Usually many practitioners focus on the compressive capacity of concrete. The tension capacity is usually ignored for design capacity purposes since it is significantly lower and also presents a very brittle failure.
However, the concrete tensile capacity can be useful for analysing crack widths as well as checking the "cracked" moment capacity of the concrete alone (without any steel reinforcement).
As a general approximate rule, the tensile capacity of concrete is usually about 10% of it's compressive strength. For example, a 30 MPa compressive strength concrete will have about a 3 MPa tensile strength.
Various equations from different codes are presented below, applicable to normal weight concrete rather than lightweight concretes. Note that fck is the concrete cylinder compressive strength.
Eurocode
In the Eurocode, the mean tensile strength of concrete is estimated as follows:
ACI
In the American concrete codes, the tensile strength is mostly referred to as the modulus of rupture, and is calculated as:
Japanese (AIJ)
In the Japanese AIJ standards, the tensile strength of concrete is as follows:
Cracking Moment
The cracking moment represents the bending moment beyond which the concrete will be cracked, so it's a useful check. In general, it can be calculated as follows:
where Z is the section modulus of the concrete (bd^2/6)