Free Online Unit Converters — Length, Weight, Volume & More (2026)
TL;DR
This page bundles every free unit converter on UtilDaily — 14 tools across six categories (length & area, weight & mass, temperature, volume, speed, specialty) plus 8 in-depth guides. Every converter uses the exact internationally-defined factor (1 inch = 25.4 mm, 1 mile = 1.60934 km, 1 kg = 2.20462 lb). All run 100% in your browser; no sign-up.
How do I convert between length and area units?
Length conversions sit at the heart of any imperial-to-metric workflow — from international shipping to printer DPI to woodworking plans translated from the other side of the Atlantic. Inches, millimetres, kilometres and miles all relate by exact, internationally-defined factors (1 inch = 25.4 mm, 1 mile = 1.60934 km), so a precise calculator gives precise results.
MM to Inches Converter
Convert millimeters to inches and inches to mm with precision.
KM to Miles Converter
Convert kilometers to miles and miles to km with a simple calculator.
Area Converter
Convert between area units: square feet, square meters, acres, hectares, square yards, square miles, and square kilometers. Includes real estate size reference table.
Aspect Ratio Calculator
Calculate pixel dimensions for any aspect ratio. Social media presets for YouTube, TikTok, Instagram, and more.
How do I convert between kilograms and pounds?
Weight conversion is the single most common metric/imperial calculation in daily life — gym scales, luggage allowances, baby weights, shipping declarations. The defined relationship (1 kg = 2.20462 lb) gives the same answer everywhere, but rounding choices matter for medical and pharmaceutical contexts where every gram counts.
How do I convert temperature between Celsius, Fahrenheit and Kelvin?
Temperature is the conversion that trips everyone up because the scales do not share a common zero. Celsius and Fahrenheit cross at -40°, Kelvin offsets from Celsius by 273.15. Cooking, weather, medical, and scientific conversions each demand precision in different ranges — our calculator covers all three at once.
How do I convert volume — gallons, litres, cups and tablespoons?
Volume conversion has the most pitfalls because the same word means different things in different countries. A US gallon is 3.785 L; an imperial gallon is 4.546 L. A US cup is 236.6 mL; an imperial cup is 284 mL. Use the right converter (or the cooking-specific one) so your bread does not turn into pancake batter.
Liquid Volume Converter
Convert between liquid volume units: US/UK gallons, liters, fluid ounces, cups, pints, and quarts. Clearly distinguishes between US and Imperial measurements.
Cooking Converter
Convert between cooking measurements: oz to ml, cups to grams, teaspoons, tablespoons, pints, and liters. Includes ingredient density selector for accurate cup-to-gram conversion.
How do I convert speed — mph, km/h, knots and m/s?
Speed conversion comes up most often when reading speed limits abroad (90 km/h ≈ 56 mph) and when sailors, pilots, or weather forecasters quote knots. One knot = one nautical mile per hour = 1.852 km/h, and the relationship to mph (≈ 1.151 mph per knot) is not the same as the simple km-to-miles ratio.
How do I convert numbers, codes and characters?
Beyond physical units, several developer-adjacent conversions show up constantly: hex colour codes to RGB for design work, decimal to binary or hexadecimal for low-level programming, integer values to written words for cheques, Roman numerals for clocks and movie credits, and Morse code for amateur radio.
Hex to RGB Color Converter
Convert hex color codes to RGB and HSL values instantly. Includes color picker, CSS variable output, and example colors.
Number Base Converter
Convert numbers between binary, decimal, octal, and hexadecimal. Includes two's complement representation and ASCII character mapping for developer reference.
Number to Words Converter
Convert numbers to English words in cardinal, ordinal, or check-writing format up to quadrillions.
Roman Numeral Converter
Convert between numbers and Roman numerals (I, V, X, L, C, D, M) with validation and date formatting.
Morse Code Converter
Translate text to Morse code and decode Morse to English instantly.
Common conversions reference table
The most-searched unit conversions, with their exact factor and a quick mental approximation. Numbers come from the SI brochure (BIPM) and NIST Handbook 44.
| From | To | Exact factor | Mental shortcut |
|---|---|---|---|
| 1 inch | millimetres | 25.4 mm | × 25 |
| 1 foot | centimetres | 30.48 cm | × 30 |
| 1 yard | metres | 0.9144 m | ≈ 0.9 |
| 1 mile | kilometres | 1.60934 km | × 1.6 |
| 1 kg | pounds | 2.20462 lb | × 2 + 10% |
| 1 oz | grams | 28.3495 g | × 28 |
| °C → °F | — | × 9/5 + 32 | × 2 + 30 |
| °F → °C | — | (°F − 32) × 5/9 | − 30, ÷ 2 |
| 1 US gallon | litres | 3.78541 L | × 3.8 |
| 1 imperial gallon | litres | 4.54609 L | × 4.5 |
| 1 US cup | millilitres | 236.588 mL | ≈ 240 |
| 1 mph | km/h | 1.60934 km/h | × 1.6 |
| 1 knot | mph | 1.15078 mph | × 1.15 |
| 1 m² | square feet | 10.7639 ft² | × 10.76 |
Frequently asked questions
What is the most accurate online unit converter?
Accuracy depends on the underlying constants the converter uses. Reliable converters use the internationally-defined exact factors — for example, 1 inch = 25.4 mm exactly (per the 1959 international yard and pound agreement). UtilDaily converters use these defined values rather than rounded approximations, so results are correct to as many decimal places as you display.
What is the difference between metric and imperial units?
Metric (also called SI) is a base-10 system where 1 km = 1,000 m = 100,000 cm. Imperial uses inherited ratios — 12 inches per foot, 3 feet per yard, 1,760 yards per mile — that are convenient for halving and quartering but awkward for arithmetic. Most of the world uses metric exclusively; the US, Liberia, and Myanmar are the three countries that still use imperial day-to-day.
How do I convert kilograms to pounds quickly?
Multiply kg by 2.205 for an answer correct to within 0.001%. For mental math, double the kg value and add 10% (for example, 70 kg × 2 = 140, + 7 = 154 lb). The exact factor is 2.20462 lb/kg.
Why do US and imperial gallons differ?
The US gallon is based on the 18th-century English wine gallon (231 cubic inches = 3.785 L), while the imperial gallon was redefined in 1824 to equal 10 lb of water (4.546 L). The two have differed by 20% ever since. Our liquid-volume converter handles both — pick the right one for the context.
How precise are these calculators?
All UtilDaily converters use IEEE-754 double-precision floating point, which gives ~15 significant decimal digits — far more than any practical measurement. The displayed result is rounded for readability; the underlying calculation is essentially exact.
Are these converters safe to use?
Yes — every conversion runs in your browser as JavaScript. Nothing is sent to a server. You can verify this by opening DevTools, switching to the Network tab, and watching that no requests fire while you convert.
Where do you get the conversion factors?
Length, mass, and temperature factors come from the SI brochure published by BIPM (the International Bureau of Weights and Measures) and from NIST Handbook 44. US customary units follow the official NIST refinements of the 1959 yard and pound agreement.
Can I convert in both directions with the same tool?
Yes. Every UtilDaily converter is bidirectional — type into either field and the other updates instantly. There is no separate 'reverse' button to find.
Do I need an account or to install anything?
No. Open the page, type a number, get the answer. No sign-up, no email, no install. The site works fully offline after the first page load.
References
- 1.The International System of Units (SI), 9th edition — BIPM
- 2.Unit Conversion — NIST
- 3.NIST Handbook 44 — NIST