About This Unit Converter

Units are agreed-upon references for measuring physical quantities—defined by science, standards bodies, or long-standing trade practice. Any measurement can be written as a number times a unit (for example, 2.54 cm or 1 inch). Because the world uses different unit systems side by side, reliable unit conversion is essential for engineering, science, education, healthcare, cooking, construction, and everyday life.

Why this site exists

Our goal is to give you a fast, accurate, and trustworthy online unit converter that works across modern SI (metric) units and widely used non-SI systems (US customary, Imperial, industry and legacy standards). Every category is backed by precise definitions and notes where context matters (e.g., gauge vs. absolute pressure, temperature offsets, or electrical power at non-unity power factor).

What you can convert here

• Length & Distance: meter, kilometer, mile, yard, foot, inch, nautical mile, angstrom, micrometer
• Area: square meter, hectare, acre, square foot, square mile
• Volume: liter, milliliter, gallon (US & UK), cubic meter, cubic inch/foot
• Mass/Weight: kilogram, gram, tonne, pound, ounce, stone
• Temperature: Celsius, Kelvin, Fahrenheit, Rankine, Reaumur
• Speed: m/s, km/h, mph, knot, ft/s
• Time: second, minute, hour, day, week, month, year (Gregorian/Julian options), millisecond -> attosecond
• Energy: joule, watt-hour, kilowatt-hour, calorie, BTU, eV, ton-TNT
• Power: watt, horsepower (mechanical/metric/electric), BTU/h, ton of refrigeration, VA/kVA/var*
• Pressure: pascal, bar, atm, psi, torr, mmHg, inH2O (with temperature-specific columns)*
• Density: kg/m³, g/cm³, lb/ft³, lb/gal
• Photometric: Luminous intensity (candela, candlepower*), luminous flux (lumen), illuminance (lux, foot-candle)

*Context-dependent entries include clear notes.

Accuracy you can trust

Exact constants (e.g., 1 inch = 0.0254 m, 1 atm = 101 325 Pa, 1 cal(th) = 4.184 J) ensure consistent results. Affine conversions are implemented where needed (e.g., temperature). Units that require extra context (e.g., VA -> W without power factor, “standard” gas volumes without T/P) are flagged to prevent misleading results.

How to use it

Pick a category (e.g., Length, Temperature, Pressure). Choose your from and to units from searchable lists. Enter a value; the site converts instantly with high-precision math. Use the copy button to grab the answer. Need more depth? Visit the All Converters pages for complete global unit lists in each category.

Who it is for

Engineers & scientists needing precise SI <-> US/Imperial conversions. Students & teachers looking for a clean, trustworthy reference. DIYers, cooks, travelers, builders—anyone who converts units day to day.

Looking for a free online unit converter that handles metric to imperial and beyond? This site includes an advanced length converter, weight converter, temperature converter, speed converter, area converter, volume converter, power converter, energy converter, pressure converter, density converter, and light/photometry converters (lux, lumen, candela). We prioritize accuracy, clarity, and real-world usability.

Quick FAQ

What is the difference between SI and Imperial/US units?
SI (metric) is the modern global standard (meter, kilogram, second). Imperial/US customary remain common in the UK/US (mile, pound, gallon). We support both—and show exact conversions.

Why do some units show notes like “at 4 °C” or “absolute vs gauge”?
Some pressure columns depend on fluid density and temperature; others depend on whether you measure relative to vacuum (absolute) or ambient air (gauge). We label those so results stay meaningful.

Can I convert horsepower to watts, or BTU/h to kW?
Yes—power conversions are supported. Context-dependent electrical terms (VA, var) are shown with clear guidance.