The Complete Unit Conversion Reference

By Michael Lip / March 20, 2026 / 20 min read

Unit conversions sit at the intersection of everyday life and technical precision. A cook doubles a recipe and needs to convert 3/4 cup to tablespoons. An engineer converts pressure from PSI to pascals for an international specification. A web developer translates a designer's pixel values to rem units. A traveler arriving in Europe needs to know that 22 degrees Celsius means they should leave the jacket in the hotel.

The conversions themselves are straightforward multiplication or division. What makes them tricky is remembering which factor to apply, which direction to multiply, and whether the relationship is linear (like meters to feet) or offset-based (like Celsius to Fahrenheit). This reference covers the most commonly needed conversions across length, weight, temperature, volume, data storage, number systems, CSS units, and currency.

For instant conversions without memorizing any of these tables, the Unit Converter handles all the categories covered here and runs entirely in your browser.

Length and Distance

The metric system uses powers of ten, which makes conversions within the system trivial. A kilometer is 1,000 meters. A centimeter is 0.01 meters. A millimeter is 0.001 meters. Moving between metric units is just shifting the decimal point.

The imperial system, still used daily in the United States, Liberia, and Myanmar, uses a collection of historically derived ratios that do not share a common base. 12 inches in a foot. 3 feet in a yard. 5,280 feet in a mile. 1,760 yards in a mile. These numbers have origins in Roman, Anglo-Saxon, and English medieval measurement systems, and they resist simple mental arithmetic.

From	To	Multiply By	Example
Inches	Centimeters	2.54	12 in = 30.48 cm
Centimeters	Inches	0.3937	30 cm = 11.81 in
Feet	Meters	0.3048	6 ft = 1.829 m
Meters	Feet	3.2808	2 m = 6.562 ft
Miles	Kilometers	1.6093	26.2 mi = 42.165 km (marathon)
Kilometers	Miles	0.6214	100 km = 62.14 mi
Yards	Meters	0.9144	100 yd = 91.44 m
Nautical miles	Kilometers	1.852	1 nmi = 1.852 km

Useful mental shortcuts speed up rough conversions. A meter is about 3 feet and 3 inches. A kilometer is about 0.6 miles (or more precisely, 5 miles is roughly 8 kilometers, which also happens to be consecutive Fibonacci numbers). An inch is almost exactly 2.5 centimeters. These approximations are close enough for everyday purposes like estimating travel distances or comparing heights.

The International Bureau of Weights and Measures (BIPM) defines the meter as the distance light travels in a vacuum in 1/299,792,458 of a second. This definition, adopted in 1983, makes the meter independent of any physical artifact. The inch is defined as exactly 25.4 millimeters, which means the imperial system is technically defined in terms of the metric system.

For specialized fields, additional length units remain in active use. Astronomers use light-years (9.461 trillion km), astronomical units (149.6 million km, the mean Earth-Sun distance), and parsecs (3.262 light-years). Mariners use nautical miles (1,852 meters, defined as one minute of latitude). Typographers use points (1/72 of an inch) and picas (12 points).

Weight and Mass

The distinction between weight and mass matters in physics (mass is intrinsic, weight depends on gravitational field strength) but not in everyday conversions. On Earth's surface, we use the terms interchangeably, and the conversion factors are stable.

From	To	Multiply By	Example
Pounds	Kilograms	0.4536	150 lb = 68.04 kg
Kilograms	Pounds	2.2046	70 kg = 154.32 lb
Ounces	Grams	28.3495	8 oz = 226.8 g
Grams	Ounces	0.03527	500 g = 17.64 oz
Stones	Kilograms	6.3503	10 st = 63.5 kg
Metric tons	Pounds	2,204.6	1 t = 2,204.6 lb
Short tons (US)	Metric tons	0.9072	1 US ton = 0.907 t

The quick mental conversion for pounds to kilograms is to halve the value and subtract 10%. So 200 pounds is approximately 100 minus 10, or 90 kg. The actual value is 90.72 kg, so the approximation is within 1%. Going the other way, double the kilograms and add 10% to get pounds.

Cooking weights create particular confusion because recipes from different countries use different measurement systems. American recipes primarily use volume measurements (cups, tablespoons) while European, British, and Australian recipes use weight (grams). Professional bakers universally prefer weight because it is more precise. A cup of flour can weigh anywhere from 120 to 160 grams depending on how it was scooped, but 150 grams of flour is always 150 grams of flour.

Troy ounces, used for precious metals, are different from avoirdupois ounces used for everything else. A troy ounce is 31.1035 grams, while a standard ounce is 28.3495 grams. When gold is quoted at "$2,050 per ounce," that is per troy ounce. This 9.7% difference matters significantly in precious metal transactions.

Temperature

Temperature conversions are the exception to the simple multiplication pattern because temperature scales have different zero points. Celsius sets zero at water's freezing point. Fahrenheit sets zero at the freezing point of a brine solution (a seemingly arbitrary choice that Daniel Fahrenheit made in 1724). Kelvin sets zero at absolute zero, the theoretical minimum temperature.

The formulas are not complicated, but they are easy to get backwards.

Celsius	Fahrenheit	Context
-40	-40	The one point where both scales match
-18	0	Freezer temperature
0	32	Water freezes
10	50	Cool autumn day
20	68	Room temperature
30	86	Warm summer day
37	98.6	Human body temperature
100	212	Water boils (at sea level)
180	356	Baking (moderate oven)
230	450	Pizza oven temperature

The quick mental trick for Celsius to Fahrenheit: double the Celsius value and add 30. This gives you 20C = 70F (actual: 68F), 30C = 90F (actual: 86F), 10C = 50F (exact). It is not precise, but it is close enough to decide what to wear.

For scientific work, Kelvin is the standard because it has no negative values and its zero point has physical meaning. A temperature of 0 K (absolute zero, -273.15C) represents the complete absence of thermal energy. It has never been achieved in a laboratory, though scientists have come within a billionth of a degree. Kelvin intervals are the same size as Celsius intervals, so a change of 1 K equals a change of 1C.

Volume and Liquid Measurements

Volume conversions between metric and imperial are complicated by the fact that there are two different "gallon" standards. The US gallon is 3.785 liters. The Imperial (UK) gallon is 4.546 liters. This 20% difference means that fuel economy figures, recipe quantities, and industrial specifications are not interchangeable between American and British standards without conversion.

From	To	Multiply By
US gallons	Liters	3.7854
Liters	US gallons	0.2642
Imperial gallons	Liters	4.5461
US fluid ounces	Milliliters	29.5735
Milliliters	US fluid ounces	0.03381
Cups (US)	Milliliters	236.588
Tablespoons (US)	Milliliters	14.787
Teaspoons (US)	Milliliters	4.929
US quarts	Liters	0.9464
Cubic inches	Milliliters	16.387

A liter of water weighs exactly one kilogram at 4 degrees Celsius and standard atmospheric pressure. This elegant relationship is a direct consequence of how the metric system was originally designed. One cubic centimeter holds one milliliter of water, which weighs one gram. This makes metric cooking conversions between weight and volume straightforward for water-based liquids, though it does not hold for other liquids (oil is lighter, honey is heavier).

US cooking measurements follow their own internal logic. 3 teaspoons make a tablespoon. 16 tablespoons make a cup. 2 cups make a pint. 2 pints make a quart. 4 quarts make a gallon. The most common conversion you need while cooking is cups to tablespoons (1 cup = 16 tbsp), which is useful when you need 3/4 of a cup and only have a tablespoon measure (answer: 12 tablespoons).

The Unit Converter handles all volume conversions between metric, US customary, and Imperial systems, including the less common units like pecks, bushels, and fluid drams.

Binary and Data Storage

Data storage units follow two parallel naming systems that have caused confusion since the early days of computing. The problem is that storage hardware manufacturers use decimal prefixes (1 GB = 1,000,000,000 bytes) while operating systems historically used binary prefixes with the same names (1 GB = 1,073,741,824 bytes).

The IEC resolved this in 1998 by creating distinct binary prefix names, but adoption has been slow. The binary prefixes use the "-bi-" infix: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB).

Decimal (SI)	Value	Binary (IEC)	Value	Difference
1 Kilobyte (KB)	1,000 bytes	1 Kibibyte (KiB)	1,024 bytes	2.4%
1 Megabyte (MB)	1,000,000 bytes	1 Mebibyte (MiB)	1,048,576 bytes	4.9%
1 Gigabyte (GB)	1,000,000,000 bytes	1 Gibibyte (GiB)	1,073,741,824 bytes	7.4%
1 Terabyte (TB)	1,000,000,000,000 bytes	1 Tebibyte (TiB)	1,099,511,627,776 bytes	10.0%
1 Petabyte (PB)	10^15 bytes	1 Pebibyte (PiB)	2^50 bytes	12.6%

This discrepancy explains why a "500 GB" hard drive shows as approximately 465 GB in Windows (which historically displayed binary sizes with decimal labels). The drive actually contains 500,000,000,000 bytes, which is 465.66 GiB. You are not being cheated. The naming convention is just inconsistent.

macOS switched to decimal (SI) display in Mac OS X Snow Leopard (2009), so a 500 GB drive shows as 500 GB in Finder. Windows 10 and 11 still display binary values with decimal labels. Linux distributions vary by distribution and file manager.

Network speeds use bits (lowercase b), not bytes (uppercase B). An internet connection advertised as 1 Gbps (gigabit per second) has a theoretical maximum throughput of 125 MB/s (megabytes per second), because there are 8 bits in a byte. The actual throughput is lower due to protocol overhead, typically achieving 110-120 MB/s on a well-configured connection.

The Binary Converter handles conversions between binary, decimal, hexadecimal, and octal number systems, which is essential for working with memory addresses, color codes, file permissions, and network configurations.

Roman Numerals

Roman numerals use seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000). Numbers are formed by combining these symbols according to specific rules.

The additive rule places larger values before smaller ones and sums them. VI = 5 + 1 = 6. CLXIII = 100 + 50 + 10 + 1 + 1 + 1 = 163. MDCCCXII = 1000 + 500 + 100 + 100 + 100 + 10 + 1 + 1 = 1812.

The subtractive rule places a smaller value before a larger one to indicate subtraction. IV = 5 - 1 = 4. IX = 10 - 1 = 9. XL = 50 - 10 = 40. XC = 100 - 10 = 90. CD = 500 - 100 = 400. CM = 1000 - 100 = 900. Only I, X, and C can be used subtractively, and only in specific pairs.

Decimal	Roman	Decimal	Roman	Decimal	Roman
1	I	10	X	100	C
2	II	20	XX	200	CC
3	III	30	XXX	300	CCC
4	IV	40	XL	400	CD
5	V	50	L	500	D
6	VI	60	LX	600	DC
7	VII	70	LXX	700	DCC
8	VIII	80	LXXX	800	DCCC
9	IX	90	XC	900	CM

The largest standard Roman numeral is MMMCMXCIX (3,999). Numbers above this require extended notation. The Roman Numeral Converter handles bidirectional conversion between decimal and Roman numeral representations.

Modern uses of Roman numerals include Super Bowl numbering (Super Bowl LX in 2026), movie sequel titles, clock faces, chapter numbering in books, monarch and pope numbering (King Charles III, Pope Francis), copyright year notices on films and television shows, and building cornerstones showing the year of construction.

An interesting edge case: the NFL skipped Roman numerals for Super Bowl 50 in 2016 because "Super Bowl L" looked awkward in branding. They returned to Roman numerals the following year with Super Bowl LI.

CSS Units for Web Developers

Web developers work with a unique set of units that map physical concepts (length, size) to the abstract space of screens with varying pixel densities, font sizes, and viewport dimensions.

Unit	Type	Relative To	Common Use
px	Absolute	Device pixel (at 1x DPR)	Borders, shadows, fine details
rem	Relative	Root element font size	Font sizes, spacing, layout
em	Relative	Parent element font size	Component-internal spacing
%	Relative	Parent element dimension	Widths, responsive layouts
vw	Relative	1% of viewport width	Full-width elements, responsive typography
vh	Relative	1% of viewport height	Full-height sections, hero areas
dvh	Relative	1% of dynamic viewport height	Mobile full-screen layouts
ch	Relative	Width of the "0" character	Limiting text line width
fr	Fraction	Available space in grid	Grid track sizing

The pixel-to-rem conversion is the most frequent one web developers perform. If the root font size is the browser default of 16px (which it is unless you have changed it), then 1rem = 16px. The conversion formula is: rem = px / 16.

Pixels	Rem (base 16px)	Pixels	Rem (base 16px)
8px	0.5rem	28px	1.75rem
10px	0.625rem	32px	2rem
12px	0.75rem	36px	2.25rem
14px	0.875rem	40px	2.5rem
16px	1rem	48px	3rem
18px	1.125rem	56px	3.5rem
20px	1.25rem	64px	4rem
24px	1.5rem	72px	4.5rem

The Pixel to Rem Converter handles this conversion for any base font size, which is useful when working with codebases that use a non-standard root font size (some frameworks set the root to 10px for easier mental math, where 1rem = 10px).

The dvh unit, introduced to solve the mobile viewport height problem, deserves specific attention. On mobile browsers, the address bar and navigation controls appear and disappear as the user scrolls. The older vh unit represents the viewport height with the browser chrome visible (the small viewport). The dvh unit dynamically adjusts as the browser chrome hides, preventing the jarring jump that occurred when elements sized with 100vh were suddenly shorter than the visible area. Use 100dvh instead of 100vh for mobile hero sections and full-screen layouts.

Currency Conversion

Currency conversion differs fundamentally from physical unit conversion because exchange rates fluctuate continuously. The rate between USD and EUR changes thousands of times per day as trades execute on foreign exchange markets with a daily volume exceeding $7.5 trillion (Bank for International Settlements, 2025 Triennial Survey).

Exchange rates are quoted as pairs. EUR/USD = 1.08 means one euro costs 1.08 US dollars. The first currency is the "base" and the second is the "quote." To convert 500 EUR to USD at this rate: 500 x 1.08 = $540. To convert $500 USD to EUR: 500 / 1.08 = 462.96 EUR.

Currency Pair	Approximate Rate (March 2026)	Daily Volume
EUR/USD	1.08	$1.87 trillion
USD/JPY	151	$1.01 trillion
GBP/USD	1.26	$714 billion
USD/CHF	0.88	$343 billion
AUD/USD	0.66	$338 billion
USD/CAD	1.36	$310 billion
USD/CNY	7.22	$295 billion

The mid-market rate, sometimes called the interbank rate, is the midpoint between the buy and sell prices on global currency markets. This is the "real" exchange rate that online converters display. When you actually exchange money at a bank, airport kiosk, or through a transfer service, you receive a worse rate that includes the provider's markup. Airport exchanges typically charge 5-12% above the mid-market rate. Banks charge 1-3%. Online services like Wise charge 0.3-1.5%.

The Currency Converter provides conversions using current mid-market rates, which is useful for estimating costs, comparing prices across countries, and understanding international financial data. For actual money transfers, always check the rate offered by your specific provider.

Purchasing Power Parity (PPP) is a different concept from exchange rates. PPP adjusts for the fact that the same goods cost different amounts in different countries. The Economist's Big Mac Index, which compares the price of a Big Mac across countries, illustrates this. A Big Mac costs $5.69 in the US and 4.35 EUR in Germany. At the current exchange rate, the German Big Mac costs about $4.70, suggesting the euro is roughly 17% undervalued against the dollar in terms of purchasing power. PPP matters when comparing salaries, GDP, or living costs across countries.

Conversion Gotchas and Common Mistakes

The Mars Climate Orbiter, a $125 million NASA spacecraft, was destroyed in 1999 because one team used imperial units (pound-force seconds) while another expected metric units (newton seconds) for thruster calculations. The spacecraft entered the Martian atmosphere at the wrong angle and burned up. Unit conversion errors have real consequences.

Fluid ounces and weight ounces share a name but measure different things. 8 fluid ounces of water weighs approximately 8.3 ounces by weight. 8 fluid ounces of honey weighs about 12 ounces. 8 fluid ounces of olive oil weighs about 7.6 ounces. The "a pint's a pound" approximation works for water but not for other liquids.

The short ton (US, 2,000 pounds), long ton (UK, 2,240 pounds), and metric ton (tonne, 1,000 kilograms or 2,204.6 pounds) are three different units with similar names. Confusing them in commodity trading, shipping, or engineering can result in 10-12% errors.

Temperature conversion mistakes happen when people apply the formulas in the wrong direction. The most common error is adding 32 when converting Fahrenheit to Celsius (you should subtract 32 first, then multiply by 5/9) or subtracting 32 when converting Celsius to Fahrenheit (you should multiply by 9/5 first, then add 32).

Bits and bytes confusion in networking contexts leads to users wondering why their "100 Mbps" connection only downloads files at 12 MB per second. The connection speed is 100 megabits. Divide by 8 to get megabytes. Then subtract about 5-10% for protocol overhead. The result, approximately 11-12 MB/s, is correct.

Dry and liquid measurements in US cooking are different. A dry cup and a liquid cup both hold 236.588 mL at the level mark, but dry measuring cups are designed to be filled to the brim and leveled off, while liquid measuring cups have extra space above the fill line to prevent spilling. Using a liquid measuring cup for flour (packing it to the line) will give you more flour than the recipe intends.

Frequently Asked Questions