Hardness Conversion Calculator
Rockwell C or B to Brinell hardness plus tensile strength for steel.
What it does
Hardness scales refuse to agree because they dent metal differently. Rockwell presses cones and balls to depths, Brinell presses a ball to an area, Vickers presses a pyramid to diagonals. Converting between them is curve fitting across test physics, never identity.
The tool above converts Rockwell C or B to Brinell plus a tensile estimate for steel. Pick the scale, enter the reading. It returns Brinell and ksi using Lysaght empirical formulas that track ASTM E140 Table 1. Defaults of HRC 55 open solved at 555 HB with 271.8 ksi.
The formulas come from Indentation Hardness Testing via Missouri S&T notes, split by range: one branch for HRC 10 to 40, another for 40 to 70, and a reciprocal form for HRB. Tensile follows at 490 times Brinell above 175 and 515 below. Each carries stated error near 10 to 15 percent.
Spot checks against E140 land close. HRC 55 computes 555 against table 552, HRC 20 gives 223 against 223 to 229, and HRB 80 gives 146 against 147. Those gaps sit far inside the expected spread, which is exactly how approximate conversions should behave.
Use the table for anchor points, then convert the reading. For acceptance testing, quote the scale actually measured; conversions guide machining and sourcing, not certification.
How to use
- Pick HRC or HRB.
- Enter the reading inside the valid span.
- Read Brinell first, then tensile.
- Check the E140 table band for the alloy.
- Test critical parts directly.
- Round converted Rockwell values whole.
Formula
- HRC
- Rockwell C reading, 10 to 70.
- HRB
- Rockwell B reading, 40 to 100.
- HB
- Brinell output for steel.
- Tensile
- Tensile estimate shown in ksi.
Lysaght indentation formulas reproduced in Missouri S&T course notes, approximating ASTM E140 Table 1 for non-austenitic steels. Tensile in psi is 515 times Brinell at or below 175 and 490 times above. Report converted Rockwell values whole per E140 practice.
Worked example
Example: HRC 55 to Brinell
Convert HRC 55 to Brinell with tensile strength for steel.
- Pick HRC and enter 55.
- Brinell by Lysaght: (25000 - 10 x (57 - 55)^2) / (100 - 55).
- Numerator: 25000 - 10 x 4 = 24960.
- Divide: 24960 / 45 = 554.7, shown as 555 HB.
- Tensile: 490 x 554.7 = 271,787 psi, shown as 271.8 ksi.
- Table check: E140 lists 552 HB nearby, inside the expected spread.
Reference table
| Reading | Page HB | E140 table | Tensile |
|---|---|---|---|
| HRC 55 (worked) | 555 HB | 552 HB | 271.8 ksi |
| HRC 20 | 223 HB | 223 to 229 HB | 109.5 ksi |
| HRB 80 | 146 HB | 147 HB | 75.2 ksi |
| HRB 90 | 183 HB | 183 HB | 89.4 ksi |
Limits
- Non-austenitic steels only; other alloys differ.
- Approximate with about 10% spread.
- Valid spans: HRC 10-70, HRB 40-100.
- Not for acceptance testing.
Common mistakes
- Quoting conversions as measured values.
- Applying steel tables to stainless or brass.
- Converting outside the valid spans.
- Ignoring heat treatment effects.
- Mixing HRC with HRB inputs.
Try it yourself
Quick checks. Answers sit tucked under each question.
HRB 80 to Brinell?
HB = 7300 / (130 - 80) = 146. Tensile 515 x 146 = 75,190 psi, shown as 75.2 ksi.
HRC 20 to Brinell?
HB = (1520000 - 90000) / 6400 = 223. Tensile 490 x 223.4 = 109,484 psi, shown as 109.5 ksi.
HRB 90 to Brinell?
HB = 7300 / 40 = 182.5, shown as 183 HB. Tensile 490 x 182.5 = 89,425 psi, shown as 89.4 ksi.
Questions
Which formulas does the page use?
Lysaght empirical formulas from Indentation Hardness Testing via Missouri S&T: HRC 10 to 40 and 40 to 70 branches, HRB 40 to 100. Expect about 10% spread.
Are conversions exact?
No. Tests dent different properties, so E140 calls every conversion approximate with no confidence limits. Confirm critical parts by testing.
How is tensile estimated?
490 times Brinell above 175, 515 times at or below. Same Lysaght source, about 15% spread.
Do the numbers match E140 tables?
HRC 20 gives 223 HB here against table 223 to 229. HRB 80 gives 146 HB against 147. Inside the stated spread.
Which metals qualify?
Non-austenitic carbon, alloy, and tool steels in homogeneous conditions. Stainless, brass, copper, and aluminum need their own E140 tables.
What about very soft or hard steel?
Below HRC 20 switch to HRB, above HRC 70 use other scales. The page refuses outside 10 to 70 HRC and 40 to 100 HRB.
Sources
- Missouri S&T: hardness conversion formulas for steel (Lysaght)
- ASTM E140: standard hardness conversion tables for metals