Unit Converter — Convert Units Accurately & Understand Pitfalls
Unit conversion is routine in travel, cooking, engineering, and software. Small mistakes in units or rounding can produce significant errors. This guide explains unit families, precision rules, temperature conversions, presentation patterns, spreadsheet and API recipes, and practical tips to avoid common mistakes.
Unit families & fundamentals
Units belong to families (length, mass, temperature, volume, time, area, energy, etc.). Always convert within the same family and pick authoritative conversion factors.
- Length: meter, kilometer, mile, foot, inch.
- Mass / weight: kilogram, gram, pound, ounce.
- Volume: liter, milliliter, gallon (US/UK differences), cup.
- Temperature: Celsius, Fahrenheit, Kelvin (affine transforms).
For engineering-critical work, reference national standards (NIST, BIPM) for exact factors and unit definitions.
Precision & rounding rules
Decide precision based on purpose. Overly aggressive rounding creates systematic bias; overly precise display can imply false accuracy.
Guidelines by context
- Everyday / shopping: 2 decimal places for currency and amounts that are presented to end-users.
- Cooking: round to sensible units (teaspoons, grams) and provide alternatives (metric & imperial).
- Engineering: display significant figures matching instrument precision (3–6+ decimals depending on tolerance).
- Analytics: store raw values at high precision and round only on presentation layers.
Always show units alongside numbers and consider showing the conversion factor or "as of" timestamp for dynamic rates (e.g., currency conversions).
Temperature — special-case conversions
Temperature conversions are affine: they require scaling and shifting. Converting an absolute temperature differs from converting temperature differences.
- Convert absolute values correctly:
- C → F: F = C × 9/5 + 32
- F → C: C = (F − 32) × 5/9
- C ↔ K: K = C + 273.15
- Converting a temperature change (Δ): convert with the scale only (multiply by 9/5 for ΔC → ΔF) — do not apply offsets.
Example mistake: "A change of 10°C is 18°F" is correct; but converting 10°C (absolute) to Fahrenheit requires adding 32 after scaling.
Common conversion recipes
- Use factor: 1 mile = 1.60934 kilometers.
- Example: 5 miles × 1.60934 = 8.0467 km → display 8.05 km for typical usage.
- Use factor: 1 lb = 0.45359237 kg.
- Example: 150 lb × 0.45359237 = 68.0389 kg → display 68.04 kg (2 decimals) or 68.0 for medical rounding depending on context.
- 1 US cup = 8 US fluid ounces = 236.588 mL.
- Check regional differences: UK/Imperial cups and gallons have different values.
Spreadsheet & script examples
Common formulas and script patterns help automate conversions at scale.
Spreadsheet example (Google Sheets / Excel)
Convert miles in A2 to kilometers: =ROUND(A2 * 1.60934, 2)
JavaScript snippet
function convertLength(val, fromFactor, toFactor) {
// where factors are expressed in meters per unit
return val * (fromFactor / toFactor);
}
// example: miles->km: milesFactor=1609.34, kmFactor=1000
Store canonical factors in one place and reuse them to avoid inconsistencies across sheets or code.
FAQ
Q: Why do imperial vs metric conversions sometimes look "off"?
A: Because rounding and display choices differ; also, some published factors (e.g., for gallons) vary by region (US vs UK). Always check which definition you're using.
Q: How do I show both units for users?
A: Present primary and secondary units (e.g., 68.04 kg (150 lb)) and place the unit the user prefers first when possible.