Unit Converter
Convert common length, weight, and volume measurements.
How to use this unit converter
- Pick a measurement category
Select length, weight, or volume from the Unit category dropdown.
- Enter the amount
Type the number you want to convert into the Value field.
- Select the source and target units
Choose the current unit in the From unit dropdown and the desired unit in the To unit dropdown.
- Read the result
The Converted value and Conversion path appear instantly below the inputs.
How this unit converter works
This unit converter handles length, weight (mass), and volume measurements by normalizing any input value to a common base unit within its measurement family and then scaling to the target unit. For length, the base unit is meters; for weight, kilograms; for volume, liters. Every supported unit has a precise conversion factor relative to its base, and the tool multiplies through in a single step: converted value = input × (source-to-base factor) × (base-to-target factor). This two-step normalization approach is the same method used by NIST and the International Bureau of Weights and Measures (BIPM) in their published conversion tables, ensuring accuracy for both metric-to-metric and metric-to-imperial transformations.
converted = value × (source unit factor ÷ target unit factor)
Equivalently: converted = value × source-to-base × base-to-target Convert 5 miles to kilometers: 1 mile = 1,609.344 meters, so 5 miles = 5 × 1,609.344 = 8,046.72 meters. Then convert to km: 8,046.72 ÷ 1,000 = 8.04672 km. Convert 10 pounds to kilograms: 1 lb = 0.45359237 kg, so 10 lb = 10 × 0.45359237 = 4.5359 kg. Convert 2 gallons (US) to liters: 1 US gallon = 3.785411784 liters, so 2 gallons = 7.5708 liters.
Convert 3.5 liters to U.S. cups: 1 liter ≈ 4.22675 cups, so 3.5 × 4.22675 ≈ 14.79 cups. Select volume as the Unit category, enter 3.5 in the Value field, choose liters as the From unit, and cups as the To unit.
Convert 180 centimeters to feet: 180 cm = 1.8 m, and 1.8 ÷ 0.3048 ≈ 5.91 feet. Select length, enter 180, and choose centimeters to feet.
- ✓ Length conversions use the international yard-and-pound agreement of 1959 (1 inch = 25.4 mm exactly), which underpins all imperial-to-metric length factors.
- ✓ Weight conversions treat mass and weight interchangeably under standard Earth gravity — the tool converts mass units (grams, pounds) without adjusting for local gravitational variation.
- ✓ Volume conversions use the metric definition where 1 liter = 1,000 cubic centimeters exactly, and U.S. customary fluid ounces / gallons are used (not Imperial UK gallons).
- ✓ Definition-based factors are exact where standards define them exactly; inverse decimal factors shown on one-way pages may be rounded for readability while still being based on the exact published relationship.
- When converting between metric prefixes (e.g. millimeters to kilometers), the tool simply shifts by powers of 10 — no rounding is introduced because the metric system is base-10 by design.
- Imperial and U.S. customary systems share some unit names (ounce, gallon, ton) but with different sizes. This tool uses U.S. customary definitions unless otherwise noted.
- For cooking measurements, note that a U.S. cup is 236.588 mL (not 250 mL), and a U.S. tablespoon is 14.787 mL — these differ from metric and Australian standards.
- Troy ounces (used for precious metals) differ from avoirdupois ounces (used for everyday weight): 1 troy oz = 31.1035 g vs. 1 avoirdupois oz = 28.3495 g.
- National Institute of Standards and Technology (NIST) — Handbook 44, Appendix C: General Tables of Units of Measurement
- Bureau International des Poids et Mesures (BIPM) — The International System of Units (SI), 9th edition
- U.S. Code Title 15, §205a–205l — Metric Conversion Act definitions
Metric versus imperial measurement systems
The metric system is built on powers of ten, making unit jumps simple shifts of a decimal point — 1 kilometer is exactly 1,000 meters, and 1 kilogram is exactly 1,000 grams. The imperial and U.S. customary systems, by contrast, evolved from older English units and use irregular ratios: 12 inches in a foot, 3 feet in a yard, 5,280 feet in a mile. Because most of the world uses metric while the United States, Liberia, and Myanmar still rely on customary units in everyday life, cross-system conversion is one of the most common practical math tasks. This tool normalizes every input to its metric base unit first, which avoids error-prone multi-step chains when converting between two non-metric units.
Why conversion factors matter
Small rounding differences in conversion factors can compound quickly when dealing with large quantities. For example, rounding the mile-to-meter factor from 1,609.344 to 1,609 introduces an error of 0.344 meters per mile — negligible for a single-mile trip but over 34 meters across 100 miles. This converter uses the full NIST-published factors, which are exact where defined by international agreement (inches, pounds, yards) and precise to at least six significant figures otherwise. For everyday tasks the extra decimal places rarely matter, but for engineering, surveying, and scientific work they prevent systematic drift.
Unit converter FAQs
Why do imperial and metric conversions sometimes differ by a tiny amount between tools?
The difference usually comes from how many decimal places the conversion factor carries. For instance, 1 inch is exactly 25.4 mm by international agreement, but some tools truncate to 25.4 while others carry more digits downstream. This converter uses the full NIST-published factors.
Is a UK gallon the same as a US gallon?
No. A UK (Imperial) gallon is approximately 4.546 liters, while a US gallon is approximately 3.785 liters — about 20% smaller. This tool uses U.S. customary definitions by default.
Can I convert square or cubic units with this tool?
This version handles linear length, weight, and volume. For area and cubic conversions, square or cube the linear conversion factor manually — for example, 1 square foot = (0.3048)² = 0.0929 square meters.
Why does the tool normalize to a base unit first?
Normalizing to a single base unit per measurement family means the tool only needs one conversion factor per unit rather than a factor for every possible unit pair. It keeps the conversion table manageable and eliminates compounding rounding errors from chaining multiple factors.
How accurate are the conversion results?
The underlying factors match NIST reference values to at least six significant figures. Results are displayed rounded to a reasonable number of decimal places, but the internal arithmetic retains full floating-point precision throughout the calculation.