I keep find references to it takes n things to empty a belt, but what does that mean? How long of a belt?
How do I use this information? Every time I look for information on how many smelters I need for my mines I get "Well, it takes n smelters to empty a belt!" Please fill me in on how this is useful information.
THanks
Empty or fill the belt

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Re: Empty or fill the belt
One tile of belt can hold a certain amount of items. Length doesn't matter. If you try to put on more there isn't enough space. So you can can only move a certain amount of items along a belt per second  depending on the speed (i.e. the tier) of the belt.
https://wiki.factorio.com/Belt_transpor ... throughput
https://wiki.factorio.com/Belt_transpor ... throughput
Re: Empty or fill the belt
It takes 48 stone furnaces to consume the iron/copper ore on a yellow belt. 15 items/sec * 3.2 sec = 48.
If you were to use 50, 72 , 144, or some other number of stone furnaces greater than 48, you will find the ones after the 48th sitting idle, doing nothing for you. Similarly, if you were to (somehow) use more than 48 stone furnaces to feed a yellow belt with iron or copper plate, you will find the later ones sitting idle as well, due to lack of place to put their plate.
Steel (and Electric) Furnaces have twice the crafting speed as a Stone Furnace, so if using those higher tiers of furnace, divide the number of furnaces needed by 2. If using a Red or Blue belt, with the double or triple throughput (30 and 45 items/sec respectively) you would multiply the number of furnaces needed by 2 or 3.
How is this useful? Well, for making your busdriven base, it is useful for figuring out how many belts of material you need to feed those machines. Blue Circuits (aka Processing units), for example, take 10 seconds to produce, and consume 20 Greens and 2 Reds to make (not counting the sulfur acid). If you wanted to fill half a yellow belt, how many machines would that be? 10 sec * 15 items/sec * 1/2 belt = 75 machines. How many Green Circuits does it take to satisfy 75 of those machines? 75 machines * 20 GC / 10 sec = 150 GC/second. How many BELTS is that? Depends on the belt. But, it would come out to 10 Yellow Belts of GC.
10 full yellow belts of GC take 15 full belts of Copper Plate and 10 full belts of Iron Plate. How much smelting does it take to produce that much plate? How much mining does it take to produce the ore needed for that much plate?
THAT is why knowing the throughput of the belts, and how many machines one belt can feed, is important. Or, if not knowing, the ability to do the math to see what it takes to feed the factory so that it produces what you want it to produce. You want to know how many machines one belt can feed, how many machines it takes to make a belt of material, how much crude you have to pump out of the ground to meet your plastic, or your sulfur, or your rocket fuel needs.
But, when they talk of emptying a belt, its not a matter of “this belt holds 8000 items on it, need to use that many items”; its a matter of “this belt carries 15/30/45 items/second, how much of it do I need to feed these machines, or how many of that machine can it feed?”
If you were to use 50, 72 , 144, or some other number of stone furnaces greater than 48, you will find the ones after the 48th sitting idle, doing nothing for you. Similarly, if you were to (somehow) use more than 48 stone furnaces to feed a yellow belt with iron or copper plate, you will find the later ones sitting idle as well, due to lack of place to put their plate.
Steel (and Electric) Furnaces have twice the crafting speed as a Stone Furnace, so if using those higher tiers of furnace, divide the number of furnaces needed by 2. If using a Red or Blue belt, with the double or triple throughput (30 and 45 items/sec respectively) you would multiply the number of furnaces needed by 2 or 3.
How is this useful? Well, for making your busdriven base, it is useful for figuring out how many belts of material you need to feed those machines. Blue Circuits (aka Processing units), for example, take 10 seconds to produce, and consume 20 Greens and 2 Reds to make (not counting the sulfur acid). If you wanted to fill half a yellow belt, how many machines would that be? 10 sec * 15 items/sec * 1/2 belt = 75 machines. How many Green Circuits does it take to satisfy 75 of those machines? 75 machines * 20 GC / 10 sec = 150 GC/second. How many BELTS is that? Depends on the belt. But, it would come out to 10 Yellow Belts of GC.
10 full yellow belts of GC take 15 full belts of Copper Plate and 10 full belts of Iron Plate. How much smelting does it take to produce that much plate? How much mining does it take to produce the ore needed for that much plate?
THAT is why knowing the throughput of the belts, and how many machines one belt can feed, is important. Or, if not knowing, the ability to do the math to see what it takes to feed the factory so that it produces what you want it to produce. You want to know how many machines one belt can feed, how many machines it takes to make a belt of material, how much crude you have to pump out of the ground to meet your plastic, or your sulfur, or your rocket fuel needs.
But, when they talk of emptying a belt, its not a matter of “this belt holds 8000 items on it, need to use that many items”; its a matter of “this belt carries 15/30/45 items/second, how much of it do I need to feed these machines, or how many of that machine can it feed?”