Common Conversion Mistakes in the SI Unit System

Why unit conversion must not be underestimated

Unit conversion looks like "simple math", but some of the most expensive mistakes in engineering history came exactly from here (see the real example below). The most frequent mistakes are generally:

Mixing up prefixes — confusing "milli" (10⁻³) with "micro" (10⁻⁶), or a one-order shift between "kilo" (10³) and "mega" (10⁶). Especially common in electrical/electronic calculations (e.g. µF vs mF).

Confusing force and mass units — "kilogram" is a unit of mass, "kilogram-force (kgf)" is a unit of force, and the two do not share the same numerical value (1 kgf ≈ 9.81 N). Using them interchangeably is a frequent source of error, especially in older or mixed-source documents.

Forgetting the exponent in compound units — when converting area (m²) or volume (m³), forgetting to square or cube the length factor. For example, while 1 m = 100 cm, 1 m² = 10,000 cm² (100², not 100) — a simple but very common mistake.

Forgetting the offset in temperature scales — when going from Celsius to Kelvin you must add 273.15, not just multiply; Fahrenheit conversion involves both a factor and an offset. Temperature, unlike most other units, is linear-offset rather than linear-proportional.

NASA's Mars Climate Orbiter spacecraft was destroyed because the developer team (Lockheed Martin) sent thrust calculations in pound-force-seconds (imperial), while NASA's orbit-calculation software expected the value in newton-seconds (metric) — causing the vehicle to enter the atmosphere at a much lower altitude than planned and break apart, a loss of about 327 million dollars. This is the best-known example of how costly a single unit-conversion assumption (not confirming which system was being used) can be in the real world.

Source: BIPM, The International System of Units (SI), 9th edition, 2019; for the Mars Climate Orbiter incident, the official NASA/JPL accident investigation report (1999).