Unit Conversion Basics: Length, Weight, and Temperature

Recipes, travel distances, DIY plans, shipping labels, and product specifications constantly mix metric units (metres, kilograms, Celsius) with imperial or U.S. customary units (feet, pounds, Fahrenheit). A wrong conversion can shrink a shelf cut, oversalt a recipe, or misjudge whether a suitcase will clear an airline weight limit.

Every conversion rests on a factor: how many of unit B equal one of unit A? Once you have the factor, you multiply or divide. Professional work often needs more significant figures; everyday cooking and travel usually need a practical rounded answer plus a sense of whether you rounded up or down.

When documents disagree, check which system the author meant. A “ton” can mean a U.S. short ton (2,000 lb), a UK long ton, or a metric tonne (1,000 kg). A “gallon” in a U.S. recipe is not the same volume as an imperial gallon. Labels and converters that name the variant prevent expensive mistakes.

Anchor facts to memorize: 1 inch = 2.54 cm exactly; 1 foot = 12 inches = 0.3048 m; 1 mile ≈ 1.609344 km. From those, 5 miles ≈ 8.05 km and 100 m ≈ 328.08 feet. For room plans, converting millimetres to inches (divide by 25.4) is common when mixing imported fixtures with local lumber.

Worked examples: a 2.4 m sofa is about 7.87 ft—check doorway clearance in inches before delivery day. A 10 km race is about 6.21 miles. A screen diagonal listed as 15.6 in is 396.24 mm. Hiking maps may mark trail distance in kilometres while your watch displays miles; convert once and keep a sticky note of the factor so you do not re-estimate every mile marker.

• 1 m ≈ 3.281 ft (roughly 3 ft 3.4 in)
• 1 km ≈ 0.621 mi; 1 mi ≈ 1.609 km
• 1 in = 2.54 cm = 25.4 mm exactly
• 1 yard = 0.9144 m (useful for fabric and sports fields)

In casual speech people say “weight” when they mean mass on Earth’s surface. The practical conversion most shoppers need is 1 kg ≈ 2.20462 lb, so 70 kg ≈ 154.3 lb and 5 lb ≈ 2.27 kg. Shipping carriers often bill by the pound in the U.S. and by the kilogram elsewhere—quote the carrier’s unit when comparing rates.

Cooking example: a European recipe calls for 250 g of flour. That is about 8.82 oz, or roughly 2 cups of all-purpose flour by volume—but volume cup measures are imprecise compared with weighing. Prefer mass when accuracy matters. Luggage example: a 23 kg airline limit is about 50.7 lb; packing to 22 kg (~48.5 lb) leaves a margin for scale differences at the counter.

Precious metal and pharmacy contexts sometimes use troy ounces or grains. Those are different from avoirdupois ounces used for groceries. If a converter offers both, pick the system named on the certificate or prescription rather than assuming “ounce” is universal.

Celsius to Fahrenheit: °F = (°C × 9/5) + 32. So 20°C = 68°F and 180°C (a common oven setting) = 356°F. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Body-temperature reference 98.6°F ≈ 37°C. Freezing is 0°C / 32°F; boiling at standard sea-level pressure is 100°C / 212°F.

Oven example: a cake recipe says bake at 175°C. That is 347°F—most home ovens would be set near 350°F. Candy or oil safety: oil at 190°C is 374°F; knowing both scales helps when one thermometer only shows Fahrenheit. Weather example: a forecast of 30°C is 86°F—hot for outdoor work without shade.

Temperature conversion is affine (scale and offset), not a simple multiply-only factor. You cannot convert 0°C to Fahrenheit by multiplying by 1.8 alone; you must add 32. Forgetting the offset is the most common error when converting by hand.

Area and volume do not use the same factor as linear length. If 1 m = 3.281 ft, then 1 m² = 3.281² ≈ 10.764 ft², not 3.281 ft². Example: a 12 m² room is about 129.2 ft². Likewise, 1 m³ ≈ 35.315 ft³. Using the linear factor without squaring or cubing understates large areas and volumes badly.

Volume liquids add another trap: 1 U.S. gallon ≈ 3.785 L while 1 imperial gallon ≈ 4.546 L. A “5 gallon” bucket in the U.S. is about 18.9 L; the imperial counterpart is larger. Paint coverage charts that mix litres per square metre with gallons per square foot need two conversions—amount and area—before you buy enough cans.

Speed: 100 km/h ≈ 62.14 mph; 60 mph ≈ 96.56 km/h. Pressure: tire labels may show psi and bar (1 bar ≈ 14.5 psi), so 2.2 bar ≈ 31.9 psi. Power: 1 horsepower is about 746 W, so a 1.5 kW motor is roughly 2.01 hp depending on which horsepower definition is used.

Digital storage uses two common conventions. Marketing “gigabytes” are often powers of 1,000 (1 GB = 10⁹ bytes), while many operating systems report gibibytes (1 GiB = 2³⁰ bytes ≈ 1.074 GB). A “500 GB” drive may show roughly 465 GiB in the OS—the difference is unit definition, not missing space.

Write the source value with its unit, name the target unit, apply the factor, then round for the purpose. Construction may round to the nearest 1/16 in; nutrition labels may round grams differently from lab notebooks. Keep more precision mid-calculation than you show in the final answer.

When converting a chain (inches → cm → m), convert in one leap if you know a composite factor to reduce rounding drift. Cross-check surprising results with a second converter or a known landmark (“a metre is a bit longer than a yard”). Consistency matters more than memorizing dozens of factors you will rarely need.

Use our Length Converter for distance between metres, centimetres, feet, inches, yards, miles, and millimetres. Use the Weight & Mass Converter for grams, kilograms, ounces, and pounds. Use the Temperature Converter for Celsius and Fahrenheit. Browse the Unit Converters category for area, volume, speed, and more—all running locally in the browser.

For critical work (medication dosing, structural loads, aviation), follow the authority’s unit system and certified instruments. Consumer converters are excellent for education, travel, DIY, and checking homework; they are not a substitute for regulated measurement practices.