Engineering Unit Converter | Imperial & SI

Engineering Unit Converter | ProEngCalc

🔧 All Engineering Disciplines

Engineering Unit Converter

Convert between Imperial and SI units across 8 engineering categories

Why Unit Conversion Matters in Engineering

Unit errors have caused catastrophic engineering failures. The Mars Climate Orbiter was lost in 1999 because one engineering team used metric units and another used Imperial — the spacecraft received incorrect thruster commands and burned up in the Martian atmosphere at a cost of $327 million. Closer to everyday engineering, unit mismatches in piping calculations, structural analysis, and machine design cause design errors, field rework, and safety incidents every year. Rigorous unit tracking is not optional — it is a fundamental engineering discipline.

📐 Engineering Best Practice: Always write units next to every number in a calculation. Use dimensional analysis to verify that units cancel correctly to give the expected output unit. If the units don’t work out, the formula is wrong or you’ve made a substitution error.

Conversion Categories Covered

📏 Length

mm, cm, m, km
in, ft, yd, miles, nautical mi

⚖ Mass

g, kg, tonne (metric)
oz, lb, ton (US), ton (UK)

🌡 Temperature

°C, °F, K, °R
(offset scale — formula required)

💧 Pressure

Pa, kPa, MPa, bar, atm
psi, psf, mmHg, inH₂O

⚡ Power

W, kW, MW
HP (mech), BTU/hr, ton refrig.

🌊 Flow Rate

m³/s, L/min, L/hr
GPM, CFM, SCFM, ACFM

⚙ Torque

N·m, kN·m
lb·ft, lb·in, oz·in, kgf·m

⚡ Energy

J, kJ, MJ, kWh
BTU, ft·lbf, cal, kcal, therm

Critical Engineering Conversion Reference

Pressure — The Most Commonly Confused Units

From	To	Multiply By	Notes
psi	kPa	6.89476	Most common US/metric conversion
psi	MPa	0.00689476	For high-pressure applications
bar	psi	14.5038	European process industry
atm	kPa	101.325	Standard atmosphere = exact
atm	psi	14.696	Standard atmosphere
mmHg	kPa	0.133322	Vacuum and medical applications
inH₂O (4°C)	Pa	249.089	HVAC duct pressure

Power

From	To	Multiply By	Notes
HP (mechanical)	W	745.700	Exact definition
HP (mechanical)	kW	0.745700
kW	HP (mechanical)	1.34102
BTU/hr	W	0.293071	HVAC and heating
Ton (refrigeration)	kW	3.51685	Cooling capacity
Ton (refrigeration)	BTU/hr	12,000	Exact definition

Temperature Formulas

Conversion	Formula	Example
°C → °F	°F = (°C × 9/5) + 32	100°C = 212°F
°F → °C	°C = (°F − 32) × 5/9	72°F = 22.2°C
°C → K	K = °C + 273.15	25°C = 298.15 K
K → °C	°C = K − 273.15	373.15 K = 100°C
°F → °R	°R = °F + 459.67	32°F = 491.67°R
K → °R	°R = K × 1.8	300 K = 540°R

Flow Rate

From	To	Multiply By
GPM (US)	L/min	3.78541
GPM (US)	m³/hr	0.227125
CFM	m³/min	0.028317
m³/s	GPM	15,850.3
L/s	GPM	15.8503

Common Mistakes Engineers Make

❌ Mistake 1 — Confusing Mass and Force (lbm vs lbf)

In the Imperial system, “pound” can mean pound-mass (lbm, a unit of mass) or pound-force (lbf, a unit of force). On Earth at standard gravity, 1 lbm weighs 1 lbf — but this identity breaks down in other gravity environments and in dynamic calculations. Always specify lbm or lbf explicitly. In SI, mass (kg) and force (N) are clearly distinct — 1 kg weighs 9.81 N at standard gravity.

❌ Mistake 2 — Gauge vs Absolute Pressure

Most pressure gauges read gauge pressure (relative to atmosphere). Thermodynamic calculations (compressor work, steam tables) require absolute pressure. P_abs = P_gauge + P_atm (≈ 101.325 kPa = 14.696 psi at sea level). A gauge reading of 0 psi (vented to atmosphere) is 14.696 psia absolute. This error causes significant mistakes in compressor sizing and thermodynamic cycle analysis.

❌ Mistake 3 — SCFM vs ACFM Confusion

SCFM (Standard CFM) references flow at standard conditions (60°F, 14.696 psia). ACFM (Actual CFM) is flow at actual operating conditions. Compressors are rated in SCFM but must deliver ACFM at the actual suction conditions. At 5,000 ft elevation (12.2 psia) and 100°F, the same compressor delivers significantly less mass flow. SCFM × (P_std/P_actual) × (T_actual/T_std) = ACFM.

❌ Mistake 4 — Using Celsius Instead of Kelvin in Gas Law Calculations

The ideal gas law (PV = nRT) and other thermodynamic relations require absolute temperature in Kelvin (or Rankine). Using 20°C instead of 293.15 K produces a result that’s wrong by a factor of ~14.6. Always convert to absolute temperature before substituting into any equation with T in the denominator or where T represents thermodynamic temperature.

Frequently Asked Questions

What is the difference between a US ton and a metric tonne?

A US short ton = 2,000 lbm = 907.185 kg. A metric tonne (t) = 1,000 kg = 2,204.6 lbm. A UK long ton = 2,240 lbm = 1,016.05 kg. In international trade and scientific work, always specify which ton — the difference between a US ton and metric tonne is about 10%, which can be significant for bulk material calculations.

What is a “standard” condition for gas flow measurement?

Standard conditions vary by industry: ISO 13443 (natural gas): 15°C, 101.325 kPa. OSHA/EPA: 25°C, 101.325 kPa. ANSI/ISA: 60°F (15.56°C), 14.696 psia. Always verify which standard is being used before comparing flow rates from different sources. A flow rate of “100 SCFM” at one standard condition is NOT the same mass flow as “100 SCFM” at another standard condition.

How do I convert between AWG wire sizes and mm² cross-sectional area?

AWG and mm² are different wire sizing systems with no exact equivalence. Standard conversion pairs: 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm², 8 AWG ≈ 10 mm², 6 AWG ≈ 16 mm², 4 AWG ≈ 25 mm², 2 AWG ≈ 35 mm². These are nearest standard sizes, not exact conversions — ampacity tables differ between NEC (North America) and IEC 60364 (Europe) standards.

What is the difference between kW and kWh?

kW (kilowatt) is a unit of power — the rate of energy use at an instant in time. kWh (kilowatt-hour) is a unit of energy — the total energy consumed over a period. A 1 kW heater running for 3 hours consumes 3 kWh of energy. Electricity bills charge for kWh (energy consumed), not kW (instantaneous demand) — though some commercial tariffs also include a demand charge based on peak kW.

How do I convert pipe pressure drop from Pa/m to psi/100ft?

1 Pa/m = 0.004335 psi/100ft. Or multiply Pa/m by 0.04335 to get psi/100ft. For example, 500 Pa/m = 500 × 0.004335 = 2.17 psi/100ft. This conversion is commonly needed when comparing European and North American piping standards or working with mixed-units hydraulic software. Keep in mind that Pa/m applies to any fluid density, while psi/100ft is often reported for water specifically.