Unit Converter: Length, Weight, Temperature

Metric vs Imperial Systems

The metric system (SI) uses base-10 relationships: 1 kilometer = 1,000 meters, 1 kilogram = 1,000 grams, 1 liter = 1,000 milliliters. Prefixes (kilo-, centi-, milli-, micro-) apply uniformly across all measurement types. This consistency makes metric calculations straightforward — moving between units is just a matter of shifting the decimal point.

The imperial system uses inconsistent ratios inherited from historical standards: 1 mile = 5,280 feet, 1 pound = 16 ounces, 1 gallon = 128 fluid ounces. These ratios are arbitrary and must be memorized individually. Converting 3.7 miles to feet requires multiplication by 5,280 (yielding 19,536 feet), a calculation that is neither obvious nor convenient without a reference.

The United States, Myanmar, and Liberia are the only countries that have not officially adopted the metric system for everyday use. However, US science, medicine, and military operations use metric units extensively. Most of the world uses metric exclusively, which means anyone working internationally — in trade, engineering, or travel — needs to convert between systems regularly.

Length and Distance Conversions

The most common length conversions bridge metric and imperial: inches to centimeters (multiply by 2.54), feet to meters (multiply by 0.3048), miles to kilometers (multiply by 1.609344). These factors are exact or close to exact — the inch is defined as exactly 25.4 millimeters, making all derived conversions (foot, yard, mile) exact multiples.

Construction and manufacturing use fractional inches (1/16", 3/8", 5/32") alongside decimal millimeters. Converting between these requires both unit conversion and fraction-to-decimal conversion. A 3/8" drill bit is 3 ÷ 8 × 25.4 = 9.525 mm. In practice, metric drill bits come in whole or half-millimeter increments (9.5 mm), so exact equivalence is not always possible — you choose the closest available metric size.

Navigation uses nautical miles (1 nautical mile = 1.852 km = 1.15078 statute miles), which are based on one minute of latitude rather than land survey measurements. Aviation altitudes use feet worldwide (even in metric countries) by international convention, creating a persistent need for feet-to-meters conversion in aerospace engineering and weather reporting.

Astronomy uses its own distance units because standard units produce unwieldy numbers. The astronomical unit (AU, Earth-to-Sun distance, about 150 million km), the light-year (9.461 trillion km), and the parsec (3.086 × 10^13 km) each serve different scale ranges. These conversions rarely come up in daily life but are essential in science education and reporting.

Weight and Mass Conversions

Pounds to kilograms (divide by 2.2046) and ounces to grams (multiply by 28.3495) are the everyday conversions. For quick mental math, 1 kg is roughly 2.2 lbs, so 70 kg is about 154 lbs. For cooking, 1 ounce is roughly 28 grams, making recipe conversion between US and metric cookbooks manageable with simple multiplication.

The distinction between mass and weight matters in scientific contexts. Mass (kilograms) is an intrinsic property; weight (newtons) depends on gravitational acceleration. On Earth's surface, a 70 kg object weighs about 686 newtons (70 × 9.81 m/s). In everyday use, "weight" and "mass" are used interchangeably because gravity is constant at Earth's surface, but engineering calculations (structural loads, vehicle dynamics) require the precise distinction.

Troy ounces (31.1035 grams) differ from avoirdupois ounces (28.3495 grams) and are used exclusively for precious metals. Gold prices quoted in "ounces" mean troy ounces. A troy pound is 12 troy ounces (373.24 grams), lighter than an avoirdupois pound of 16 avoirdupois ounces (453.59 grams). This historical anomaly catches people converting gold weights using standard ounce-to-gram factors.

Temperature Conversions

Temperature conversion between Fahrenheit and Celsius is the most commonly needed conversion for travelers and anyone reading international weather reports. The formulas are: C = (F - 32) × 5/9 and F = C × 9/5 + 32. Key reference points: water freezes at 0C / 32F, boils at 100C / 212F, and normal body temperature is 37C / 98.6F.

Mental math shortcuts: for a rough Fahrenheit-to-Celsius conversion, subtract 30 and divide by 2. This gives approximate results: 80F → (80-30)/2 = 25C (actual: 26.7C). For Celsius-to-Fahrenheit, double and add 30: 20C → 40+30 = 70F (actual: 68F). These approximations are within a few degrees and sufficient for weather and cooking contexts.

Kelvin uses the same degree size as Celsius but starts at absolute zero (-273.15C). Converting is simple: K = C + 273.15. Kelvin is the SI standard and is used in all scientific calculations involving temperature. Rankine is the absolute scale for Fahrenheit (R = F + 459.67) and appears occasionally in US engineering specifications, particularly in thermodynamics.

Cooking temperatures illustrate where conversion precision matters. A recipe calling for 350F translates to 176.7C, which you would round to 175C or 180C depending on your oven's markings. A recipe at 425F is 218.3C, rounded to 220C. Most ovens are not calibrated precisely enough for the rounding to matter, but baking (where temperature control affects chemical reactions) is more sensitive than roasting.

Convert Your Units

Our Unit Converter handles length, weight, temperature, volume, area, speed, and digital storage units. Select the category, choose source and target units, and enter a value to see the instant conversion. The tool shows the conversion formula alongside the result so you understand the math behind the number. All calculations run in the browser with no data sent to any server.