Engineering unit conversions

Pressure

The SI unit of pressure is the pascal (Pa), which is one newton per square meter. That's a tiny amount — atmospheric pressure at sea level is about 101,325 Pa — so engineers usually work in kilopascals (kPa), megapascals (MPa), or bar.

• 1 bar = 100,000 Pa = 100 kPa
• 1 atmosphere (atm) = 101,325 Pa ≈ 1.01325 bar
• 1 psi = 6,894.757 Pa ≈ 6.895 kPa
• 1 MPa ≈ 145.038 psi
• 1 mmHg (torr) ≈ 133.322 Pa

Worked example. A car tire inflated to 32 psi contains 32 × 6.895 ≈ 220.6 kPa of gauge pressure, which is about 2.21 bar above atmospheric.

Force

One newton is the force needed to accelerate a 1-kg mass at 1 m/s². Outside the SI, force is often expressed using a unit mass under standard gravity (9.80665 m/s²), giving rise to the kilogram-force (kgf) and pound-force (lbf).

• 1 kgf = 9.80665 N (exact, by definition)
• 1 lbf = 4.4482216152605 N
• 1 kip = 1,000 lbf ≈ 4.448 kN
• 1 dyne = 10⁻⁵ N (CGS unit, still seen in older physics)

Energy

The joule (J) is the SI unit of energy: one newton-meter of work, or one watt-second of electrical energy. Larger units show up because the joule is small relative to everyday quantities — a single kilowatt-hour is 3.6 million joules.

• 1 kWh = 3,600,000 J = 3.6 MJ
• 1 calorie (thermochemical) = 4.184 J (exact)
• 1 kilocalorie (food Calorie) = 4,184 J = 4.184 kJ
• 1 BTU (IT) = 1,055.05585 J
• 1 therm = 100,000 BTU ≈ 105.506 MJ

Worked example. A 60-watt bulb left on for 24 hours uses 60 × 24 = 1,440 watt-hours, or 1.44 kWh — about 5.18 MJ of electrical energy.

Power

Power is energy per unit time. The watt (W) is one joule per second. The mechanical horsepower used in the United States is defined as exactly 550 foot-pounds per second; metric (PS) horsepower used in continental Europe is slightly smaller.

• 1 hp (mechanical) = 745.69987 W
• 1 PS (metric horsepower) = 735.49875 W
• 1 ton of refrigeration = 3,516.85 W
• 1 BTU/hr ≈ 0.293071 W

Torque and bending moment

Torque shares dimensions with energy but is not interchangeable with it — convention reserves the joule for energy and the newton-meter (N·m) for torque. American mechanical work usually uses pound-feet (lb·ft) or pound-inches (lb·in).

• 1 lb·ft = 1.3558179483 N·m
• 1 lb·in = 0.1129848290 N·m
• 1 kgf·m = 9.80665 N·m

A note on precision and significant figures

Most engineering disasters that trace back to units involve mixing systems silently, not rounding too early. The 1999 loss of NASA's Mars Climate Orbiter, for example, was caused by one team supplying thrust impulse data in pound-seconds while the navigation software expected newton-seconds. The factor of 4.448 was simply never applied. The lesson is to label every quantity with its unit at every step, and to convert only at well-defined boundaries.

ConvertMaster's interactive tool keeps full double-precision internally and displays up to six significant decimals. For finite-element work or fatigue analysis where rounding error compounds, copy the factor and apply it once at the end of your calculation rather than round-tripping through the UI.