The cosmic distance ladder (or cosmological distance ladder, or extragalactic distance scale) is a series of methods for measuring distance to galaxies, the longest range methods of distance determination methods. The more accurate methods, typically limited in distance, are used to calibrate additional methods which handle longer distances, step-by-step, like climbing a ladder. The precision and accuracy of the methods generally decline with increasing distance, some methods being presumed no more accurate than 50%.
The shortest of the following methods are limited to the Milky Way, but are key "lower" steps in the ladder. To gain as much accuracy as possible, efforts are made to confirm distances and calibrations using multiple methods.
Among the standard candles:
Since redshift-derived distance depends upon the Hubble constant, which is subject to improvement, cosmological distances are sometimes cited divided by the Hubble parameter, assisting future readers in accounting for future improved Hubble constant determinations.