How to Convert Units: Length, Weight, and Temperature Without Guesswork

Two systems, two logics

The metric system is built on tens: millimeters to centimeters to meters to kilometers, each step a factor of 10 or 1000. That's why converting within metric is just moving the decimal point — 1.5 kilometers is 1,500 meters is 1,500,000 millimeters.

Imperial units (used in the US) grew from history, not math: 12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile, 16 ounces to a pound. No decimal pattern — which is why you need conversion factors rather than point-moving.

The anchors worth memorizing

A handful of conversions covers almost every everyday need:

• 1 inch = 2.54 cm exactly (this one is defined, not measured)
• 1 mile ≈ 1.61 km — so mph × 1.6 ≈ km/h
• 1 foot ≈ 30 cm; a 6-foot person is about 183 cm
• 1 kilogram ≈ 2.2 lb; 1 lb ≈ 454 g (roughly half a kilo)
• 1 ounce ≈ 28 g; a 100 g chocolate bar is about 3.5 oz
• 1 US cup ≈ 240 ml — useful for recipes

Temperature works differently

Length and weight are scales that start at zero the same way, so a factor converts between them. Temperature scales have different zero points — 0 °C is 32 °F — so converting requires a formula, not a multiplier.

Celsius to Fahrenheit: multiply by 9, divide by 5, add 32. So 20 °C → 20 × 9 ÷ 5 + 32 = 36 + 32 = 68 °F — a comfortable room temperature. Fahrenheit to Celsius: subtract 32, then multiply by 5 and divide by 9. 98.6 °F → (98.6 − 32) × 5 ÷ 9 = 37 °C, normal body temperature. Kelvin, the scientific scale, is simply Celsius plus 273.15.

Estimating when precision doesn't matter

For weather and casual conversation, rough rules are enough: double the Celsius and add 30 to get a ballpark Fahrenheit (20 °C → 70 °F, real answer 68). For kilometers to miles, take off about a third (60 km ≈ 40 mi).

Never estimate for baking, medicine doses, or anything with a specification sheet — those need exact factors. A 5% error in flour, or in a pediatric dose, is not a rounding difference.

Let the tool handle the rest

The Unit Converter handles length (millimeters to miles), weight (grams to tonnes), and temperature (Celsius, Fahrenheit, Kelvin) with exact factors — type a value, pick the units, and get the answer to four decimal places.

It's also a good way to check your mental estimates: try 20 °C and see 68 °F, or 10 kg and see 22.05 lb. After a few checks, the anchors above start to feel natural.