Plan live event streaming capacity from one number: peak concurrent viewers multiplied by the audience-weighted bitrate of your ladder, plus 50% headroom. A 400,000-viewer event on a typical 1080p ladder blends to about 3.9 Mbps per viewer. That means the CDN must sustain roughly 1.55 Tbps at peak, and you should reserve about 2.3 Tbps. Building the plan below takes about half a day with your last event's analytics open. The load test adds roughly one more day.
For HLS and DASH live streams in 2026, peak CDN egress equals peak concurrent viewers times the blended bitrate of the bitrate ladder. At 400,000 concurrent viewers and a 3.875 Mbps blended ladder, that is 1.55 Tbps. A 3-hour event at 60% average concurrency delivers about 1,256 TB.
Forecast peak concurrent viewers as expected unique viewers multiplied by a peak concurrency ratio, then plan to the P90 forecast rather than the median. Unique viewers come from registrations, ticket sales or past events. For multi-hour events the ratio typically lands between 20% and 40% (our estimate), with short keynotes and finals at the high end.
Before you start, collect these inputs:
Step 1: build two forecasts. For the worked example, 1.6 million expected unique viewers at a 25% concurrency ratio gives a P50 of 400,000. Use a P90 that reflects how wrong your last forecast was. Why: capacity is bought against the tail, and launch-day surprises go up far more often than down.
Model the join surge separately. Between 40% and 60% of peak audience can arrive in the first 5 to 10 minutes (estimate). That surge hits multivariant playlists, DRM license servers and authentication endpoints far harder than it hits segment delivery.
Step 2: compute the blended bitrate from rendition share, not from the top rung. Why: a 6 Mbps top rung does not mean 6 Mbps per viewer. Mobile and constrained networks pull the average down, and the average is what your egress bill and your capacity reservation follow.
| Rung | Video bitrate | Assumed viewer share | Contribution to blend |
|---|---|---|---|
| 1080p60 | 6.0 Mbps | 30% | 1.800 Mbps |
| 1080p30 | 4.5 Mbps | 25% | 1.125 Mbps |
| 720p30 | 3.0 Mbps | 20% | 0.600 Mbps |
| 540p30 | 1.8 Mbps | 15% | 0.270 Mbps |
| 360p30 | 0.8 Mbps | 10% | 0.080 Mbps |
| Blended | 100% | 3.875 Mbps |
Even with 30% of viewers on the top rung, the blended bitrate is about 65% of the top-rung bitrate, so sizing capacity from the top rung overbuys by roughly half.
Add audio and container overhead (fMP4 or MPEG-TS, plus HTTP and TLS framing) on top of video bitrate. A planning allowance of 5% to 8% is a reasonable estimate. If your shares come from a past event, rerun them per device class, because a connected-TV-heavy audience shifts the blend upward.
Step 3: turn the forecast into the four numbers your CDN providers and origin team need. Why: providers reserve capacity in Gbps per region, origin teams size for requests per second, and finance budgets in TB. A plan that gives only one of the three leaves two teams guessing.
| Metric | Arithmetic | Result |
|---|---|---|
| Peak edge egress | 400,000 viewers times 3.875 Mbps | 1.55 Tbps |
| Capacity to reserve | 1.55 Tbps times 1.5 headroom | about 2.33 Tbps |
| Edge segment requests | 400,000 viewers divided by 4 s segments | 100,000 per second, plus a similar playlist rate |
| Origin requests, shielded and coalesced | 5 renditions times 2 objects times 3 shield nodes, divided by 4 s | about 7.5 per second, independent of audience size |
| Event volume | 240,000 average viewers times 3.875 Mbps times 10,800 s, divided by 8 | about 1,256 TB |
With origin shield and request coalescing working, origin load depends on renditions and shield count rather than audience, so a 10x audience surprise lands on the CDN, not on your packager.
That origin row is the most useful number in the plan, because it shows what must never break. If origin requests grow with viewers during the load test, coalescing or the cache key is wrong. LL-HLS changes the edge picture: 1 s partial segments plus blocking playlist reloads raise edge request rates by roughly 4x. Edge capacity in requests per second then matters as much as Gbps.
Step 4: run two complete contribution chains that produce interchangeable segments. Why: a backup that emits different segment numbers or timestamps forces players to rebuffer or reload during failover, which viewers experience as an outage.
Step 5: send each CDN your per-region forecast 2 to 4 weeks ahead, then set cache rules so the edge absorbs the join surge. Why: confirmed regional capacity prevents silent saturation in one region, and correct playlist and segment TTLs keep the origin flat while 400,000 players reload every few seconds.
| Object | Edge TTL | Cache key rule | Why |
|---|---|---|---|
| Media playlists (m3u8, live MPD) | Half the target duration, 1 to 2 s | Path only, auth token excluded | Fresh enough for players, cacheable enough for coalescing |
| Media segments and parts | 24 hours or longer | Path only, auth token excluded | Segment names are immutable once published |
| Init segments | 24 hours or longer | Path only | Requested by every joining player during the surge |
| Multivariant playlist | 30 to 60 s | Path only | Short enough to change hosts during an incident |
Excluding per-viewer tokens from the cache key is the single rule that decides whether 400,000 viewers produce 7.5 or 100,000 origin requests per second.
Validate tokens at the edge, but never let them fragment the cache. For multi-CDN, set weighted DNS so each provider receives no more than its confirmed regional capacity divided by your headroom factor. BlazingCDN can run as that second provider through multi-CDN integration with a weighted DNS split, with origin shield and request coalescing included on every plan. It delivers the packaged HLS, LL-HLS and DASH segments, but it does not accept RTMP or RTSP ingest, so contribution stays on your own packagers.
Step 6: load test at 10% to 20% of peak with synthetic players in every major region, running against the production CDN hostnames. Separately, hit the origin at twice the expected shielded rate with coalescing bypassed, so you know the worst case if a rule fails.
Watch these during the load test and the first ten minutes of the real event:
For a manual check, request the same segment twice from two regions and read the cache status response header. The second request in each region should report a hit.
Decide your degradation order before the event, with the egress effect of each lever already calculated.
| Lever | Effect at 400,000 viewers | Viewer cost |
|---|---|---|
| Shift DNS weight to the second CDN | Moves load and leaves total egress unchanged | None if the second CDN has headroom |
| Remove the 1080p60 rung | 30% share drops to 4.5 Mbps: minus 0.45 Mbps per viewer, about 180 Gbps (11.6%) | Lower motion fidelity |
| Fall back from LL-HLS to standard HLS | Edge request rate falls by roughly 4x | Several seconds of added latency |
Shifting traffic between CDNs costs viewers nothing, so it comes first; ladder trimming is the lever that actually reduces total egress.
Headroom is the expensive trade-off. Reserving 2.33 Tbps for a 1.55 Tbps forecast buys insurance against a P90 miss. Commit-based contracts charge for that insurance even when the audience falls short. Metered billing with confirmed burst capacity shifts the risk back to usage.
Shorter segments cut latency but multiply requests and shrink the time a shield has to coalesce a miss. Multi-CDN removes a single point of failure, but it doubles the cache rules, token schemes and dashboards you must keep identical. After the event, compare actual rendition share against the table above and feed the corrected blend into the next forecast.
Multiply peak concurrent viewers by the blended bitrate of your ladder, then add about 50% headroom. For 400,000 viewers at a 3.875 Mbps blend, that means 1.55 Tbps sustained and roughly 2.3 Tbps reserved. Take the blended bitrate from rendition share in player analytics, not from the top rung, or you will overbuy by about half.
Multiply expected unique viewers by a peak concurrency ratio, typically 20% to 40% for multi-hour events (estimate). Base the ratio on per-minute curves from comparable past events. Build P50 and P90 forecasts, plan capacity to P90, and model the join surge separately, because many viewers can arrive within the first ten minutes.
Yes. LL-HLS barely changes bandwidth but multiplies edge requests, roughly 4x with 1 s partial segments and blocking playlist reloads. Plan edge capacity in requests per second as well as Gbps. Keep a tested fallback to standard HLS as a brownout lever, accepting several seconds of extra latency in exchange for lower request load.
Segment hit ratio at the edge should reach 98% or higher within the first minute of a live event. Live segments are immutable and requested by thousands of viewers within seconds. A lower ratio usually means per-viewer tokens or query strings are fragmenting the cache key, or request coalescing is missing at the shield tier.
This week, pull rendition share and the per-minute concurrency curve from your last event. Compute your real blended bitrate and the origin requests-per-second formula above. Then run a 10% synthetic load test while watching whether origin requests stay flat as players join. If they climb, fix the cache key before you negotiate any capacity, because no reservation survives a shield that does not coalesce.
If your packagers already produce HLS, LL-HLS or DASH segments, the BlazingCDN HLS Streaming CDN page describes how it delivers them, either as a primary CDN or as a weighted second provider for peak audiences.