This section contains 8 free converters across 4 categories. Unit Converters handle length, weight, height, and temperature. Speed Converters include the full multi-unit speed hub plus dedicated pair pages for mph to km/h, knots to mph, m/s to km/h, and more. Currency and Text Tools cover live exchange rates, text case, and OCR.
All conversion tools
Every tool is free, runs entirely in your browser, and produces results as you type. No button press, no registration, no data sent to any server.
Which converter to use
Not sure which tool you need? Use this table to match your conversion task to the right tool.
| What you are converting | Examples | Tool |
|---|---|---|
| Distance and length | km to miles, feet to metres, inches to cm, nautical miles | Length Converter |
| Body weight or mass | kg to lbs, stone to kg, grams to ounces, metric tons | Weight Converter |
| Height (person or object) | cm to feet and inches, 5'11" to cm, meters to feet | Height Converter |
| Temperature | °C to °F, Kelvin to Celsius, Fahrenheit to Kelvin | Temperature Converter |
| Speed - any units | mph to km/h, knots to mph, m/s to km/h, ft/s - all 10 units | Speed Converter |
| Speed - specific pair | mph ↔ km/h, knots ↔ mph, knots ↔ km/h, m/s ↔ km/h, m/s ↔ mph, ft/s ↔ mph | mph to km/h, knots to mph, knots to km/h, and more |
| Money and exchange rates | USD to EUR, GBP to JPY, live mid-market rates | Currency Converter |
| Text case | Sentence case, UPPER CASE, Title Case, camelCase, alternating | Case Converter |
| Text in an image or scanned PDF | Extract text from PNG, JPG, WEBP, TIFF, scanned PDF | Image to Text (OCR) |
Metric vs imperial
Most unit conversion confusion comes from the fact that the world uses two main measurement systems that are not compatible with each other. Knowing which system a value comes from is the first step in converting it correctly.
The metric system (SI)
The International System of Units (SI) is the modern form of the metric system. It is the official measurement standard in almost every country in the world and the only system used in science and engineering internationally. All units scale by powers of 10 - a kilometre is exactly 1,000 metres, a kilogram is exactly 1,000 grams. This makes arithmetic straightforward: moving between metric units is just moving a decimal point.
The imperial and US customary systems
The United States uses a system derived from the old British imperial system. The UK officially moved to metric in the 1960s but retains imperial units in daily life: miles for roads, pints for beer, stone for body weight, and Fahrenheit in some contexts. The US never made the switch and continues to use miles, pounds, gallons, and Fahrenheit as primary units.
US customary and imperial units look similar but are not identical. The US gallon (3.785 L) is smaller than the UK imperial gallon (4.546 L). This distinction matters for fuel economy, brewing, and shipping calculations.
| Measurement | Metric unit | Imperial/US unit | Conversion |
|---|---|---|---|
| Length | metre (m) | foot (ft) | 1 m = 3.28084 ft |
| Distance | kilometre (km) | mile (mi) | 1 km = 0.621371 mi |
| Mass | kilogram (kg) | pound (lb) | 1 kg = 2.20462 lb |
| Speed | km/h or m/s | mph | 1 mph = 1.609344 km/h |
| Temperature | Celsius (°C) | Fahrenheit (°F) | °F = °C × 9/5 + 32 |
| Volume (liquid) | litre (L) | US gallon (gal) | 1 L = 0.264172 US gal |
| Short distance | centimetre (cm) | inch (in) | 1 in = 2.54 cm exactly |
When conversion errors matter most
Most of the time, a small rounding error in a unit conversion is inconsequential. But in some contexts, precision matters:
- Medical dosing: Drug concentrations and body weight affect dosing calculations. A kg/lb mix-up can result in a dose 2.2 times too large or too small.
- Engineering and construction: A foot/metre mix-up in structural calculations caused the loss of the Mars Climate Orbiter in 1999 - a $327 million spacecraft.
- International shipping: Customs declarations require accurate weight in the destination country's units. Errors can cause clearance delays or fines.
- Currency in invoicing: Using an outdated exchange rate or confusing similar currencies (such as USD vs AUD or CAD) can create significant discrepancies in international invoices.
Accuracy and rounding
All unit converters use exact conversion factors as defined by the International Bureau of Weights and Measures (BIPM) and relevant standards bodies. For example, 1 inch is defined as exactly 2.54 centimetres by international agreement - there is no approximation involved. Results are displayed with enough decimal places for practical use, without showing false precision.
For currency, rates are sourced from a financial data provider and updated once per day. They reflect the mid-market rate - the rate before any bank or broker markup is applied. For time-sensitive or high-value transactions, always confirm the rate with your actual payment provider.
FAQs
Are these converters free?
Yes, completely. No registration, no usage limits, no premium tiers. Every tool on this site is free to use.
Do the converters work on mobile?
Yes. Every tool is fully responsive and works on phones and tablets without an app download.
How accurate are the unit converters?
All converters use exact conversion factors as defined by BIPM and relevant standards bodies. For example, 1 inch = 2.54 cm exactly; 1 pound = 0.45359237 kg exactly. Results are rounded to a practical number of decimal places to avoid false precision.
What is the difference between metric and imperial?
The metric system (SI) uses units that scale by powers of 10 and is the international standard for science and most everyday use worldwide. Imperial units (miles, pounds, Fahrenheit) are primarily used in the United States and partially in the UK. The two systems are not compatible - you always need a conversion factor to move between them.
Can I convert between metric and imperial?
Yes. All unit converters support both metric and imperial units in the same tool. Pick your source and target unit from the dropdowns, enter a value, and the result appears instantly.
Why does the height converter have a separate tool from length?
The height converter includes a dedicated feet-and-inches input mode (5 ft 11 in as two separate fields) and a reference table of common adult heights. This makes it faster and less error-prone for the specific task of converting personal height, which has different UX needs from general length conversion.