802.11 Airtime Calculator

How long a frame actually occupies the medium. Frame counts and airtime answer different questions.

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Transmit time
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PHY rate
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Max frames / sec
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Airtime at that rate

Where the airtime goes (one exchange)

The same frame at every rate

PHY / rateTXTIMEvs fastest

Why this matters. A management frame sent at a low basic rate occupies the medium for many times longer than the same frame at a high rate. Count frames and you conclude one thing; measure airtime and you conclude another. On a real capture it is entirely normal for management frames to be a modest share of the frame count and the overwhelming share of the airtime, in which case the fix is the basic rate set, not the thing the frame count pointed you at.

How this is calculated

DSSS / HR-DSSS IEEE 802.11-2024 16.3.4
TXTIME = Preamble + PLCPHeader + ceil(8 × LENGTH / RATE)
Long preamble is 144 µs sync plus a 48 µs header at 1 Mb/s. Short preamble is 72 µs sync plus a 24 µs header at 2 Mb/s.

OFDM 17.4.3
TXTIME = 16 + 4 + 4 × ceil((16 + 8 × LENGTH + 6) / N_DBPS) + SignalExtension
Signal extension is 6 µs for ERP-OFDM in 2.4 GHz and 0 in 5 GHz.

HT and VHT 19.3.4 · 21.4.3
Preamble is the legacy portion plus the HT or VHT training fields, then N_SYM × T_SYM data symbols. N_DBPS = N_SD × N_BPSCS × R × N_SS, with N_SD 52, 108, 234 and 468 subcarriers at 20, 40, 80 and 160 MHz.

HE and EHT Clause 27 · Clause 36
Single user. The data symbol is 12.8 + GI with GI 0.8, 1.6 or 3.2 µs. Preamble is the legacy portion plus RL-SIG, HE-SIG-A (HE) or U-SIG and EHT-SIG (EHT), the STF, and N_LTF HE/EHT-LTF symbols whose length follows the 1×, 2× or 4× setting. N_DBPS = N_SD × N_BPSCS × R × N_SS, with N_SD 234, 468, 980, 1960 and 3920 at 20, 40, 80, 160 and 320 MHz. HE adds 1024-QAM (MCS 10-11); EHT adds 4096-QAM (MCS 12-13) and 320 MHz.

Scope: HE and EHT are single user only. OFDMA and MU-MIMO are not modelled, because airtime per user under a shared resource unit or spatial stream is a scheduling outcome, not a single number, so a value would be a guess. Packet Extension and the 2.4 GHz signal extension are excluded. Every PHY is implemented from the printed formulas and the values match the published rate tables.