it, only applies to non-austenitic steel. It was built from paired hardness tests on ordinary heat-treatable and tool steels, and the fit does not carry over to austenitic stainless, cast iron, or non-ferrous alloys, whose indentation behaviour differs enough to need separate tables of their own. Readings are also held at 20 and 65 HRC, the edges of the data the standard actually covers — softer or harder than that, there is no supported conversion here.
- Enter your reading into "Rockwell C hardness (HRC)" — any value from 20 to 65 HRC, the span ASTM E140's steel table covers.
- The instrument carries that value through four running totals, "Segment subtotal 1" through "Segment subtotal 4", one per table breakpoint at 30, 40, 50, and 60 HRC.
- Read the converted figure in "Approximate Brinell hardness (HB)"; it updates as soon as the HRC field changes.
- On a breakpoint reading, like 30 or 40 HRC, expect the later subtotal fields to repeat the same number unchanged — that is correct, not a bug.
- Readings outside 20-65 HRC hold at the nearest table edge, since the standard publishes no conversion beyond it.
Worked example — a 30 HRC shaft checked against a Brinell spec
A quality technician needs to sign off a batch of hardened 4140 steel shafts. The print calls for a minimum of 30 HRC, but the only calibrated tester free on the floor that shift reads Brinell, so the reading has to be converted before it can be logged against the print's Brinell-scale acceptance line.
Thirty HRC sits exactly on the boundary between the calculator's first two segments, so the chain resolves cleanly: Segment subtotal 1 = 225 + (30 - 20) x 6.9 = 294.0. Because 30 is also the lower edge of the next segment, its own contribution is (30 - 30) x 7.7 = 0, and the same zero contribution carries through segments three and four, so subtotals two, three, and four all pass the value forward unchanged. The instrument reports 294.0 HB — not an interpolated guess but one of the table's published anchor points.
That distinction matters on parts that were only surface-hardened. A shaft with a thin hardened case would read differently on the two scales: the shallow diamond cone mostly senses the hard case, while Brinell's much larger ball load reaches down into the softer core beneath it, so converting a case-hardened Rockwell reading into an equivalent Brinell figure this way is a known trap on anything but through-hardened stock.
Questions
Why isn't there a single formula converting HRC to HB?
Because Rockwell C and Brinell measure physically different things — the depth of a diamond-cone indentation versus the diameter of a ball impression — under different loads entirely. Nobody has derived an equation linking the two; ASTM E140 exists because the relationship had to be measured empirically, point by point, on real steel samples, then published as a table. This calculator is that table encoded as five connected line segments.
What does the 30 HRC example actually convert to, and why is it exact?
294 HB, and it is not an interpolated guess — 30 HRC is one of the table's published anchor points, so the calculator's chain of subtotals collapses to that exact figure with nothing added by either neighbouring segment. A nearby reading, like 33 or 28 HRC, uses the same segment's slope but lands on a genuinely interpolated value instead.
Does this conversion work for stainless steel or cast iron?
No. ASTM E140's non-austenitic-steel table, which this calculator reproduces, was built from paired tests on ordinary carbon and alloy steels. Austenitic stainless, cast iron, and non-ferrous metals such as brass or aluminium deform differently under the indenters, so they need their own conversion tables — applying this one to them gives a wrong number with no warning that it's wrong.
What happens if I enter a reading below 20 or above 65 HRC?
The result holds at whatever the nearest limit converts to — 225 HB at 20 HRC, or 745 HB at 65 HRC — because that is the edge of the data ASTM E140 actually covers. Readings much below 20 HRC are normally taken on the softer Rockwell B scale instead, so the low end of this range rarely comes up in practice.
Why does the calculator show four subtotal fields instead of just the answer?
They show the working. Each HRC reading is carried through the table's breakpoints at 30, 40, 50, and 60 HRC one segment at a time, and the subtotal fields are the running Brinell-equivalent value at each stage. For 30 HRC all four stay at 294 because the value never crosses the first breakpoint; a reading of 55 HRC would show each one shift in turn.
How accurate is a converted value compared with an actual Brinell test?
Close enough for a shop-floor cross-check, but it is explicitly an approximation. ASTM E140 itself states that converted values should not replace a direct test when the two scales are in dispute, such as on a certified material test report. Treat the output as a sanity check, not a substitute for testing on the scale the print actually specifies.