Which Unit Should I Use?
A conversion only helps if you pick the unit your situation expects. Here are the common “which unit” decisions, with the why behind each.
Weight: pounds (lb) or kilograms (kg)?
Use the US unit when…
Everyday US weight — body weight, groceries, shipping scale weight, gym plates.
Use the metric unit when…
Medicine, science, international shipping, and metric-standard fitness apps or research.
Both are exact: 1 lb = 0.45359237 kg. Switch to kg whenever the field expects SI or you are comparing across countries.
Temperature: Fahrenheit (°F) or Celsius (°C)?
Use the US unit when…
US weather forecasts, oven temperatures, and daily comfort.
Use the metric unit when…
Science labs, medical body temperature, and most of the world.
°C = (°F − 32) × 5/9. Use °C in any clinical or scientific context where precision and SI are expected.
Length: feet/inches or cm/meters?
Use the US unit when…
US construction, room dimensions, height, and real estate listings.
Use the metric unit when…
Science, medicine, engineering drawings, and international product specs.
1 inch = 2.54 cm exactly. Use cm/m when the spec sheet or standard is metric.
Volume: cups or milliliters / liters?
Use the US unit when…
US recipes (cups, tablespoons, teaspoons) and fuel/large containers (gallons).
Use the metric unit when…
Baking by weight, science labs, and medical doses (mL).
Volume measures are inconsistent for flour and sugar — bakers convert to grams. Never use cups for medication; use mg/mL clinical converters.
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