LoRa Airtime & Duty-Cycle Calculator

Quick answer

LoRa "airtime" (Time-on-Air) is how long a single packet occupies the radio channel. It is set by the spreading factor, bandwidth, coding rate and payload size — not by distance. This calculator uses the exact Semtech AN1200.13 Time-on-Air formula to give you the airtime per packet in milliseconds, the effective bitrate, and — critically for mesh planning — how many packets per hour you can legally send under your region's duty-cycle cap. On a shared Meshtastic channel, airtime is the scarce resource: slow long-range presets burn it fast.

On Long Fast (SF11/250 kHz) a 16-byte packet takes about 354 ms on air. Under an EU868 1% duty cap that is only ~101 packets/hour from one node — so keep payloads small and node counts modest, or the channel saturates.

Meshtastic preset Spreading factor (SF) Bandwidth Coding rate Payload (bytes) Region / duty-cycle cap Mesh nodes (N) Msgs / node / hour

Time-on-Air / packet—

Symbol time (Tsym)—

Effective bitrate—

Max packets/hour (duty cap)—

Mesh channel utilization (estimate)—

Mesh airtime / hour (estimate)—

Formula & constants (Semtech AN1200.13)

Tsym        = 2^SF / BW                      (s; BW in Hz)
Tpreamble   = (n_preamble + 4.25) · Tsym     (n_preamble = 16, Meshtastic)
payloadSymb = 8 + max( ceil( (8·PL − 4·SF + 28 + 16·CRC − 20·IH)
                              / (4·(SF − 2·DE)) ) · (CR+4), 0 )
ToA         = Tpreamble + payloadSymb · Tsym

PL  = payload bytes        CRC = 1 (Meshtastic CRC on)
IH  = 0 (explicit header)  CR  = 1..4 for 4/5..4/8
DE  = low-data-rate-optimize = 1 for SF11/SF12 @ BW125 (Tsym ≥ 16 ms), else 0

Reference check — Long Fast (SF11, 250 kHz, 4/5, 16-byte payload):
  Tsym = 8.192 ms · payload symbols = 23 · ToA = 354.304 ms

Source: Semtech SX127x / AN1200.13 LoRa Modem Designer's Guide and the Meshtastic radio-settings docs (CC BY 4.0). The N-node mesh utilization figure is a planning estimate assuming a 3-hop rebroadcast flood, not a guarantee; real airtime depends on hop limit, retransmissions, ACKs and topology.

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Last reviewed June 23, 2026.