"Hard drive density is measured in bits per square inch, the highest of which are currently (5/2013) 750 gigabits per square inch. This means that a gigabyte of data will take up about 6.88 millimeter2. The weight of an area of a platter consists of the substrate (usually glass and ceramic) and the magnetic layer which actually holds the magnetic grains storing the data. The magnetic layer is usually made of a mostly cobalt alloy of 10-20 nm thickness. Assuming 10nm thickness to make the math easier, This gives us about 6.88 * 1013 nm3 of magnetic layer material for one gigabyte.
Given the density of cobalt, this means that we can approximate the weight at 0.612471 micrograms.
I'm not sure how much the substrate weighs, but it's almost certainly more than that.
2012 Update: This is all about drives that are shipping now - there is a lot of buzz about Seagate getting to 1 terabit per square inch recently, but that is a tech demo and not shipping quite yet.
2013 Update: It looks like the areal density of Hard Drive platters is stagnating, according to an interesting IBM report on the subject of Areal density. TDK says that they can approach the 1.5Tbits/inch2 mark, but they won't show up in the market until 2014. The Seagate tech touted in last year is supposed to show up in 2014 as well. Next year should be exciting for the weight of gigabytes.
Previously on ""How much does a gigabyte weigh on a hard disk?""
2009: Areal Density 400 Gbit/in2 = 1.1518 micrograms (ref) 2010: Areal Density 541.4 GBit/in2 = 0.84817 micrograms (ref) 2011: Areal Density 625 GBit/in2 = 0.734966 micrograms (ref) 2012: Areal Density 744 GBit/in2 = 0.617411 micrograms (ref)"
"On the disk an individual bit weighs nothing, it's just a change in magnetic polarity; see TheTXI's answer for a more elaborate explanation of this.
In RAM, however, bits are comprised of electrons (or lack thereof) and they do have a mass which is about 9.10938215 × 10−31 kg. So for a GiB of memory, assuming equal distribution for zero and one bits, we get around
4294967296 n × 9.10938215 × 10−31 kg
4294967296 would be the number of one bits in memory (assumed to be 50 %) and n would be the number of electrons that are on average in one bit. I have found one source1 that specified this number at around 105.
So we can give an estimate of how much mass 1 GiB (or 1 GB) of memory would have:
1 GiB, half filled with ones ≈ 3.91 × 10−16 kg = 391 femtograms
1 GiB, completely filled with ones ≈ 7.82 × 10-16 kg = 782 femtograms
1 GB, half filled with ones ≈ 3.64 × 10−16 kg = 364 femtograms
1 GB, completely filled with ones ≈ 7.29 × 10−16 kg = 729 femtograms
So in general you can assume that weight to be pretty unnoticeable (or, with hard disks to be downright nonexistant).
Yes, hard drives store data by flipping poles in magnetic domains on the disk--at first glance this means nothing is added or subtracted and thus no weight.
However, that's not the whole picture. The orientation of those domains matter. There is less total field energy when the domains are 1010101010 than when they are 11111111 or 00000000. I'm sure everyone is familiar with e=mc^2. Putting energy into the domains DOES mean mass, albeit an incredibly small amount of it.
My physics isn't up to even trying to estimate the mass but I'm sure it's beyond anything the most sensitive scale could possibly measure." "It depends on the data.
Yes, hard drives store data by flipping poles in magnetic domains on the disk--at first glance this means nothing is added or subtracted and thus no weight.
However, that's not the whole picture. The orientation of those domains matter. There is less total field energy when the domains are 1010101010 than when they are 11111111 or 00000000. I'm sure everyone is familiar with e=mc^2. Putting energy into the domains DOES mean mass, albeit an incredibly small amount of it.
My physics isn't up to even trying to estimate the mass but I'm sure it's beyond anything the most sensitive scale could possibly measure."
"Depends on where you're doing the weighing. One of the answers immediately jumps into discussing femtograms, which are not a measure of weight, but instead measure mass.
On the moon things weigh less, on Jupiter they weigh more. In space they weigh nothing.
