Weighing scale

A weighing scale, also called a balance, mass scale, weight scale, massometer, or weight balance, is an instrument for measuring weight or mass. These devices are common in commerce because many goods are sold and packaged based on mass.

The classic balance scale has two pans or bowls suspended at equal distances from a central pivot. An object of unknown mass is placed on one pan, while known reference masses are added to the other until the beam levels off, indicating mechanical equilibrium. At this point, the masses on both sides are equal, and the scale rests neutral. A spring scale, by contrast, uses a spring with a known stiffness. According to Hooke's law, the spring stretches by a set amount for a given mass, with heavier objects causing greater extension. Other scales rely on different physical principles. Some devices can be calibrated to read in force units like newtons instead of mass units like kilograms.

The balance scale is so simple that it likely predates clear evidence of its use. Archaeologists identify early scales mainly through carved stones used as standard masses, since the balance itself could determine relative mass long before absolute mass was measured. The oldest confirmed evidence comes from Egypt's Fourth Dynasty, around the reign of Sneferu (c. 2600 BC), where deben balance weights have been found. Carved stones marked with mass values and the Egyptian hieroglyph for gold suggest merchants used a standardized mass system for tracking gold shipments or mine yields. Though no actual scales from that era survive, many sets of weighing stones and murals depicting balance scales indicate widespread use.

Examples from roughly 2400–1800 BC have been uncovered in the Indus River valley. Uniform, polished stone cubes found in early settlements were likely used as mass-setting stones. These cubes bear no markings, but their masses are multiples of a common denominator. They are made from various stones with different densities, showing that mass—not size or other traits—guided their creation. In China, the earliest excavated weighing balance comes from a Warring States period tomb of the state of Chu (3rd–4th century BC) at Mount Zuojiagong near Changsha, Hunan. This wooden balance used bronze masses.

By around 400 BC, variations like the cheap, inaccurate bismar (an unequal-armed scale) became common among small merchants and customers. Many scale designs with different advantages appeared throughout history, with inventors such as Leonardo da Vinci contributing to their development. Despite these advances, all scales until the 17th century were variations on the balance scale. Governments were deeply concerned with standardizing weights and ensuring traders used correct ones.

The original balance had a beam with a central fulcrum. For high accuracy, the fulcrum was a sharp V-shaped pivot set in a shallower V-shaped bearing. To find an object's mass, reference masses were hung on one beam end and the unknown object on the other. For precision work like chemistry, the center beam balance remains one of the most accurate technologies and is used to calibrate test masses. Bronze fragments from central Germany and Italy were used as early currency during the Bronze Age, and merchants across regions from Great Britain to Mesopotamia used standard weights of 8 to 10.5 grams.

The balance—also called a beam balance or laboratory balance—was the first mass-measuring instrument. In its traditional form, it has a pivoted horizontal lever with equal arms (the beam) and a pan suspended from each arm. The unknown mass goes in one pan, and standard masses are added to the other until the beam is nearly level. In precision balances, a sliding mass along a graduated scale gives a more accurate reading. A decimal balance uses a lever where the weight arm is ten times longer than the object arm, allowing lighter weights to measure heavy objects. A centesimal balance uses a 1:100 ratio.

Unlike spring scales, balances measure mass precisely because their accuracy is unaffected by local gravitational variations (which on Earth can differ by ±0.5% between locations). Moving a balance does not change the measured mass, since gravity affects both sides of the beam equally. A center beam balance gives accurate mass readings anywhere with constant gravity or acceleration. Very precise measurements require the fulcrum to be essentially frictionless.