Bits per second (bps) is the unit for the rate at which data moves across a link. Internet circuits, Wi-Fi, Ethernet ports and WAN services are rated and sold in bps, almost always with a decimal prefix: kilobits (kbps), megabits (Mbps), gigabits (Gbps) or terabits (Tbps) per second. Because file sizes, storage and many usage bills are counted in bytes, and a byte is eight bits, a link speed and a file transfer rate describe the same thing in units that differ by a factor of eight.
At a glance
- bps measures rate, not quantity: 100 Mbps means up to 100 million bits each second.
- Network prefixes are decimal, so 1 Gbps is 1,000 Mbps and 1,000,000,000 bits per second.
- Divide by eight to convert to bytes: 1 Gbps is at most 125 MB/s, and 100 Mbps is at most 12.5 MB/s.
- Real throughput is lower than the line rate because of protocol overhead, latency, congestion and end-system limits.
- Bandwidth is usually priced in bps, while egress and transfer charges are usually priced per GB, so comparing them needs a conversion.
What problem it solves
Buyers meet the same data described in two units. A circuit quote says 500 Mbps. A cloud bill charges per GB of data transfer. A backup tool reports MB/s, and a speed test reports Mbps. Without a clear grasp of bps, those figures look inconsistent, and decisions about bandwidth, migration timelines and transfer costs rest on guesses.
Understanding bps, and how it relates to the byte, lets you check whether a link is performing as sold, estimate how long a transfer will take, and compare rate-based pricing with volume-based pricing.
How it works
Why bits. A network link sends data as a serial stream of signals, one bit after another, so its capacity is naturally a count of bits per second. Telecommunications used bps long before personal computers, and carrier products such as T1 lines were defined by bit rates. Computers organize memory and files in bytes, so storage kept the byte. The two conventions have stayed separate.
Converting rate to volume. Bytes per second equals bits per second divided by eight. From there:
- 1 Gbps is at most 125 MB/s, or 450 GB per hour.
- 100 Mbps is at most 12.5 MB/s, or 45 GB per hour.
- A 10 TB migration over a fully used 1 Gbps link takes at least about 22 hours, and longer in practice.
These use decimal megabytes and gigabytes. Operating systems that report sizes with binary prefixes will show somewhat smaller numbers for the same data.
Line rate versus throughput. The advertised rate is the line or port rate. Each packet carries headers for Ethernet, IP and TCP or UDP, and encryption and tunneling add more. On a well-behaved local link, large transfers can come close to the line rate, but long-distance paths with higher latency, packet loss or limited TCP window sizes can fall well short, as can slow disks or servers at either end.
Committed versus peak rates. A service may have a port speed, a committed information rate and a burst rate, all in bps. Which one applies to your traffic, and when, depends on the service terms.
When it matters for buyers
- Comparing bandwidth and egress pricing. Circuits and 95th percentile billing charge for a rate in Mbps. Cloud data egress fees charge for volume in GB. As a rule of thumb, 1 Mbps sustained for a 30-day month moves about 324 GB, which lets you compare the two models.
- Sizing a circuit. Translate backup windows, replication volumes and video traffic into bps, then add headroom. Our internet access page covers how buyers size and compare circuits, including dedicated internet access.
- Checking performance claims. When users complain that a “1 Gig” link downloads at about 110 MB/s, that is close to normal.
- Planning migrations. Divide the data volume by the realistic throughput, not the line rate, to get a timeline.
Questions to ask vendors
- Is the quoted speed the port rate, the committed rate or a burst rate, and is it symmetrical?
- What throughput should we expect for a single large transfer, and between which points is it measured?
- Is usage billed as a rate in Mbps, such as 95th percentile, or as a volume in GB, and how is each measured?
- For data transfer charges, is a GB 1,000,000,000 bytes or 1,073,741,824 bytes?
- What overhead do your encryption, tunneling or SD-WAN features add on this link?
How it differs from bandwidth
Bandwidth is the capacity of a connection, the maximum it can carry. Bits per second is the unit that capacity, and actual traffic, are measured in. A link may have 1 Gbps of bandwidth while carrying only 300 Mbps of traffic at a given moment; both figures are in bps, but one is the ceiling and the other is current use.