Burstable bandwidth is a circuit where you pay a fixed price for a committed amount of bandwidth and may use more, up to the port size, paying extra for the overage. The overage is usually measured with 95th percentile billing: usage is sampled every five minutes, the top 5% of samples are thrown away, and you are billed on the highest sample left. Short spikes cost nothing. Busy stretches that last more than about a day and a half in a month do.
Most office internet is sold at a flat monthly rate. Burstable billing shows up mainly on IP transit, larger dedicated circuits and services delivered in data centers, where traffic swings a lot and buyers want room to grow without paying for all of it every month. It is a fair model once you understand it, and an expensive surprise when you do not.
Three numbers on a burstable quote
| Term | What it means | Example wording |
|---|---|---|
| Port size | The physical or provisioned ceiling. You can never use more than this. | 1 Gbps port, 10 Gbps port |
| Commit | The bandwidth you pay for every month whether you use it or not. | 200 Mbps commit |
| Burst (overage) rate | The price per Mbps for billed usage above the commit. | A stated price per Mbps above commit |
A quote might read "1 Gbps port, 200 Mbps commit, burstable to line rate." You pay for 200 Mbps each month. If your billed usage comes in at 350 Mbps, you pay for the 200 at the commit rate and the extra 150 at the burst rate. The burst rate is often higher per Mbps than the commit rate, which is how the carrier is paid for keeping capacity free for you. Some carriers price burst at the same rate as the commit. It is worth asking, because it changes how high you should commit.
How 95th percentile billing is calculated
- Sampling. The carrier reads the traffic counters on your port at a fixed interval, most commonly every five minutes, and works out the average rate for each interval.
- Sorting. At the end of the billing month, all the samples are ranked from highest to lowest.
- Discarding. The top 5% of samples are removed.
- Billing. The highest remaining sample becomes your billed rate for the month.
The arithmetic is worth doing once. A 30-day month has 8,640 five-minute samples. Five percent of that is 432 samples, which is 36 hours. So in a 30-day month, your busiest 36 hours of traffic are ignored for billing, however high they go. Hour 37 is the one you pay for. A 31-day month gives you a little over 37 hours.
Inbound, outbound or both
Traffic flows in both directions, and contracts differ on which one counts. A common approach is to calculate the 95th percentile for inbound and outbound separately and bill on whichever is higher. Some contracts add the two together, and some bill each direction on its own. For an office, inbound is usually larger; for a hosting company or a video platform, outbound usually is. The wording changes the bill, so find it in the contract rather than the proposal.
What a sample actually measures
Each sample is an average over its five minutes. A ten-second burst at line rate barely moves the five-minute figure. That is why short spikes from a software update or a single large download rarely show up in a 95th percentile bill. Long, steady transfers do.
A worked example
Hypothetical figures. The prices are invented round numbers for arithmetic, not market rates or a FiberX quote.
A small hosting business buys a 1 Gbps port with a 300 Mbps commit. For the arithmetic, assume committed bandwidth costs $1.00 per Mbps per month and burst costs $1.50 per Mbps. The commit alone is $300 a month.
- A normal month. Traffic peaks around 410 Mbps most afternoons, and the 95th percentile lands at 410. Billed burst is 110 Mbps at $1.50, so $165, plus the $300 commit: $465.
- A 30-hour spike. A client launch pushes traffic to 800 Mbps for 30 hours. That is 360 samples, fewer than the 432 that get discarded, so the spike drops out of billing. The 95th percentile is still about 410, and the bill is still about $465.
- A 72-hour spike. The same launch runs for three days. Now 864 samples sit near 800 Mbps, more than the 432 discarded, so the 95th percentile lands near 800. Billed burst is about 500 Mbps at $1.50, so $750, plus the commit: about $1,050.
The same peak, held for a day and a half longer, more than doubles the bill. That is the most useful single thing to understand about 95th percentile billing: duration matters more than height.
Burstable, flat rate and usage-based compared
| Model | How you pay | Suits | Watch for |
|---|---|---|---|
| Flat rate | One price for the full bandwidth, used or not | Offices and most single-site businesses with steady patterns | Paying for headroom you rarely touch |
| Burstable (95th percentile) | Commit, plus overage on the billed rate | ISPs, hosting, media and data center traffic that varies by day | Long busy periods, steep burst rates, which direction is billed |
| Usage-based (data transferred) | A price per gigabyte or terabyte moved | Cloud egress and some content delivery services | Costs that grow with every byte, including backups and replication |
For a typical office, a flat-rate dedicated internet circuit is simpler and usually the right buy. Burstable billing earns its place when traffic genuinely swings, or when you want a large port for growth without paying for all of it from the first month.
Choosing a commit
The aim is a commit close to your normal 95th percentile, so you are not paying every month for capacity you never use, and not paying burst rates every month either.
- Use your own data. If you have six to twelve months of traffic graphs, calculate the 95th percentile for each month. Most routers, firewalls and monitoring tools can export the samples.
- Commit near the typical month, not the worst one. If most months sit around 400 and one sat at 700, a commit near 400 usually wins. The occasional heavy month costs some burst; the other eleven do not carry extra commit.
- Compare commit and burst rates together. A low commit rate with a steep burst rate can cost more than a higher commit with a gentle one, depending on how often you go over.
- Check whether the commit can change mid-term. Raising it is usually easy. Lowering it often is not.
Keep your own graphs
The carrier's usage report is the evidence behind the bill, and it is worth checking against your own. Ask how the carrier samples, at what interval, and whether you can see the per-sample data rather than just the final figure. Then graph the port yourself from your router or monitoring system, at the same interval. Small differences caused by sampling times are normal. Large ones are worth raising before you pay.
Also watch for traffic you did not plan. An offsite backup moved into working hours, a misconfigured replication job or a compromised device sending traffic out can sit at high usage for days and land squarely in the billed range. Attack traffic is a particular risk on burstable ports, since it counts as usage unless the contract says otherwise; our guide to DDoS protection covers the options for keeping it off your port.
Aggregated and multi-port billing
Businesses with several ports from the same carrier, for example in two different colocation facilities, can sometimes have usage combined for billing. Whether the carrier sums the samples first and then takes the 95th percentile, or takes each port's 95th percentile and adds them, makes a real difference. Summing first is usually cheaper for you, because peaks on different ports rarely line up exactly. Ask which method the contract uses before you assume the cheaper one.
If you run your own IP space with BGP across two carriers, you also have some control over how traffic splits between them, which in turn shapes each carrier's 95th percentile. Our article on static IPs and BGP explains the basics.
Questions to ask your provider
- What are the port size, the commit and the burst rate per Mbps?
- What sampling interval do you use, and do you bill on inbound, outbound, the higher of the two, or the sum?
- Can I see the per-sample usage data behind each bill?
- Can I raise or lower the commit during the term, and with how much notice?
- Is burst capped below the port size, or can I use the full port at any time?
- How is traffic from a denial-of-service attack treated for billing?
- If I have more than one port, is usage aggregated, and by which method?
Get burstable quotes compared properly
Burstable quotes are hard to compare by eye, because a cheaper commit rate can hide a steeper burst rate. When carriers bid on your port, we line up port size, commit, burst rate and billing method, and run your own usage through each one so you see the likely monthly cost rather than the headline. Carriers pay FiberX, not you, so nothing is marked up, and the lowest-price promise on our pricing page applies: bring a lower qualified quote for the same service and we beat it. Reach Phil Morales through the contact page, call 478-758-8091 or text (347) 870-0965. If your current bill carries a usage line you cannot explain, how to read a business internet bill is a good place to start.
