Hydraulic and sealing specifications cross US customary, metric and ISO units daily. A groove drawing in millimeters, a system pressure in bar, a customer RFQ in psi, and a material datasheet in MPa create conversion errors that propagate into wrong compression calculations and incorrect material selection.
This reference consolidates the conversions seal engineers use most often when specifying O-rings, preparing quote requests, and reviewing supplier datasheets.
Pressure Conversions
Seal datasheets and hydraulic system specs mix psi, bar, and MPa. Use these exact factors (not rounded shortcuts) for specification documents.
| From | To | Multiply by | Example |
|---|---|---|---|
| psi | bar | 0.0689476 | 3,000 psi = 206.8 bar |
| bar | psi | 14.5038 | 250 bar = 3,626 psi |
| MPa | bar | 10 | 35 MPa = 350 bar |
| MPa | psi | 145.038 | 35 MPa = 5,076 psi |
| bar | MPa | 0.1 | 414 bar = 41.4 MPa |
| kPa | bar | 0.01 | 1,000 kPa = 10 bar |
Quick mental checks
- 1 bar ≈ 14.5 psi (atmospheric reference)
- 1 MPa = 10 bar ≈ 145 psi
- 3,000 psi ≈ 207 bar ≈ 20.7 MPa (common mobile hydraulic ceiling)
When comparing O-ring extrusion resistance to system pressure, always convert to a single unit before checking gap tables. A 0.15 mm extrusion gap safe at 150 bar is not safe at 150 MPa.
Temperature Conversions
Material temperature limits are published in °C or °F depending on supplier region.
| From | To | Formula |
|---|---|---|
| °C | °F | °F = (°C × 9/5) + 32 |
| °F | °C | °C = (°F − 32) × 5/9 |
| K | °C | °C = K − 273.15 |
Common seal temperature anchors
| °C | °F | Context |
|---|---|---|
| −40 | −40 | Automotive cold soak |
| −18 | 0 | Freezer reference |
| +100 | +212 | Boiling water / steam reference |
| +150 | +302 | High-temp NBR limit |
| +200 | +392 | FKM continuous max (dry heat) |
| +260 | +500 | PTFE / FFKM upper range |
Always specify continuous vs peak temperature when converting — a seal rated to +150°C continuous may tolerate +175°C for minutes only.
Force and Load (Bolt / Gland)
Face seal and flange designs sometimes express preload in force rather than torque.
| From | To | Notes |
|---|---|---|
| lbf | N | × 4.448 |
| N | lbf | × 0.2248 |
| kgf | N | × 9.80665 |
| ft·lbf | N·m | × 1.35582 |
| in·lbf | N·m | × 0.112985 |
Torque-to-clamp-force depends on thread pitch, friction coefficient, and washer stack — do not convert torque to O-ring compression percentage without a joint diagram.
Hardness Scales
O-rings are specified in Shore A (ASTM D2240). Some European datasheets use IRHD (ISO 48).
| Shore A (approx.) | IRHD (approx.) | Typical use |
|---|---|---|
| 50 | 45 | Soft conforming seals |
| 70 | 67 | Standard industrial default |
| 80 | 77 | High-pressure dynamic |
| 90 | 87 | Extrusion-resistant, pump heads |
Shore A and IRHD are not identical scales — for critical specifications, require supplier measurement on the same scale (Shore A Type A durometer, 1 kg indenter).
PTFE is measured on Shore D, not Shore A. A PTFE "90 Shore A equivalent" in marketing literature usually means ~55–65 Shore D.
Length and Groove Dimensions
| From | To | Multiply by |
|---|---|---|
| inch | mm | 25.4 |
| mm | inch | 0.0393701 |
| mil (0.001 in) | mm | 0.0254 |
AS568 sizes are defined in inches; ISO 3601 in millimeters. When cross-referencing, convert both ID and CS — do not mix inch CS with metric ID on the same specification line.
Surface finish conversions:
| Ra (µm) | Ra (µin) | Typical seal application |
|---|---|---|
| 0.2 | 8 | Dynamic rod/bore, premium |
| 0.4 | 16 | Dynamic general |
| 0.8 | 32 | Static face seal |
| 1.6 | 63 | Static flange, acceptable |
| 3.2 | 125 | Static maximum (rough) |
See the surface finish guide for Ra recommendations by motion type.
Volume Swell (Fluid Compatibility Testing)
ASTM D471 immersion results are often reported as % volume change. Interpretation guide:
| Volume change | Typical rating |
|---|---|
| 0 to +5% | Excellent |
| +5 to +15% | Acceptable (verify compression) |
| +15 to +25% | Marginal — shorten service interval |
| > +25% | Unacceptable — wrong material |
Negative volume change (shrinkage) can be equally damaging — the seal loses contact stress. Always record hardness change and tensile change alongside volume swell.
Related Resources
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Q1: How do I convert 3,000 psi hydraulic pressure to bar for an O-ring spec sheet?
Multiply by 0.0689476: 3,000 × 0.0689476 = 206.8 bar (or ~20.7 MPa). Many European datasheets list extrusion gap limits in bar — use the converted value before comparing to your system pressure.
Q2: What is 70 Shore A in IRHD?
Approximately 67 IRHD — close but not identical. For procurement specifications, state "70 ± 5 Shore A per ASTM D2240 Type A" rather than converting to IRHD unless your customer requires ISO 48.
Q3: How many bar is 1 MPa?
10 bar exactly. This is the most common conversion error in multinational RFQs — always verify whether the document uses MPa or bar before specifying backup rings or hardness upgrades.
Q4: How do I convert groove dimensions from inches to millimeters for ISO 3601?
Multiply each dimension by 25.4. Example: 0.103" CS → 2.62 mm. Round only after conversion; do not round intermediate values when calculating compression percentage.
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Preparing an RFQ with mixed units? Request a quote with your groove drawing — we accept inch, metric, and AS568 dash numbers and return specifications in your preferred unit system.