Convert length, weight, temperature, volume, area and speed between Metric, US Customary, UK Imperial and nautical units — built to work no matter which system your country uses.
Enter a length value to convert between units
Enter a weight value to convert between units
Enter a temperature to convert between scales
Enter a volume value to convert between units
Enter an area value to convert between units
Enter a speed value to convert between units
This free online unit converter and conversion calculator converts values between length, weight, temperature, volume, area, and speed units in one tool — covering Metric (SI), US Customary, UK Imperial, and nautical systems. Acting as a universal unit converter and a metric to imperial converter for virtually any everyday measurement, it works as a dedicated length converter, weight converter, temperature converter, volume converter, area converter, and speed converter in a single place — a true all-countries conversion calculator rather than six separate tools.
Each of the six tabs (length, weight, temperature, volume, area, speed) lets you enter a value, pick a "from" unit, and instantly see it converted into every other unit in that category — km to miles, kg to pounds, liters to gallons, meters to feet, inches to cm, stone to pounds, or knots to mph. The result panel groups converted values by measurement system, so you always see at a glance which figures are Metric, US Customary, UK Imperial, or Nautical.
It's built for anyone who regularly works across mixed measurement systems — travelers checking distances or temperatures abroad, students and engineers converting metric to imperial specs, cooks following recipes from another country, and shoppers comparing product sizes or fuel-economy figures listed in unfamiliar units.
The math behind every conversion, from fixed multiplication factors to linear temperature scales
For length, weight, volume, area, and speed, every unit in a category is stored as a fixed multiplication factor relative to one base unit — meters for length, grams for weight, milliliters for volume, square meters for area, and meters per second for speed. To convert, the calculator first multiplies your input by the "from" unit's factor to reach a base-unit value, then divides by every other unit's factor:
For example, converting 1 mile to kilometers uses the factors mi = 1,609.344 m and km = 1,000 m, giving (1 × 1609.344) ÷ 1000 = 1.609344 km. Because these factors come from exact international definitions — NIST defines 1 inch as exactly 0.0254 m — every conversion is deterministic and reproducible to the precision your input allows.
Temperature is handled separately because Celsius, Fahrenheit, and Kelvin don't share a zero point, so a single multiplier can't capture the relationship. The calculator uses the two standard linear conversion formulas:
Kelvin starts at absolute zero (−273.15°C), and the calculator rejects any temperature entered below that physically impossible floor.
Unit mismatches cause real problems: a US gallon and a UK gallon differ by about 20%, a US ton and a metric tonne differ by roughly 10%, and confusing them in a recipe, shipping quote, or engineering spec can lead to meaningful errors. Side-by-side conversions across systems — from a simple km to miles lookup to a full stone to pounds or knots to mph conversion — remove the guesswork when working across borders or unit conventions.
From picking a category to reading the full result table
Click Length, Weight, Temperature, Volume, Area, or Speed at the top of the calculator to open the category you need.
Type the number you want to convert into the Value field for that category. The Weight and Length tabs start at 1, while Temperature starts at 20°C, so you can press Convert immediately for a quick default.
Select the unit your value is currently in from the From Unit dropdown — for example, Miles for length or Pounds for weight. Units are grouped by system to make them easy to find.
The calculator converts your value to the base unit for that category, then instantly shows it converted into every other unit in the category, grouped by measurement system.
Scan the grouped table for the unit you need. Your original unit's row is highlighted, so you can verify the input was read correctly at a glance. Use Reset to clear the result and start a new conversion.
Three sample conversions, worked the same way the calculator does it
| Tab | Input | Base value | Key conversions |
|---|---|---|---|
| Weight | 1 kilogram | 1,000 g | 2.205 lb · 35.274 oz · 0.157 stone |
| Volume | 1 US gallon | 3,785.41 mL | 3.785 L · 4 qt · 8 pt · 0.833 UK gal |
| Area | 1 acre | 4,046.86 m² | 0.405 ha · 43,560 ft² · 0.0016 mi² |
| Speed | 1 mph | 0.447 m/s | 1.609 km/h · 0.869 knots · 1.467 ft/s |
Explanation: Every tab follows the same two-step logic — convert the input to the category's base unit, then divide by each target unit's factor. Only temperature swaps the factor method for the linear formulas above, because its scales lack a common zero point. The figures in this section were computed with the exact factors this calculator's JavaScript uses, so entering them here reproduces the table exactly.
How to read the result table and what each element means
| Result Element | What It Means |
|---|---|
| Grouped sections | Converted values are separated into Metric, US Customary, UK/Imperial, and Nautical groups so you never mistake a US gallon for a UK gallon |
| Highlighted "from" row | Your original unit's row has a teal background so you can verify the input unit was read correctly |
| Decimal precision | Values under 1 show up to 6 decimal places; values from 1–999 show up to 4; large values show 2 — so tiny conversions like mg → t stay readable without misleading rounding |
| Thousands separators | Large results like 43,560 ft² are formatted with commas for easy scanning |
| Absolute-zero guard | The Temperature tab refuses any value below −273.15°C, which is physically impossible |
| Placeholder before Convert | The results panel stays hidden until you click Convert, so an empty read never gets mistaken for a result |
Why measurement systems differ around the world — and how this tool keeps them apart
Almost every country uses the metric system (meters, kilograms, liters, Celsius) officially. The US primarily uses feet, pounds, gallons and Fahrenheit, while the UK commonly mixes both. This calculator groups units by system so you always know which you're using.
Celsius is used worldwide for weather and science, Fahrenheit is standard in the US, and Kelvin is the scientific base unit starting at absolute zero (−273.15°C). All three convert instantly and warn you if a result would fall below absolute zero.
A US gallon (3.785 L) and a UK/Imperial gallon (4.546 L) are about 20% different — the same applies to pints and fluid ounces. This calculator lists US Customary and UK/Imperial volume units in separate groups to prevent costly mix-ups in recipes and fuel economy figures.
Nautical miles and knots are the international standard for maritime and air navigation, based on the Earth's geometry rather than a national standard — making them consistent regardless of which country is operating the vessel or aircraft.
When to use this all-in-one tool versus NeftCal's single-category converters
| Tool | What It Converts | Best For |
|---|---|---|
| Conversion Calculator (this page) | All six categories in one tabbed tool | Quick lookups that span several categories without switching pages |
| Length Converter | Length and distance units only | Focused length work with more in-depth guidance |
| Weight Converter | Weight and mass units only | Focused weight work such as body-weight or postage conversions |
| Temperature Converter | Celsius, Fahrenheit, Kelvin | Weather, cooking, and science temperature conversions |
| Volume Converter | Volume and capacity units | Recipes, fuel, and liquid-measurement conversions |
| Area Converter | Area and land-measurement units | Real estate, flooring, and land-area conversions |
| Speed Converter | Speed units including knots | Driving, aviation, and maritime speed conversions |
| Storage Converter | Digital storage sizes (KB, MB, GB, TB) | Computer file and disk sizes — not physical measurements |
For everyday mixed conversions this page is the fastest option — it converts every unit in a category at once. Reach for the single-category converter when you need a deeper explainer focused on one measurement type.
Real situations where switching measurement systems comes up
Convert distances, speeds, and temperatures between the units of your home country and your destination — miles to kilometers for driving, mph to km/h for rental-car speed limits, and °F to °C for weather forecasts.
Scale a recipe that lists cups, ounces, and °F when your kitchen measures in grams, milliliters, and °C. The Volume and Weight tabs handle the US vs UK unit differences that trip up most cooks.
Convert between square meters and square feet, acres and hectares, or meters and feet when reading international floor plans, land listings, or building material specs.
Compare parts and products sourced across countries by converting torque, distance, and weight specs — for example, inches to centimeters or pounds to kilograms — before you order.
Convert between knots, mph, and km/h, and between nautical miles and statute miles, when planning routes or reading navigational charts in mixed units.
Check homework or lab conversions across metric prefixes — milligrams to grams, milliliters to liters, or meters to kilometers — and verify temperature conversions between scales.
What this conversion calculator does well, and where it has boundaries
Avoid the unit mix-ups that cause most conversion errors
Common questions about unit conversion across measurement systems
Official standards bodies behind the conversion factors used on this page
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