Pound (lb)

Definition

The pound (symbol: lb) is a unit of mass used in the imperial and US customary systems. The most common version today, the international avoirdupois pound, is defined as exactly 0.45359237 kilograms. One avoirdupois pound is made up of 16 ounces.

History & Origin

The pound traces back to the Roman libra, and over time, many different versions were used. The avoirdupois system, developed in the 13th century, became the standard and was officially updated in 1959. It was based on a prototype physical weight divisible into 16 ounces, chosen likely for its convenient factors (8, 4, 2). This may have contributed to its popularity over other systems that divided units into 10, 12, or 15 parts.

Current Use

The pound is widely used in the United States for everyday weight measurements, especially body weight. In the United Kingdom, despite the adoption of the metric system, pounds are still used in specific contexts like food labeling (metric units must also be shown). It’s also common in the UK to measure body weight in stones, where one stone equals 14 pounds.

Kilogram (kg)

Definition

The kilogram (symbol: kg) is the base unit of mass in the International System of Units (SI). Since 2019, it has been defined using the fixed numerical value of the Planck constant (h), which is exactly 6.62607015 × 10⁻³⁴ J·s (or kg·m²·s⁻¹). Despite this redefinition, the actual size of the kilogram has remained unchanged.

History & Origin

The word kilogram comes from the French kilogramme, combining Greek for “a thousand” and Latin gramma, meaning “a small weight.”

Originally defined in 1794 as the mass of one liter of water at freezing, the kilogram later relied on a physical prototype — a platinum-iridium cylinder. However, even this standard was imperfect, as variations between the original and its copies were found (up to 50 micrograms difference).

The 2019 redefinition ties the kilogram to universal constants like the Planck constant and the second, providing greater accuracy and stability over time.

Current Use

As an SI base unit, the kilogram is used globally across nearly all fields — science, industry, education, and commerce. While the United States still commonly uses pounds in daily life, kilograms are regularly used in science, government, and military settings.