Physicist Paul Davies has asserted that "There is now broad agreement among physicists and cosmologists that the Universe is in several respects 'fine-tuned' for life". However, he continues, "the conclusion is not so much that the Universe is fine-tuned for life; rather it is fine-tuned for the building blocks and environments that life requires."

Martin Rees formulates the fine-tuning of the Universe in terms of the following six dimensionless physical constants.[12]

N, the ratio of the strength of electromagnetism to the strength of gravity for a pair of protons, is approximately 1036. According to Rees, if it were significantly smaller, only a small and short-lived universe could exist.[12]

Epsilon (ε), the strength of the force binding nucleons into nuclei, is 0.007. If it were 0.006, only hydrogen could exist, and complex chemistry would be impossible. If it were 0.008, no hydrogen would exist, as all the hydrogen would have been fused shortly after the big bang.[12]

Omega (Ω), also known as the Density parameter, is the relative importance of gravity and expansion energy in the Universe. It is the ratio of the mass density of the Universe to the "critical density" and is approximately 1. If gravity were too strong compared with dark energy and the initial metric expansion, the universe would have collapsed before life could have evolved. On the other side, if gravity were too weak, no stars would have formed.[12]

Lambda (λ) is the cosmological constant. It describes the ratio of the density of dark energy to the critical energy density of the universe, given certain reasonable assumptions such as positing that dark energy density is a constant. In terms of Planck units, and as a natural dimensionless value, the cosmological constant, λ, is on the order of 10−122.[13] This is so small that it has no significant effect on cosmic structures that are smaller than a billion light-years across. If the cosmological constant was not extremely small, stars and other astronomical structures would not be able to form.[12]

Q, the ratio of the gravitational energy required to pull a large galaxy apart to the energy equivalent of its mass, is around 10−5. If it is too small, no stars can form. If it is too large, no stars can survive because the universe is too violent, according to Rees.[12]

D, the number of spatial dimensions in spacetime, is 3. Rees claims that life could not exist if there were 2 or 4.[12]

An older example is the Hoyle state, the third-lowest energy state of the carbon-12 nucleus, with an energy of 7.656 MeV above the ground level. According to one calculation, if the state's energy were lower than 7.3 or greater than 7.9 MeV, insufficient carbon would exist to support life; furthermore, to explain the universe's abundance of carbon, the Hoyle state must be further tuned to a value between 7.596 and 7.716 MeV. A similar calculation, focusing on the underlying fundamental constants that give rise to various energy levels, concludes that the strong force must be tuned to a precision of at least 0.5%, and the electromagnetic force to a precision of at least 4%, to prevent either carbon production or oxygen production from dropping significantly