Unit Conversion Best Practices for Developers
Incorrect unit conversions have caused spacecraft crashes and medical errors. This guide covers best practices for implementing unit conversions in software, including precision handling and common pitfalls.
Key Takeaways
- NASA's Mars Climate Orbiter was lost in 1999 because one team used metric units while another used imperial.
- Store all measurements in SI base units internally and convert for display only.
- Temperature conversions are not simple multiplication:
- Define conversion factors as constants, not inline numbers.
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Unit Conversion Failures
NASA's Mars Climate Orbiter was lost in 1999 because one team used metric units while another used imperial. The $125 million spacecraft burned up in Mars's atmosphere due to a simple unit mismatch.
Fundamental Principles
Use SI Internally
Store all measurements in SI base units internally and convert for display only. This prevents conversion errors in calculations and simplifies testing.
Floating Point Precision
Floating point numbers cannot represent all decimal values exactly. 0.1 + 0.2 = 0.30000000000000004 in IEEE 754. For financial and scientific calculations, use decimal types or integer arithmetic.
Common Conversion Categories
Temperature
Temperature conversions are not simple multiplication:
- Celsius to Fahrenheit: F = C x 9/5 + 32
- Celsius to Kelvin: K = C + 273.15
Length
- 1 inch = 25.4 mm (exact)
- 1 mile = 1.609344 km (exact)
- 1 nautical mile = 1.852 km (exact)
Data Storage
Be clear about SI vs binary prefixes:
- 1 KB (kilobyte) = 1,000 bytes (SI)
- 1 KiB (kibibyte) = 1,024 bytes (binary)
Implementation Tips
- Define conversion factors as constants, not inline numbers.
- Include units in variable names:
distance_meters, not justdistance. - Write tests that verify round-trip conversions.
- Document which unit system your API uses.
κ΄λ ¨ λꡬ
κ΄λ ¨ κ°μ΄λ
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