Engineering Units

Engineering Unit Conversions

Essential SI and imperial unit conversions for pipeline operations. Covers pressure, temperature, flow rates, and standard conditions per GPSA and API standards.

View Calculators Pressure conversions

Pressure conversion

1 psi = 6.895 kPa

Most common pressure conversion in midstream operations.

Temperature

°C = (°F - 32) / 1.8

Linear conversion between Fahrenheit and Celsius scales.

Flow rate

1 MMscfd = 28,317 Sm³/d

Million standard cubic feet per day to standard cubic meters per day.

Contents

Use this guide when you need to:

  • Convert between imperial and metric units.
  • Ensure unit consistency in calculations.
  • Understand standard condition differences.

1. SI Unit System Overview

The International System of Units (SI) is the modern form of the metric system and the most widely used system of measurement worldwide. Understanding both SI and imperial (US Customary) units is essential for midstream engineers working on international projects.

Base units

Seven fundamentals

meter, kilogram, second, ampere, kelvin, mole, candela.

Derived units

Combinations

Pascal (pressure), Joule (energy), Watt (power) from base units.

Decimal system

Powers of 10

kilo (10³), mega (10⁶), milli (10⁻³), micro (10⁻⁶) prefixes.

Global standard

ISO compliance

ISO 80000 series defines SI units for engineering applications.

SI Base Units

Quantity SI Unit Symbol Imperial Equivalent
Length meter m 3.281 feet
Mass kilogram kg 2.205 pounds
Time second s 1 second (same)
Temperature kelvin K °R (Rankine) / 1.8
Amount of substance mole mol 1 lb-mol = 453.6 mol

Common SI Prefixes

Prefix Symbol Factor Example (Pipeline/Gas)
giga G 10⁹ GPa (pipe yield strength, 1 GPa = 145,000 psi)
mega M 10⁶ MPa (pressure, 1 MPa = 145 psi)
kilo k 10³ kPa (pressure, 1 kPa = 0.145 psi)
centi c 10⁻² cm (diameter, 1 cm = 0.394 in)
milli m 10⁻³ mm (pipe wall thickness, 1 mm = 0.0394 in)
micro µ 10⁻⁶ µg (mercury concentration, 1 µg = 1 × 10⁻⁶ g)
nano n 10⁻⁹ ng (LNG mercury spec, 1 ng = 1 × 10⁻⁹ g)
Unit consistency: Mixing imperial and metric units in the same calculation is the most common source of engineering errors. Always convert all inputs to a consistent unit system (all SI or all imperial) before performing calculations, then convert results to desired output units.
SI metric prefixes scale diagram showing nano through giga with pipeline engineering examples including wall thickness in mm, pressure in kPa and MPa, and steel strength in GPa
SI metric prefixes for pipeline engineering with practical examples at each scale level

Imperial (US Customary) System

The imperial system remains prevalent in United States pipeline and midstream operations:

  • Length: inch (in), foot (ft), mile (mi)
  • Mass: pound (lb), ton (2000 lb)
  • Force: pound-force (lbf) — distinct from pound-mass (lbm)
  • Pressure: psi (lb/in²), psig (gauge), psia (absolute)
  • Temperature: Fahrenheit (°F), Rankine (°R = °F + 459.67)
  • Volume: cubic foot (ft³), gallon (gal), barrel (bbl = 42 gal)

2. Length & Area Conversions

Length conversions are fundamental for pipe sizing, distance measurements, and area calculations in pipeline engineering.

Length Conversion Factors

Exact Definitions: 1 inch = 25.4 mm (exact, by definition since 1959) 1 foot = 0.3048 m (exact) 1 mile = 1.609344 km (exact) Derived conversions: 1 meter = 39.37 inches = 3.281 feet 1 kilometer = 0.6214 miles 1 millimeter = 0.03937 inches

Comprehensive Length Conversion Table

From To Multiply By Example
inches millimeters 25.4 24 in × 25.4 = 609.6 mm (24" pipe OD)
inches centimeters 2.54 36 in × 2.54 = 91.44 cm
feet meters 0.3048 100 ft × 0.3048 = 30.48 m (pipe segment)
miles kilometers 1.60934 50 mi × 1.60934 = 80.47 km (pipeline length)
millimeters inches 0.03937 12.7 mm × 0.03937 = 0.500 in (wall thickness)
meters feet 3.28084 10 m × 3.28084 = 32.81 ft
kilometers miles 0.621371 100 km × 0.621371 = 62.14 mi

Area Conversions

Area Conversion Factors: 1 in² = 645.16 mm² 1 ft² = 0.092903 m² 1 acre = 4,046.86 m² = 0.404686 hectares 1 mile² = 2.58999 km² Common pipeline cross-section: Pipe ID = 20 inches Imperial area: A = π × (20/2)² = π × 100 = 314.16 in² Metric area: A = 314.16 in² × 645.16 mm²/in² = 202,683 mm² = 0.2027 m² Or directly: ID = 20 × 25.4 = 508 mm = 0.508 m A = π × (0.508/2)² = 0.2027 m² ✓

Pipe Size Conversions (NPS to DN)

NPS (Nominal Pipe Size) OD (inches) DN (Diameter Nominal) OD (mm)
2" 2.375 DN 50 60.3
4" 4.500 DN 100 114.3
6" 6.625 DN 150 168.3
8" 8.625 DN 200 219.1
12" 12.750 DN 300 323.9
16" 16.000 DN 400 406.4
20" 20.000 DN 500 508.0
24" 24.000 DN 600 609.6
36" 36.000 DN 900 914.4
42" 42.000 DN 1050 1066.8
Pipe size notation: NPS (Nominal Pipe Size) and DN (Diameter Nominal) are not exact conversions. NPS is in inches, DN is approximately in millimeters, but both are nominal designations. Always use actual OD/ID dimensions for calculations, not nominal sizes.
NPS vs DN pipe size comparison showing cross-sections of NPS 2 through 24 inch pipes with corresponding DN designations and OD dimensions in both inches and millimeters
NPS vs DN pipe size comparison — nominal designations are not exact conversions; use actual OD/ID for calculations

3. Pressure & Temperature Conversions

Pressure and temperature conversions are critical for all pipeline and process calculations. Understanding absolute vs gauge pressure and temperature scale offsets is essential.

Pressure Conversion Factors

Fundamental Pressure Conversions: 1 psi (lb/in²) = 6.89476 kPa (exact to 5 decimals) 1 bar = 100 kPa = 14.5038 psi 1 atm (atmosphere) = 101.325 kPa = 14.696 psi 1 MPa = 1000 kPa = 145.038 psi Common approximations: 1 psi ≈ 6.895 kPa (0.01% error) 1 bar ≈ 14.5 psi (0.03% error) 1 MPa ≈ 145 psi (0.03% error) Gauge vs Absolute: psia = psig + 14.7 (at sea level) kPa(a) = kPa(g) + 101.325 bar(a) = bar(g) + 1.01325

Pressure Conversion Table

From To Multiply By Example (Pipeline)
psi kPa 6.89476 1000 psig × 6.895 = 6895 kPa(g)
psi bar 0.0689476 1000 psi × 0.06895 = 68.95 bar
psi MPa 0.00689476 1440 psi × 0.006895 = 9.93 MPa (ANSI 900#)
kPa psi 0.145038 10,000 kPa × 0.145 = 1450 psi
bar psi 14.5038 70 bar × 14.5 = 1015 psi (pipeline MAOP)
MPa psi 145.038 15 MPa × 145 = 2176 psi
in H₂O kPa 0.248844 100 in H₂O × 0.249 = 24.9 kPa (orifice ΔP)
mm Hg kPa 0.133322 760 mm Hg × 0.133 = 101.3 kPa (1 atm)

Temperature Conversions

Temperature Scale Conversions: Celsius to Fahrenheit: °F = (°C × 1.8) + 32 Fahrenheit to Celsius: °C = (°F - 32) / 1.8 Absolute temperature scales: K = °C + 273.15 °R = °F + 459.67 Kelvin to Rankine: °R = K × 1.8 Common reference points: - Water freezes: 0°C = 32°F = 273.15 K = 491.67°R - Water boils: 100°C = 212°F = 373.15 K = 671.67°R - Absolute zero: -273.15°C = -459.67°F = 0 K = 0°R Example conversions: 60°F = (60 - 32)/1.8 = 15.56°C (standard conditions) 100°C = (100 × 1.8) + 32 = 212°F (boiling water)

Common Temperature Conversions

°F °C K °R Typical Application
-40 -40 233.15 419.67 Low-temperature gas processing
32 0 273.15 491.67 Hydrate formation reference
60 15.56 288.71 519.67 US standard conditions
70 21.11 294.26 529.67 Typical ambient (design)
100 37.78 310.93 559.67 Summer pipeline temperature
150 65.56 338.71 609.67 Compressor discharge (typical)
212 100 373.15 671.67 Water boiling point (1 atm)
Absolute vs gauge pressure: Gas law calculations (PV = nRT) require absolute pressure and absolute temperature. Using gauge pressure or relative temperature scales (°C, °F) will produce incorrect results. Always convert to psia/kPa(a) and °R/K for thermodynamic calculations.
Pressure and temperature scale comparison showing relationships between psig, psia, kPa, bar for pressure and Fahrenheit, Celsius, Kelvin, Rankine for temperature with key reference points
Pressure and temperature unit scale comparison with key reference points and conversion formulas

Worked Example: Combined Conversion

Problem: Convert pipeline conditions from imperial to metric: P = 950 psig, T = 80°F Solution: Step 1: Convert pressure to absolute P(psia) = 950 + 14.7 = 964.7 psia Step 2: Convert to kPa P(kPa) = 964.7 × 6.89476 = 6,651 kPa(a) Step 3: Convert temperature to Celsius T(°C) = (80 - 32) / 1.8 = 26.67°C Step 4: Convert to Kelvin for gas calculations T(K) = 26.67 + 273.15 = 299.82 K Final answer: 6,651 kPa(a), 26.67°C (or 299.82 K for gas law)

4. Flow Rate & Volume Conversions

Flow rate and volume conversions are essential for production reporting, pipeline capacity calculations, and custody transfer.

Volumetric Flow Rate Conversions

Gas Flow Rate Conversions: 1 scf (standard cubic foot) = 0.0283168 Sm³ (standard cubic meter) 1 MMscfd = 1,000,000 scfd = 28,316.8 Sm³/d 1 MMscfd = 1,179.9 Sm³/h = 0.3278 Sm³/s European notation: 1 Nm³ (normal cubic meter) ≈ 1 Sm³ (standard cubic meter) (Note: Nm³ at 0°C, 1 atm; Sm³ at 15°C, 1 atm — small difference) Common conversions: scfd to Sm³/d: multiply by 0.0283168 MMscfd to Sm³/h: multiply by 1,179.9 Sm³/d to scfd: multiply by 35.3147

Gas Flow Rate Conversion Table

From To Multiply By Example
scfd Sm³/d 0.0283168 100,000 scfd × 0.0283 = 2,832 Sm³/d
MMscfd Sm³/d 28,316.8 50 MMscfd × 28,317 = 1,415,840 Sm³/d
MMscfd Sm³/h 1,179.9 10 MMscfd × 1,180 = 11,800 Sm³/h
scfm Sm³/h 1.699 1,000 scfm × 1.699 = 1,699 Sm³/h
Sm³/d scfd 35.3147 10,000 Sm³/d × 35.31 = 353,147 scfd
Sm³/h MMscfd 0.000847 50,000 Sm³/h × 0.000847 = 42.4 MMscfd

Liquid Flow Rate Conversions

Liquid Flow Rate Conversions: 1 barrel (bbl) = 42 US gallons = 0.158987 m³ = 158.987 liters 1 bbl/d = 0.00658 m³/h 1 gpm (gallon per minute) = 0.227125 m³/h = 3.785 L/min Common oil/NGL flow rates: 1,000 bbl/d × 0.158987 = 159 m³/d 10,000 bbl/d = 69.4 m³/h = 1,590 m³/d Condensate production: 100 bbl/MMscf × 1 MMscfd = 100 bbl/d = 100 × 0.158987 = 15.9 m³/d = 15.9 m³/d ÷ 28,317 Sm³/d = 0.561 m³/1000 Sm³

Volume Conversions

From To Multiply By Application
ft³ m³ 0.0283168 Vessel volumes, pipeline inventory
barrel (bbl) m³ 0.158987 Oil/NGL storage, production
gallon (US) liter 3.78541 Chemical injection, small volumes
in³ cm³ (mL) 16.3871 Laboratory samples
m³ ft³ 35.3147 Reverse conversion
m³ barrel 6.28981 Reverse conversion
liter gallon 0.264172 Reverse conversion

Mass Flow Rate Conversions

Mass Flow Rate: 1 lb/h = 0.000126 kg/s = 0.4536 kg/h 1 ton/h (US) = 2,000 lb/h = 907.2 kg/h 1 tonne/h (metric) = 1,000 kg/h = 2,204.6 lb/h Example: Compressor throughput Flow = 100 MMscfd natural gas (SG = 0.65, MW = 18.8) At standard conditions (60°F, 14.7 psia): ρ = (14.7 × 18.8) / (10.73 × 519.67) = 0.0496 lb/ft³ Mass flow = 100 × 10⁶ scfd × 0.0496 lb/ft³ = 4,960,000 lb/d = 206,667 lb/h Convert to kg/h: 206,667 lb/h × 0.4536 kg/lb = 93,744 kg/h = 93.7 tonne/h
Standard conditions matter: Gas flow rates are always referenced to standard conditions, but these differ by region (60°F/14.7 psia in US, 15°C/101.325 kPa in ISO). A 100 MMscfd flow at US standard = 103.6 MMscf at ISO standard due to temperature difference. Always specify which standard is used.
World map showing gas measurement standard conditions by region: US standard at 60°F and 14.696 psia, ISO standard at 15°C and 101.325 kPa, with volume correction factors between standards
Gas measurement standard conditions by region — always specify which standard applies in contracts

5. Standard Conditions & Unit Consistency

Standard conditions for gas measurement vary by region and industry. Understanding these differences is critical for accurate flow rate reporting and contract compliance.

Standard Condition Definitions

Standard Temperature Pressure Application
US (API, GPSA) 60°F (15.56°C) 14.696 psia (101.325 kPa) US pipeline contracts, API standards
ISO 13443 15°C (59°F) 101.325 kPa (14.696 psia) International standard, LNG contracts
Normal (European) 0°C (32°F) 101.325 kPa (14.696 psia) European pipeline systems
CAGI (compressed air) 68°F (20°C) 14.5 psia (100 kPa) Compressed air systems (not natural gas)
SPE (petroleum) 60°F (15.56°C) 14.65 psia (101 kPa) Some reservoir engineering applications

Impact of Standard Condition Differences

Volume Correction Between Standards: Using ideal gas law: V₁/T₁ = V₂/T₂ (at same pressure) Convert US standard to ISO standard: T₁ = 60°F = 519.67°R T₂ = 15°C = 288.15 K = 518.67°R V_ISO / V_US = T_ISO / T_US = 518.67 / 519.67 = 0.9981 Therefore: 100 MMscfd (US) = 99.81 MMscfd (ISO) Convert US standard to Normal (European): T_normal = 0°C = 273.15 K = 491.67°R V_normal / V_US = 491.67 / 519.67 = 0.9461 Therefore: 100 MMscfd (US) = 94.61 Nm³/d × 35.31 scf/Sm³ = 3,340 Nm³/d Wait, this is confusing. Let's restart: 100 MMscfd (US) = 100 × 10⁶ scfd × 0.0283168 Sm³/scf = 2,831,680 Sm³/d at 60°F Correct to 0°C (Normal): V_Nm³ = V_Sm³ × (T_normal / T_standard) V_Nm³ = 2,831,680 × (273.15 / 288.71) = 2,679,400 Nm³/d Difference: 5.4% lower at 0°C standard vs 15°C standard

Unit Consistency in Calculations

Common pitfalls and how to avoid them:

Example: Orifice Meter Flow Calculation Given: ΔP = 100 in H₂O ρ = 0.045 lb/ft³ d = 3.5 inches WRONG (mixed units): Q = √(ΔP / ρ) [This will give nonsense units] CORRECT (convert all to consistent system): Imperial approach: Convert ΔP to psi: 100 in H₂O × 0.03613 psi/in H₂O = 3.613 psi Keep ρ in lb/ft³: 0.045 lb/ft³ Keep d in inches: 3.5 in Now use AGA-3 orifice equation with all imperial units. Metric approach: Convert ΔP to kPa: 100 in H₂O × 0.2488 kPa/in H₂O = 24.88 kPa Convert ρ to kg/m³: 0.045 lb/ft³ × 16.0185 kg/m³ per lb/ft³ = 0.721 kg/m³ Convert d to mm: 3.5 in × 25.4 mm/in = 88.9 mm Now use ISO 5167 orifice equation with all metric units.

Density Conversion Factors

From To Multiply By Common Value
lb/ft³ kg/m³ 16.0185 Natural gas: 0.05 lb/ft³ = 0.80 kg/m³
lb/gal kg/L (or SG) 0.11983 Water: 8.34 lb/gal = 1.0 kg/L
API gravity SG (specific gravity) SG = 141.5/(API+131.5) 40°API oil = 0.825 SG
lb/bbl kg/m³ 2.8531 Condensate: 300 lb/bbl = 856 kg/m³

Energy Conversions

Energy and Heating Value: 1 Btu = 1.05506 kJ = 0.000293 kWh 1 MMBtu = 1.05506 GJ = 293.1 kWh 1 therm = 100,000 Btu = 105.506 MJ Gas heating value: 1,000 Btu/scf × 0.0373 MJ/Sm³ per Btu/scf = 37.3 MJ/Sm³ Typical natural gas: 1,030 Btu/scf = 38.4 MJ/Sm³ (HHV) Energy content of gas volume: 100 MMscfd × 1,030 Btu/scf = 103,000 MMBtu/d = 108,670 GJ/d Power: 1 HP (horsepower) = 0.7457 kW = 2,545 Btu/h 1 ton refrigeration = 12,000 Btu/h = 3.517 kW

Viscosity Conversions

From To Multiply By Example
cP (centipoise) Pa·s 0.001 Gas: 0.011 cP = 1.1 × 10⁻⁵ Pa·s
lb/(ft·s) Pa·s 1.48816 Oil: 0.001 lb/(ft·s) = 0.00149 Pa·s
cSt (centistokes) m²/s 1 × 10⁻⁶ Kinematic: 10 cSt = 1 × 10⁻⁵ m²/s
Best practice for unit consistency: Create a reference table at the start of each calculation showing all input values in BOTH unit systems. Perform calculation entirely in one system, then convert final result to desired output units. Never mix units mid-calculation — this is the #1 cause of engineering calculation errors.
Engineering unit conversion workflow flowchart showing four steps: identify inputs, convert all to single system, perform calculation, convert results, with common mistakes highlighted
Proper engineering unit conversion workflow — never mix units mid-calculation

Common Conversion Mistakes

  • Forgetting absolute pressure: Using 1000 psig in gas law instead of 1014.7 psia (1.5% error)
  • Mixing °F and °R: Using °F directly in PV = nRT (causes ~65% error at 60°F)
  • Confusing scf and cf: Standard cubic feet (scf) at base conditions vs actual cubic feet (acf) at flowing conditions
  • Ignoring standard condition differences: Assuming US standard when contract specifies ISO (0.2% error)
  • Mass vs force units: Using lbm and lbf interchangeably (requires g_c = 32.174 lb·ft/(lbf·s²) conversion)
  • Area conversions: Using linear conversion for area (1 ft = 0.3048 m, but 1 ft² = 0.0929 m², not 0.3048 m²)

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