国际米原器，为国际单位制米在1960年之前所使用的标准器。1米的最初定义为经过巴黎的四分之一经线（北极点至赤道）总长度的1000万分之一。由于该标准较难复现，1799年，法国制作了一个铂制成的米原器，于1799年6月22日存放在国家档案馆内。由于精确测量米原器从首端到尾端的距离非常困难，1889年，重新制作一个由铂铱合金制作的米原器，其断面为X型。米的定义重新为0°C时该合金米原器的长度。米原器保存在法国巴黎的国际度量衡局总部。

1960年，第11届国际计量大会决定放弃以米原器为标准的米定义，并采用氪86光谱作为计量依据，1米定义为氪86原子在2P10到5d5能级之间跃迁的辐射在真空中波长的1650763.73倍。1983年又将1米定义为「光在真空中于1/299792458秒内行进的距离」。

In the aftermath of the French Revolution (1789), the old units of measure that were associated with the Ancien Régime were replaced by new units. The livre was replaced by the decimal franc, and a new unit of length was introduced which became known as the metre. Although there was initially considerable resistance to the adoption of the new metric system in France (including an official reversion to the mesures usuelles (French for: "normal units") for a period of time), the metre gained following in continental Europe during the mid nineteenth century, particularly in scientific usage, and was officially adopted as an international measurement unit by the Metre Convention of 1875.