WiFi Airtime & Capacity
Define the clients (PHY rate and demand) and I compute the airtime each one consumes, the cell total and whether it saturates. See why a slow client hogs the medium (airtime fairness).
Cell clients
Each row is a WiFi client. Set its PHY rate (Mbps, the one
negotiated with the AP) and its actual traffic demand (Mbps).
The airtime it consumes is demand / (PHY × efficiency).
Total cell airtime
—
Per-client breakdown
| Client | PHY | Demand | Airtime | Real throughput |
|---|
How it is computed · airtime, saturation and the slow client
1. For each client, the airtime (fraction of air time
it occupies) is airtime = demand(Mbps) / (PHY(Mbps) × efficiency).
Efficiency (MACeff) accounts for 802.11 overhead: preambles, ACKs, contention and
inter-frame spaces. Typically 0.65 on a/g/n, up to 0.75 on ax
thanks to OFDMA; in crowded 2.4 GHz it drops to ~0.5.
2. The cell's total airtime is the sum of all of them:
total = Σ airtime_i. The air is a shared resource, so only 100%
is available.
3. If total ≤ 100% the cell does not
saturate: every client gets its demand and some airtime is left free. If
total > 100% the cell saturates: there is not enough air and each
client's real throughput is scaled by 1/total (proportional to demand).
4. Slow-client effect: a client at a low PHY rate
consumes a lot of airtime even with modest demand. E.g. 5 Mbps at 6 Mbps PHY
→ 5/(6×0.65) = 128%, it alone saturates the cell. That client hogs the medium
and slows everyone down (which is why raising its rate or moving it off the channel helps).
5. Simplified model: it does not include retransmissions, real collisions, MU-MIMO or aggregation (A-MPDU). It is meant to show the airtime-fairness dynamics, not as an exact capacity figure.
Runs locally in your browser · no sign-up · nothing leaves your browser
How it works
The tool models a WiFi cell as what it really is: a shared medium where only 100% of the air time exists. You define each client with its negotiated PHY rate (Mbps) and its actual traffic demand (Mbps), plus an adjustable MAC efficiency (40–90%), and it computes the airtime fraction each one consumes, the cell total, whether the cell saturates and the real throughput left for every client.
The per-client formula is airtime = demand / (PHY × efficiency). MAC efficiency discounts the IEEE 802.11 overhead (preambles, ACKs, contention, interframe spaces): 65% is typical for a/g/n, up to 75% on 802.11ax thanks to OFDMA, and around 50% on congested 2.4 GHz. Total airtime is the sum over all clients; above 100% the cell is saturated and each client throughput is cut by 1/total. It is a simplified model (no retransmissions, MU-MIMO or A-MPDU aggregation), built to show the dynamics of airtime fairness.
Example: two fast clients and one slow camera (65% efficiency)
- Laptop at 433 Mbps PHY demanding 50 Mbps:
50 / (433 × 0.65) ≈ 17.8%of the air. Phone at 433 Mbps demanding 30 Mbps: 10.7%. - Legacy camera at 6 Mbps PHY asking for only 5 Mbps:
5 / (6 × 0.65) = 128.2%— it alone needs more air than exists. - Total airtime: 156.6% > 100% → saturated cell. Real throughput scales by 1/1.57: the laptop gets ~31.9 Mbps, the phone ~19.2 and the camera ~3.2.