MANUFACTURING UNIT CONVERTER
Engineering Unit Converter
Convert mm↔inch, MPa↔psi, N↔lbf, N·m↔lbf·ft, Ra↔µin, HRC↔HB, and more — all the units mechanical engineers and CNC machinists use daily. Live bidirectional conversion with exact factors per NIST and ISO 80000.
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Free Online Tool
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NIST / ISO 80000 Standard
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10 Conversion Categories
Quick Unit Converter
Select a category, enter a value, and get instant bidirectional conversion.
From Value
25.4 mm
To Value
1.000 inch
Conversion Factor
× 0.03937
Mental Shortcut
1 inch = 25.4 mm exactly
ADVANCED CONVERTER
Multi-Category Unit Converter
Convert between any units in length, pressure, force, torque, surface finish, hardness, temperature, mass, and speed.
From Value
25.4 mm
To Value
1.000 inch
Conversion Factor
× 0.03937
Formula & Shortcut
1 inch = 25.4 mm exactly | Mental: 1″ ≈ 25 mm
HOW IT WORKS
Engineering Unit Conversion Principles
Understand the conversion factors that drive manufacturing — and the mental shortcuts that prevent costly errors.
Length — The Exact One
1″ = 25.4 mm
Exact by international agreement (1959)
1 mm ≈ 0.040″
Temperature — The Offset Trap
°F = (°C × 1.8) + 32
20°C = 68°F (ISO reference)
Forgetting the +32 is the #1 error
Worked Example — From mm to inch
A drawing calls out a hole at 12.7 mm diameter. What's the size in inches?
12.7mm
÷
25.4mm per inch
=
0.500″Exact size
→
½″Common reference
From mm
12.7 mm
To inch
0.500″
Mental Shortcut
1 mm ≈ 0.040″
Why Conversion Accuracy Matters
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Exact vs. Approximate
Length (25.4) is exact. Most other conversions are approximations with varying precision. Hardness conversions are empirical tables, not formulas.
NIST SP 811 | ISO 8000002
Temperature Offset
°F = (°C × 1.8) + 32. Forgetting the +32 is the most common conversion error. Standard inspection temperature: 20°C = 68°F.
Always use the offset03
Hardness Conversions
HRC ↔ HB conversions come from ASTM E140 empirical lookup tables. They are not exact. Use them for comparison, not for critical specifications.
±2 HRC typical accuracyShop Floor Mental Shortcuts
1″ = 25.4 mm
1 mm ≈ 0.040″. 10 mm ≈ 0.394″ ≈ 3/8″. Exact — use it for all length conversions.
1 MPa ≈ 145 psi
1 ksi ≈ 7 MPa. 100 MPa = 14.5 ksi. Quick material spec checks.
1 N ≈ 0.225 lbf
1 kN ≈ 225 lbf ≈ 100 kg force. Cutting forces, clamp forces.
1 N·m ≈ 0.74 lbf·ft
10 N·m = 7.4 lbf·ft. M10 bolt torque ≈ 50 N·m ≈ 37 lbf·ft.
1 µm Ra ≈ 40 µin Ra
Standard CNC finish: 1.6 µm Ra = 63 µin Ra. Fine: 0.8 µm Ra = 32 µin.
°C × 1.8 + 32 = °F
20°C = 68°F. 100°C = 212°F. Standard reference temperature for inspection.
APPLICATIONS
Where Unit Conversion Matters in Manufacturing
From the shop floor to the inspection room — getting units right at every stage.
Drawing Interpretation
Metric drawings vs. imperial drawings — converting dimensions correctly avoids scrapped parts. A 10 mm hole is 0.394″, not 0.400″.
- Length conversions are exact
- Check the drawing title block
- Never assume unit system
Material Selection
Material yield strengths in MPa or ksi. 6061-T6 = 276 MPa = 40 ksi. 4140 alloy = 655 MPa = 95 ksi. Convert incorrectly and the part fails.
- 1 MPa ≈ 145 psi
- 1 ksi ≈ 7 MPa
- Always verify material spec units
Machining & Inspection
Spindle speeds in m/min or SFM. Surface finish in µm Ra or µin Ra. A conversion error scraps the part before it comes off the machine.
- 1 m/min ≈ 3.3 SFM
- 1 µm Ra ≈ 40 µin Ra
- Double-check every conversion
Ready to manufacture — in any unit system?
Baetro accepts drawings in mm or inches, MPa or psi. Upload your CAD and get an instant quote in under 60 seconds. Full CMM inspection in the unit system you specify.
Unit Conversion FAQs
Frequently Asked Questions
1 inch = 25.4 mm exactly, by international agreement (1959). To convert inches to mm, multiply by 25.4. To convert mm to inches, divide by 25.4. A thou (0.001″) = 0.0254 mm = 25.4 µm. This is the only length conversion that is exact by definition.
Multiply MPa by 145.038 to get psi. For quick estimates: 1 MPa ≈ 145 psi. Common CNC reference values: 6061-T6 aluminum yield = 276 MPa ≈ 40 ksi. 4140 alloy steel yield = 655 MPa ≈ 95 ksi. Titanium Grade 5 yield = 880 MPa ≈ 128 ksi.
Ra is the arithmetic average roughness — the standard callout on engineering drawings. RMS (Root Mean Square) is approximately 1.11 × Ra. If a drawing specifies "63 RMS," expect approximately 57 Ra. At the standard CNC finish range (32–125), Ra and RMS are close enough that many shops treat them interchangeably.
No. Hardness conversions are empirical — they come from ASTM E140 lookup tables, not exact formulas. The same material at the same HRC value can have different HB values depending on alloy and heat treatment. Use conversions for approximate comparison (±2 HRC typical accuracy), and verify critical hardness specs with direct measurement.
Celsius and Fahrenheit have different zero points. 0°C = water freezing; 0°F = a brine solution freezing. The conversion is °F = (°C × 1.8) + 32. A temperature change of 1°C = a change of 1.8°F, but for absolute temperatures, the +32 offset is required. Forgetting the offset is the single most common conversion error in engineering.
1 N·m = 0.7376 lbf·ft. 1 lbf·ft = 1.356 N·m. For shop estimates: 1 N·m ≈ 3/4 lbf·ft; 10 N·m ≈ 7.4 lbf·ft. Automotive lug nuts typically tighten to 100–150 N·m (74–110 lbf·ft). An M10 bolt tightens to ~50 N·m (~37 lbf·ft).
All conversion factors per NIST Special Publication 811 and ISO 80000-1 standards. Length conversions are exact. Temperature conversions use the standard offset formula. Hardness conversions use ASTM E140-12b empirical values. For critical engineering applications, always verify conversions against your organization's quality procedures.
PRECISION MANUFACTURING SUPPORT
From Conversion to Machined Part
You've converted the units. Now get the parts machined to your specifications — in mm or inches, MPa or psi. Baetro delivers precision CNC machining from our ISO 9001 / AS9100 certified facility, with full CMM inspection in the unit system you specify. Upload your CAD for an instant quote in under 60 seconds.
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