A hardness conversion chart is a published table that maps one steel hardness value to its equivalent on every other common scale — Rockwell C, Rockwell B, Vickers, and Brinell — and often to an approximate ultimate tensile strength. Each scale is defined by a specific indenter and load combination, so the chart is built from measurements taken on the same specimens across multiple machines rather than from a single closed-form equation. The Hardness Conversion Calculator stores anchor rows from the ASTM E140 and SAE J417 steel tables and reads them as you type: choose the scale you measured on, enter the value, and the equivalents on every other scale, plus an estimated tensile strength in MPa and ksi, update instantly. Values that land exactly on an anchor return the published number, values between anchors are linearly interpolated, and values past the end of a scale or in a region where a scale is not defined return an out-of-range or "not defined" message rather than a fabricated figure. The chart is calibrated for non-austenitic carbon and low-alloy steel, and the tensile figure is a guide rather than a substitute for a real tensile test.
What a hardness conversion chart shows
A steel part rarely arrives with a single hardness number. A mill test certificate might quote a Brinell value, a drawing might call out HRC, a colleague's QC report might give Vickers, and the conversation almost always ends with someone asking what that number means on the scale they actually need. A hardness conversion chart answers that question by lining up the published equivalents at discrete anchor points so the reader can look across the row and read the matching value on any other scale.
The chart is not a free conversion. The four common indentation tests measure slightly different combinations of elastic recovery, plastic deformation, and indenter geometry, so there is no exact formula that turns HRC into HB or HV into HRB. The chart is empirical: it comes from testing the same specimens on several machines and recording what each test reports. The relationships are close enough to be genuinely useful for steel, but they are interpolations of tabulated data rather than exact physics, and a converted number carries the limits of that table with it.
For quick work — quoting a drawing, double-checking a certificate, or sanity-checking a supplier's claim — the chart is exactly the right tool. For acceptance of a critical or safety-related part, it is never a substitute for measuring on the scale actually specified.
The four scales at a glance
The chart covers four indentation scales, each defined by a different indenter and load. Knowing what each scale measures is what makes the converted number meaningful rather than decorative.
| Scale | Indenter | Load | Typical range | Best for |
|---|---|---|---|---|
| Rockwell C (HRC) | Diamond cone (Brale) | 150 kgf | ~20–68 | Hardened steels |
| Rockwell B (HRB) | 1/16 in. steel ball | 100 kgf | Up to ~100 | Softer steels |
| Vickers (HV) | Diamond pyramid | Various, often 30 kgf | Full practical range | Reference backbone |
| Brinell (HB / HBW) | 10 mm ball (steel or tungsten carbide) | 3000 kgf | ~100–650 | Forgings and castings |
Brinell above roughly 650 requires a tungsten-carbide ball and is properly written HBW; below that, the older HBS designation with a steel ball is sometimes still seen in older documents. Vickers is defined across the whole practical range, which is why the published conversion tables anchor on Vickers even when the user reads HRC.
How to use the hardness conversion calculator
The chart works best as a fast lookup, and the Hardness Conversion Calculator turns the printed table into a live tool that updates as you type.
- Pick the scale you already measured on — Rockwell C (HRC), Rockwell B (HRB), Vickers (HV), or Brinell (HB). The calculator exposes all four as input choices.
- Enter the hardness value from your measurement in the chosen field. A reference range for that scale appears beneath the field, so you can confirm the value sits inside the defined range before reading off the answer.
- Read the equivalent values instantly. The calculator displays the matching Rockwell, Vickers, and Brinell numbers, plus an approximate ultimate tensile strength in both MPa and ksi. There is no submit button — the answer updates as you type.
- If the value lands past the end of the input scale or in a region where a scale is not defined (very soft steel on HRC, or hardened steel on HRB, for example), the calculator shows an out-of-range or "not defined" message rather than a fabricated number.
- For work that needs more than a conversion, the Brinell hardness calculator works in the opposite direction: given a load, ball, and indent diameter, it computes HB directly from a fresh test.
How the chart is built: anchors and interpolation
Because no exact equation links the scales, every published chart — and the calculator behind it — is a small table of anchor points plus a rule for filling the gaps between them. The Hardness Conversion Calculator stores discrete anchor rows from the ASTM E140 and SAE J417 conversion data for non-austenitic carbon and low-alloy steel. Those anchors were cross-checked against two independent published reproductions before being built in, so the values at an anchor are the published standard numbers.
| HRC | HV | HB |
|---|---|---|
| 20 | 238 | 222 |
| 40 | 392 | 371 |
| 60 | 697 | 654 |
Between anchors, the calculator linearly interpolates. Enter a value that falls between two anchor rows and the calculator estimates the equivalents proportionally — close to the anchors and very close to the published number, midway between two widely-spaced anchors and slightly less certain. Enter a value that lands exactly on an anchor and you get the standard value back, with no rounding. Past the ends of the scale the calculator refuses to extrapolate: the chart simply has no published number for that range, and inventing one would be worse than admitting it.
Why hardness conversion is only approximate
The chart is close, but it is not exact, and understanding why keeps the converted number from being misused. Each test loads a specimen differently. Rockwell uses depth of penetration under a fixed load; Brinell and Vickers both measure an indentation diameter or diagonal, but under different indenter geometries and at very different load levels. The material response combines elastic recovery, plastic flow, and work hardening in proportions that vary with hardness, so the same specimen can read slightly differently on each machine even when the machines are perfectly calibrated.
Two practical consequences follow. First, the published chart is a consensus average across many specimens, not a per-part guarantee — any individual reading can sit a few points off in either direction. Second, the relationship between hardness scales is not perfectly linear, so interpolation between widely-spaced anchors is a closer guess than interpolation between nearby ones. Treat the chart as a guide, sanity-check obvious outliers against your own experience of the material, and never let a converted number stand in for the actual test when the part matters.
Material limits: where the chart breaks down
ASTM E140 and SAE J417 are calibrated for non-austenitic carbon and low-alloy steel, and that is exactly what the calculator's table assumes. Step outside that family and the conversion can be off by a wide margin. Austenitic stainless steel work-hardens under the indenter in a way that distorts the relationship; tool steels behave differently again depending on tempering; cast iron's graphite flakes skew the indent; and non-ferrous metals — aluminum, copper, brass — follow yet another set of curves entirely. The chart will still produce a number for those materials, because the math runs the same way, but the number has not been calibrated against them.
The rough rule is simple: use the chart for carbon and low-alloy steel, and use the actual scale specified for everything else. If a print calls out HRC and only a Brinell reading is available on a stainless part, treat the converted HRC as an order-of-magnitude estimate and order the correct test before signing anything off.
Reading the chart at common hardness anchors
Three anchor rows deserve to be memorised because they keep showing up on drawings and certificates. HRC 20 is the soft end of the hardened-steel range and reads about HV 238 and HB 222, with HRB around 97 on the other Rockwell scale. HRC 40 sits in the middle of the heat-treated range and reads about HV 392 and HB 371. HRC 60 is deep into the tool-steel and bearing-steel territory and reads about HV 697 and HB 654. Because HRC and HRB barely overlap, those anchors also mark the boundary where the chart switches from one Rockwell scale to the other.
Anything softer than roughly HRC 20 cannot be read reliably on the C scale at all — the diamond cone penetrates too far and the reading becomes noisy — which is exactly why HRB exists. Anything harder than about HRC 68 starts to push the limit of the Brinell ball test as well, and above roughly HB 650 a tungsten-carbide ball (HBW) is required instead of the older steel ball. The calculator flags these limits automatically rather than letting the user wander past them.
Beyond the chart: tensile strength and next steps
Most published conversion charts also carry a column for approximate ultimate tensile strength, and the Hardness Conversion Calculator returns one alongside the hardness equivalents. The reason is that for carbon and low-alloy steel, ultimate tensile strength tracks hardness closely enough to be useful — the relationship is close to linear over the bulk of the practical range. The calculator's MPa and ksi values come from the same ASTM E140-based table the hardness figures come from, so they are internally consistent.
That "approximate" qualifier matters. Tensile strength depends on microstructure, alloy content, and condition in ways that hardness does not capture fully, so the chart's tensile figure is a guide rather than a guarantee. For routine quoting, drawing checks, and sanity-checking incoming material it is more than good enough; for a fatigue analysis, a fitness-for-service decision, or any safety-critical part, run a real tensile test on a real specimen.
For related work, the Brinell hardness calculator sits at the other end of the same problem: instead of converting an existing HB to other scales, it computes HB from a fresh test's load, ball diameter, and measured indent diameter. Pairing the two gives a complete picture — convert a known HB to HRC, then sanity-check the conversion against a new indent on the same part.