GC-content is usually expressed as a percentage value, but sometimes as a ratio (called G+C ratio or GC-ratio). GC-content percentage is calculated as
whereas the AT/GC ratio is calculated as  
The GC-content percentages as well as GC-ratio can be measured by several means, but one of the simplest methods is to measure the melting temperature of the DNA double helix using spectrophotometry. The absorbance of DNA at a wavelength of 260 nm increases fairly sharply when the double-stranded DNA molecule separates into two single strands when sufficiently heated. The most commonly used protocol for determining GC-ratios uses flow cytometry for large numbers of samples.
In an alternative manner, if the DNA or RNA molecule under investigation has been reliably sequenced, then GC-content can be accurately calculated by simple arithmetic or by using a variety of publicly available software tools, such as the [http://www.basic.northwestern.edu/biotools/oligocalc.html free online GC calculator].