Unit Converter SI/US Calculator
Calculate unit siuscalculator with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Converted Value = Input Value x (From Unit SI Factor / To Unit SI Factor)
Each unit has a conversion factor to its SI base unit. To convert between any two units, multiply the input by the source unit factor, then divide by the target unit factor. Temperature is the exception, requiring offset formulas: C = (F-32) x 5/9, K = C + 273.15. This approach allows conversion between any pair of units in the same category through the SI intermediary.
Worked Examples
Example 1: Marathon Distance Conversion
Problem:A marathon is 42.195 kilometers. Convert to miles, feet, and meters.
Solution:42.195 km x 1000 = 42,195 meters\n42.195 km / 1.60934 = 26.2188 miles\n42,195 m / 0.3048 = 138,435 feet\nVerification: 26.2188 x 5280 = 138,435 feet
Result:42.195 km = 26.219 miles = 42,195 m = 138,435 ft
Example 2: Body Temperature in Different Scales
Problem:Normal body temperature is 98.6 degrees Fahrenheit. Convert to Celsius and Kelvin.
Solution:Celsius = (98.6 - 32) x 5/9 = 66.6 x 5/9 = 37.0 C\nKelvin = 37.0 + 273.15 = 310.15 K\nVerification: 37.0 x 9/5 + 32 = 66.6 + 32 = 98.6 F
Result:98.6 F = 37.0 C = 310.15 K
Frequently Asked Questions
How do significant figures affect unit conversions?
Your converted result should have the same number of significant figures as your original measurement. If you measure 5.2 inches (2 significant figures), converting to centimeters gives 13 cm, not 13.208 cm. Using excessive decimal places implies false precision.
What are the most common unit conversion mistakes?
Common errors include confusing fluid ounces with weight ounces, mixing up miles and nautical miles, forgetting that UK and US gallons differ (UK is 20% larger), using the wrong temperature formula, and not accounting for the difference between troy and avoirdupois ounces.
Why do some countries use different unit systems?
Most countries adopted the metric system after the French Revolution standardized it in the 1790s. The US, Liberia, and Myanmar still primarily use imperial/customary units due to historical inertia, though US science and military use metric.
How precise should my unit conversions be?
Match precision to your application. Cooking tolerates rough conversions (1 cup is about 240 mL). Engineering may need 4-6 decimal places. Scientific work requires exact conversion factors and proper significant figure handling. More precision than your measurement accuracy is meaningless.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy