Concrete grades are determined by concrete's compressive strength 28 days after it has been poured. The strength is measured in psi, or pounds per square inch.
The greater the psi rating on a mix of concrete, the stronger the concrete will be after it has been poured and set. The required strength of the grade itself is largely dependent on its intended use. For most residential uses, the minimum concrete grade is 2,500 psi. Commercial work, on the other hand, will typically require a grade of 4,000 psi or higher. Any type of concrete that is rated higher than 6,000 psi is considered to be high-performance concrete. This grade is typically set aside for heavy-duty commercial projects, although special projects such as skyscrapers can require concrete that has a grade of at least 10,000 psi.