钾 - 氩年代测定法，简称K-Ar测年，是在地质年代学和考古学中，利用放射性来测定年代的方法。它是利用钾（K）的同位素会经由放射性衰变变成氩（Ar）的性质来测量。钾是一种常见的元素，存在于在许多物质中，如云母，粘土矿物，火山灰。40Ar存在于液态的岩石时，会稳定的存在其中而无法逃脱，但当岩石凝固（再结晶）时，40Ar便会开始衰变。通过测量40K剩余量的比例来计算从结晶到现在的时间长度。40K有很长半衰期，使用的方法能计算年龄长达几千年的样本。

在快速冷却的熔岩中，由于冷却过程很快的降低到铁的居里温度以下，使得钾 - 氩年代测定法能够以其为十分理想的样品求出当时的磁场的方向和强度。由于这个方法十分理想，地磁极性的时间和尺度主要使用K-Ar测年来进行校准。

Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites. In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air. Time since recrystallization is calculated by measuring the ratio of the amount of Ar accumulated to the amount of K remaining. The long half-life of K allows the method to be used to calculate the absolute age of samples older than a few thousand years.