How big would an asteroid or comet have to be to end life on Earth?

NEO=Near Earth Object. The Absolute Magnitude (H) on the bottom axes is a measure of a body’s brightness and is correlated with size. The larger an asteroid is, the brighter it will appear, assuming that all asteroids have the same surface composition (which they don’t, but the assumption is likely good enough to give a rough estimate of the body’s size)

This is an excellent diagram by astronomer Alan Harris that provides a lot of information on the relationship between asteroid size, damage, and frequency of impact.

The K-T impact event that caused the extinction of about ¾ of the Earth’s animal and plant species was likely caused by an impact from a body with a diameter of about 10 km. The good news is that there’s an inverse relationship between the size of the impactor and the number of years between events of similar magnitude (in this case about a 100 million years for a mass-extinction event). The bad news is that we’re likely due for another event fairly soon (The K-T event was 66 million years ago.)

To end all life on Earth, an impactor with a diameter between 10-100 km would likely do the job.

Source: Quora