There are three types of concentration that come with metta bhavana:
1. parikamma samadhi – preliminary concentration
2. upacara samadhi – access concentration
3. appana samadhi – fixed concentration

1. PARIKAMMA SAMADHI
Parikamma bhavana is the stage of doing the preliminary exercises. Some degree of one-pointedness or calmness is reached when radiating metta to the person as one is involved with active recitations.

2. UPACARA SAMADHI
When it comes very close as if about to sink or merge with the object we may consider it as access concentration. By then one has overcome the hindrances as it is close to fixed absorption. The mind has reached a very subtle and sleeplike state. If one is not careful one may fall asleep. One has to be mindful to maintain the flow of metta and yet not too energetic that it stirs it up to a restless state. At this state visions may creep in but one has to be mindful enough to maintain the flow of metta. The Visuddhimagga describes this state as a state when the barriers are broken. That is, at that time one's metta is developed to the state that one is as if one with the person. One cannot be said to have any less or more metta one has for oneself than another or a close one from a hostile one.
As concentration develops, the object of mind likewise becomes more refined and steady. It may be very gross ideas of the person at preliminary concentration to fine, transparent-like visualisations at access concentration. However the development of the object is not as obvious as in kasinas. So in initial stages the states of mind and metta are more obvious and important criteria for checking one's development. With frequent meditation one can also be aware of the nature of this fine object. I remember that the first time I noticed it was as the person outlined on a crystal-clear surface. It has appeared again in another way.

3. APPANA SAMADHI
When the mind becomes fixed onto the object it sinks and merges into it to become as if one. The result is the development of a different form of consciousness called (jhana citta) absorption. Very often people say this is like falling into a state deeper than sleep. Yet on emerging one is aware that at that period one is in bliss and is still having metta for the person. It has been claimed that the state is so sleep-like that one may not be aware that one has entered into it, especially when it first occurs in only very short moments. However with frequency it should become obvious. How long one takes to reach up to this level is very much of an individual capability. If we go into intensive meditation, it should not take too long. Those who have undergone vipassana meditation, whose mind is already flexible and wholesome, should be even quicker. There are 4 types of these absorptions (in the 5-fold classification) in metta bhavana. They are called the 1st jhana, 2nd jhana, 3rd jhana, and 4th jhana. As for the 5th jhana, it can be attained only in the development of equanimity – upekkha bhavana.