Zerg vs. Terran Timings
From Liquipedia Starcraft Wiki
Bio Terran vs. Zerg is a fast paced match up with the intensity of the Terran Ball facing off against the mobility and numbers of the Zerg army. Throughout the match up Terran and Zerg have different opportunities to exploit each others' weaknesses. This article explores the Timing of these strengths and weaknesses as well as strategies for players to exploit and shore up these positions.
 Game Flow Overview
The Zerg vs. Terran match up can be grouped in three game phases: Early, mid, and late. These three game phases correspond well to the three tiers of Zerg Tech: the Hatchery, Lair, and Hive Tech phases. To summarize these three phases:
- Early Game is Zerglings against Marines and Medics.
- Mid game adds Lurkers or Mutalisks versus Marines and Medics with static defenses.
- Late game is Ultralisks and Zerglings against Marines and Medics with tanks. Late game is supported by Terran and Zerg's respective spellcasters.
 Early Game
Early game consists of early Zergling pressure followed by Mutalisk or Lurker pressure out into mid game.
In modern play Zerg does not often scout with a Drone. Instead, Zerg will often lead off by sending their Overlord to scout and then move out with Zerglings. Also, because of the timing of Zerg build orders, Zerg do not really have the ability to change their build order in response to what Terran is doing with the exception of some sort of cheese. However, Zerg can choose how much they plan to pressure Terran depending on their scouting within the limits of the build order they chose to lead off with. That being said, Zerg's scouting time will depend on when their build order spawns their first Zerglings. The exception of this would be if Zerg expected some sort of cheese and moved out with a Drone to search for it.
Adaptations in Zerg vs. Terran will read how the Zerg should adapt their build order to fit his scouting intelligence rather than changing his build order entirely.
 Two Barracks before Expansion
If Zerg scouts two Barracks he can probably expect some kind of early game pressure once Academy Tech comes out. Until then, the Zerg will need to decide how much they want to pressure or if they plan on pressuring at all.
 9 Pool
A 9 Pool is still an effective opening against a two Barracks build as the Terran will not have many units out when the Zerglings arrive. Nevertheless, Zerg must be wary of the Terran once the Terran reaches Academy Tech as, depending on how much damage the Zerglings did, the Terran may be able to apply a lot of pressure to Zerg's attempt to take their Natural Expansion.
 Two Hatchery
With Terran leading off with two Barracks, the Zerg will not be able to deal much damage to Terran's early game economy as so few Zerglings can hardly push up Terran's ramp or pass through their choke. Zerg can however disrupt Terran's attempts to take their Natural Expansion by keeping Zerglings near the Terran's Natural. The Zerg will be able to keep up this kind of pressure until Terran reaches Academy Tech at which point Zerg will need to back off and prepare for some kind of early game pressure.
 Adapting a Three Hatchery
Zerg will be unable to pressure Terran with so few Zerglings against a two Barracks build. Furthermore, once Terran reaches Academy Tech, they will be able to pressure Zerg's Natural Expansion and even attempt some kind of Sunken breaking build. Zerg will need to keep an eye on Terran to make sure that they do not attempt to break Zerg's natural and be ready to place down Sunken Colonies if they suspect a push.
 One Barracks Before Expansion
If Zerg scouts one Barracks they are free to apply as much early game pressure as their opening build will allow. One Barracks for Terran can be risky because Zerg can potentially damage Terran's economy so much. Zerg would do well to exploit this temporary weakness until Academy tech comes out, at which point Zerg will need to back off until mid game tech.
 Adapting A 9 Pool
A 9 pool is a very effective build against a one barracks build so little adaptation is needed. It is important once Terran reaches Academy Tech for Zerg to begin backing off and moving to take their Natural Expansion.
 Adapting a Two Hatchery
With Terran leading off with one Barracks, Zerg will still be able to disrupt Terran with the Zerglings from a two Hatchery. At the very least Zerg will be able to stall Terran's attempts at taking their Natural until they reach Academy tech. Zerg also does not have to fear early pressure as much because it is unlikely that Terran will be able to break Zerg's natural by going one Barracks build.
 Adapting a Three Hatchery
This is the most common situation for Zerg, as it is the most standard type of ZvT. Depending on how many Zerglings you made, you will only have to make 0-2 Sunken Colonies if he follows up with a Factory and eBay after his first two Barracks which is most common. 3 Hatchery Mutalisk or Lurker is continued normally.
 Mid Game
Mid game in Terran vs. Zerg is defined as Lair tech pressure from Zerg (either Mutalisks or Lurkers) followed by a Terran Timing Push. Scouting for Zerg in mid game does not become much of an issue as they are able to dominate the playing field with their mid game tech. However, Zerg needs to be aware of how far along Terran's tech is as Science Vessels will mark the end of Zerg's ability to harass.
Once Zerg is unable to continue their harass, they will be susceptible to a Timing Push for the duration of their transition to Hive Tech. Zerg's goal at this point will be to shorten this transition as much as possible. They also should attempt to slow down the Push by sniping Marines and Medics and particularly Science Vessels. Once Zerg reaches Hive Tech, they have completed their transition into late game.
 Late Game
Late game ZvT is too situational and dependant on early/midgame outcomes to determine fixed timings.