// reference · mcs rate calculator

802.11 MCS Rate Calculator

Select standard, channel width, spatial streams, MCS index, and guard interval. Get the exact data rate, modulation type, minimum RSSI, and the full IEEE-spec calculation showing every step of the math.

— Shankar K. · Rates per IEEE 802.11-2020 / 802.11be-2024

Quick Device Preset

Standard

Channel Width

Spatial Streams

MCS Index

Guard Interval

// data rate

—

Mbps

// modulation

—

Coding rate —

// min rssi

—

dBm minimum

signal requirement

-90 dBm -40 dBm

// field note

// fec forward error correction

// how this rate is calculated - ieee 802.11 spec

// phy rate vs real throughput

PHY Rate (spec)

—

Realistic (70%)

—

Conservative (60%)

—

Under ideal RF conditions, actual throughput is typically 60-70% of the PHY rate due to Wi-Fi overhead: IFS spacing, MAC headers, ACK frames, random backoff, half-duplex constraint, rate adaptation cycles, and protocol retransmissions. Source: Candelatech Session 2c - PHY Data Rates and Throughput.

// why ax/be have more subcarriers per mhz

Wi-Fi 4 / Wi-Fi 5 (OFDM)

Subcarrier spacing: 312.5 kHz

20 MHz ÷ 312.5 kHz = 64 tones

52 usable data subcarriers

Wi-Fi 6 / Wi-Fi 7 (OFDMA)

Subcarrier spacing: 78.125 kHz

20 MHz ÷ 78.125 kHz = 256 tones

234 usable data subcarriers (4× more)

The 4× narrower subcarrier spacing in Wi-Fi 6/7 is why ax/be achieve ~20% higher rates than ac at the same channel width and stream count - more data subcarriers per MHz means more bits per symbol period, even before accounting for longer symbol duration (12.8 µs vs 3.2 µs). When OFDMA is used, the 234 subcarriers in a 20 MHz channel are divided into Resource Units (RU) of 26, 52, 106, 242, 484, or 996 tones - each RU assigned to a different client. Per-device rate is then calculated on RU subcarriers, not full-channel subcarriers.

// all mcs - 

MCS	Modulation	Code	Data Rate	Min RSSI

// how to read and analyse an mcs table

// the 5 columns - what each one means

MCS

Index Number

A single number (0–13 for Wi-Fi 7) that summarises the entire modulation + coding combination. Higher = faster but needs stronger signal. Think of it as a gear number.

MOD

Modulation

How many bits per symbol. BPSK=1 bit, QPSK=2, 16-QAM=4, 64-QAM=6, 256-QAM=8, 1024-QAM=10, 4096-QAM=12. Higher QAM = more data per symbol but needs a much cleaner signal.

CODE

Coding Rate

The ratio of actual data bits to total transmitted bits. 3/4 means 3 data bits for every 4 sent - 1 bit is error correction overhead. Lower = more protection, lower rate. Higher = more efficiency, less protection.

RATE

Data Rate

The theoretical PHY rate in Mbps or Gbps. This is the layer 1 speed - before Wi-Fi overhead. Real application throughput is typically 60-70% of this value. Never quote this number to users as expected speed.

RSSI

Min Signal

The minimum received signal level (dBm) needed to sustain this MCS. If client RSSI drops below this threshold, the AP will rate-adapt down to a lower MCS automatically.

// 5-step pcap analysis workflow

1

Read the MCS index from the radiotap header

In Wireshark: radiotap.mcs.index (Wi-Fi 4), radiotap.vht.mcs.0 (Wi-Fi 5), radiotap.he.data_rate (Wi-Fi 6). Note the MCS index and number of spatial streams.

2

Look up the MCS index in this table

Use the selector above - set your standard, channel width, and spatial streams. Find the MCS row. Note the modulation and coding rate. A Samsung GS10 at 80 MHz ax, 2SS, MCS 11 = 1024-QAM 5/6, 0.8 GI = 1200.9 Mbps PHY rate (real throughput ~840 Mbps).

3

Compare RSSI to the minimum threshold

Check radiotap.dbm_antsignal vs the Min RSSI column. If RSSI is above the threshold but MCS is still low, the problem is interference, not distance. This is the most common misdiagnosis in Wi-Fi troubleshooting.

4

Check for MCS consistency across frames

A healthy link holds a stable MCS. Frequent drops from MCS 9 to MCS 3 and back = rate adaptation thrashing = marginal signal or interference. Occasional drops during initial association are normal. Sustained low MCS after association is not.

5

Translate PHY rate to expected throughput

Use the 60-70% rule from the throughput estimator above. If a user complains they are only getting 400 Mbps on a 600 Mbps PHY rate link - that is normal. If they are getting 150 Mbps on a 600 Mbps link - that is a real problem worth investigating (check retransmit rate, CRC errors, and channel utilisation).

// quick diagnosis - what is your mcs telling you?

High MCS + Good RSSI

Healthy link

Client is close, RF is clean, modulation is high. Expected behaviour. Design target for any well-deployed network.

Low MCS + Weak RSSI

Distance / coverage gap

Client is too far from AP. Fix: add an AP, adjust transmit power, or move the client. This is the expected cause of low MCS.

Low MCS + Strong RSSI

Interference - not distance

Most common misdiagnosis. Good signal but bad modulation = co-channel interference, hidden node, or high channel utilisation. Check the spectrum, not the coverage map.

MCS Thrashing

Marginal or unstable link

MCS jumping up and down across frames. Rate adaptation algorithm is struggling. Often caused by a client near the edge of two cells - roaming trigger not firing. Check 802.11k/v/r configuration.

MCS 0 / BPSK Only

Link is barely alive

BPSK 1/2 is the fallback of last resort - 1 bit per subcarrier, maximum error correction. If a client is stuck here, either coverage is critically poor or there is severe interference. Check for FCS errors alongside low MCS.

High MCS but slow app

Overhead or congestion issue

MCS looks great but user is complaining? Check actual throughput vs PHY rate (60-70% rule). If gap is larger - inspect retransmit rate, channel utilisation, packet size, and whether ARP/DHCP storms are consuming airtime.

// worked example - reading a real pcap

Device: Samsung Galaxy S10 connecting to an Aerohive AP630 at 80 MHz on Wi-Fi 6.
Wireshark shows: radiotap.he.data_rate = 1200.95 Mbps | RSSI: -55 dBm | NSS: 2

Step 1: match the rate

1200.9 Mbps at 80 MHz, 2SS → MCS 11

Step 2: read the row

1024-QAM · coding 5/6 · GI 0.8 µs

Step 3: check RSSI

-55 dBm vs min -51 dBm → within range

Step 4: expected throughput

1200.9 × 70% ≈ 840 Mbps realistic

Conclusion: this is a healthy, well-performing Wi-Fi 6 link. MCS 11 is the highest index for 802.11ax. The client is sustaining the peak modulation scheme with adequate signal. If the user reported poor performance at this MCS and RSSI, the investigation would move to channel utilisation and congestion - not RF.

// five rules every engineer should know

MCS is a quality grade, not a speed setting. The AP and client negotiate MCS dynamically based on current RF conditions. You cannot force a client to use MCS 11 - you can only improve the RF environment so the algorithm selects it.

Strong RSSI + low MCS = interference, every time. Do not add more APs. Check for co-channel interference, non-Wi-Fi interference sources, or a hidden node problem first.

Real throughput is 60-70% of PHY rate under ideal conditions. In a real deployment with multiple clients, contention, and retransmits, expect 50-60%. Never quote PHY rate as user-facing speed.

The weakest device sets the ceiling. If one client on the BSS is sending at MCS 0 (BPSK), it consumes much more airtime than a client at MCS 11 to send the same amount of data. Low-MCS legacy clients degrade performance for everyone sharing the channel.

MCS 9 is N/A at 20 MHz for 802.11ac. This is not a bug - 256-QAM 5/6 over 52 subcarriers does not produce an integer number of coded bits per OFDM symbol. The spec explicitly marks this combination as invalid. Any tool showing a rate for this combination is wrong.

// read mcs in wireshark

radiotap.mcs.index

MCS index - 802.11n frames

radiotap.vht.mcs.0

MCS index - 802.11ac frames

radiotap.he.data_rate

Data rate - 802.11ax frames

radiotap.dbm_antsignal

RSSI - compare to min above

See MCS per frame in a real PCAP

Upload a capture - WiFi Analyser decodes MCS index, spatial streams, RSSI, and guard interval per frame automatically.

try it free ↗

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reference

Wireshark display filter reference

SK

— Shankar K., Wi-Fi engineer, Irving TX
 Building WiFi Analyser V2 · CWNA-109 in progress · one post every two weeks 

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