A well-planned SCBW game has, well, a plan! A plan might be "I want to make 4 lurkers and 18 zerglings in the quickest way possible and then break down the terran's natural base and win the game right there." Or it might be "I want to survive into the late game so that I have 5 bases with 8 hatcheries, producing 2 ultralisks, 1 defiler, 1 lurker, 1 pair of scourge, and the rest zerglings and overlords non-stop." Economy management is how you get to the end goal of your plan. And good economy management is getting there in the most efficient way possible.
I'll start by breaking down the first example plan so you can get an idea of what economy management is. The first thing you need to do is get the numbers. Building times, how often larva pop out, and how many minerals and gas are mined by a single drone are the main numbers you need to know for this. You can jump into a game and work them out yourself, or you can go onto the StarCraft wikia and get some of the numbers there.
+ Show Spoiler [Some Data] +
All times use in-game seconds. There are 0.65 game seconds in 1 real life second.
Spawning Pool = 80 seconds
Drone = 20 seconds
Overlord = 40 seconds
Lair = 100 seconds
Hatchery = 120 seconds
Larva Spawn Time = 20 seconds
From http://www.teamliquid.net/forum/viewmessage.php?topic_id=78828 (my inspiration for this thread in some ways)
How many drones do I need to maintain constant production of a ..... from 1 hatchery?
Drone: 3 drones mining minerals
Zergling: 3 drones mining minerals (duh, it costs 50 minerals just like the drone)
Overlord: this unit is unique since you don't constantly produce it....you just need to start a new one when the old one pops out. 2 drones per overlord is good, if later in the game you need to make 2 overlords every round, then 4 drones, no more will be needed, ever.
Hydralisk: 6 drones on minerals + 1 drone on gas ( remember, we are talking about 1 hathcery)
Mutalisk: 4 drones on minerals + 3 drones on gas. It should be noted that this combo gives you the same amount of minerals as gas, 100/100 for muta, and the same combo is used for lurker and ultralisk.
Note, in zvz for muta/ling combo, it's 6 drones on minerals + 3 drones on gas per hatchery, mainly because mutas are limited by gas, not larva, so you will have excess larva and no gas for the zergling.
Scourge: 1 drone on mineral + 3 drones on gas. This unit is only constantly produced in the ling/scourge strategy in zvz. And for that 3 drones on minerals + 3 drones on gas should maintain constant production of ling/scourge from 1 hatchery.
Queen: Omitted from experiment due to lack of use.
Lurker: 4 drones on minerals + 3 drones on gas. This includes the hydralisk cost.
Defiler: 3 drones on minerals + 3 drones on gas, you get a little excess minerals, but 2 drones on minerals is not enough.
Ultralisk: 4 drones on minerals + 3 drones on gas.
The example we'll be breaking down is Jaedong vs Dolphin. The replay is available here.
In this replay, Jaedong does a fairly standard 3 hatch muta build. What we'll be investigating is when and why he builds certain buildings to reach his checkpoints along the way, and we'll also see just how far along his economy management goes. Most people (that I know of) only follow a build order for the first ten minutes or so of a game. After that, they start to make it up as they go. While this isn't inherently terrible, you can be much more effective if you have precise knowledge of just how many workers you need to produce X amount of units from Y hatcheries, and so on. The game is a ZvT, and we'll be looking at the Zerg side of things, but the same concept applies to the other races.
Let's take a look at his build order. He goes for a standard 12 hatch opening, with the pool at 12 as well.
14 second hatchery
@100 gas lair
22 second gas
One more pair of lings
24 Ling speed
4 more pairs of lings when the overlord finishes
@31 when lair finishes, he makes his spire.
He then drones until 34, and makes 3 overlords. At this stage he has 22 drones on minerals, and 6 on gas.
He makes one more drone with his remaining 1 supply, bringing him up to 35 with 3 overlords on the way.
Marines push out so he makes two sunken colonies at his natural, bringing him down to 22 mineral mining drones and 5 on gas, with 9 mutalisks on the way. He uses one of his gas mining drones to make one of the sunken colonies.
He then makes a hydralisk den at his main. Down to 21 on minerals, 5 on gas. He makes one more drone and two more mutalisk, back up to 22 on minerals and 6 on gas. He then makes an overlord and 2 pairs of lings.
He researches lurker aspect as soon as he has the gas for it. Then he makes two hydralisks and a queens nest, and 2 drones. 24 on minerals, 5 on gas. But he takes his 3rd so 22 on minerals.
Then 4 pairs of zerglings, and 2 more drones.
2 more hydralisk, 2 more pairs of zerglings
Hive the moment he has minerals for it. Lurker aspect finishes and he immediately morphs two lurkers, which is all he has gas for, with the third lurker morphing once he has gas.
Then 4 pairs of zerglings, and 2 more drones. He continues morphing lurkers as he gets gas
He gets two more hydras, his evo chamber and another hatchery in his main. At his third he starts making drones, while at his main he makes one more drone and an overlord and another hydralisk. He makes 3 more drones at his main and maynards over to his third. He starts his defiler mound when hive finishes and he has resources.
At this stage things start to get a bit messy, but he keeps his drone count at 42/43 for several minutes at this stage, replacing any he loses. This means he has 8 on gas and 33/34 on minerals.
Once he gets his fourth base up, he makes 8 more drones, bringing him up to 50/51. 3 on gas, so 38/39 on minerals.
The game ends shortly after this, as Jaedong loses his main.
And now we analyze just what happened.
For the first stage of the game, he's working hard on getting his total drone count to 23, give or take a couple drones depending on what buildings he places. For now we'll ignore the initial mutalisk part, other than a small aside:
The first 11 mutalisks and the build up to them are perfectly timed to be able to get the exact amount of mutas you want and not have more nor less drones than you need, with any excess larva being devoted to zerglings.
Carrying on, what can you produce with 23 drones? You have 6 drones on gas, and around 17 on minerals (again, that number varied slightly). You can produce a maximum of 2 lurkers at a time, and are limited by gas. That was what Jaedong was doing, and we have remaining 9 drones on minerals. We need to produce an overlord every round, which requires 2 drones mining minerals, so we have 7 drones left on minerals. With that, we can produce 2 sets of zerglings at a time and have an excess of minerals, which we need anyway for placing other buildings and hatcheries. Jaedong has the precise amount of drones he needs, no more, no less.
Later on he goes up to 38 drones, varying slightly depending on the situation. Now, he took one of his natural gas drones off to make a sunken. He may have made a mistake. I could be reading this wrong. However, that's 8 on gas, with 29 left mining minerals. And he's producing (per round, roughly): one defiler, one lurker, and sometimes a pair of scourge, with some upgrades consuming his excess gas. That leaves us with all the gas used up precisely, and 22 mineral mining drones. Subtract 4 for 2 overlords per round, and we have 18. With this, he's producing 5 pairs of zerglings, leaving 3 extra drones on minerals which earn minerals for extra hatcheries and nydus canals and such. Again, precise drone amounts, no more, no less.
And when he gets his next base, he increases his defiler production if I see correctly. Now, he's not producing exactly those units every round, but it's pretty close to it. He's good enough to know when he can vary it, but not good enough to spend his minerals and gas perfectly, so he does have a few hundred excess minerals near to the end. His gas usage is pretty accurate though. He adds on his natural gas drone at around 110 supply. Again, he may have made a mistake and was wondering the whole game why he didn't have enough gas, it is possible.
Why is this so useful?
Maximization. If you only produce the drones you need and the hatcheries you need to achieve your production goals, not only will you not float resources (looks so pro!) but you will be able to produce everything you need without running out of resources. Too often when I play without a definite build, I'll either never have enough resources, or I'll start floating resources massively. Not only that, when I float resources too high I have not maximized the size of my army somewhere down the line. That means a potential window of oppurtunity for my opponents.
This helps you when you're doing a particular build, because you'll be able to work out exactly how many drones you need to do whatever you want, and you'll also have a nice framework to know what units you can afford to purchase. This makes macro easier because you don't have to think too hard about what to produce, you just produce whatever you've already planned for. This also means you don't have too many lings and too much gas and not enough lurkers, because you're always producing roughly what you planned for.
This is also extremely useful for when you create a build, so that you can maximize the efficiency of the build. For example, here's a cheese/allin build I use often in ZvT which was created from scratch using my knowledge of building times and resource requirements.
Birdie's 2 hatch lurker rush
12 hatch and scout
drone up to 13
Move three drones onto gas the moment it completes.
@Spawning pool completion - Lair
Produce 2-4 lings (usually I produce 4).
Use one ling to sit outside the opponent's base in case of any early pushes, and the rest to chase away the scout. If possible, you want the scout out of your main and then keep your lings on the ramp so that it can't get back up.
16 - Overlord
@Lair 60% completion - Hydralisk Den
3 more drones
@Den and lair completion - Lurker Aspect
At this stage you don't need to produce any more drones. Make 4 hydralisks, and one more overlord, and use the rest of the larva and minerals on Zerglings. Make sure you stop production a few seconds before the lurker aspect completes so that you have enough minerals for the 4 lurkers to morph straight away. Continue ling production and overlord production.
@your next 100 gas, start ling speed.
Soon after, the lurkers complete and you attack his base with the lings in front and lurkers behind. Burrow the lurkers and watch the chaos ensue.
I hope this article was useful to you. If you think anything in it is incorrect, feel free to let me know. If it helped you at all or you want any advice, feel free also to let me know I realize that nothing here is particularly new, but an article like this would have been useful to me when I was learning to play.