tools / RF & SDR
LoRa Airtime (Time on Air) Calculator
Set the spreading factor, bandwidth, coding rate and payload and get the exact time on air, straight from the Semtech formula. It also shows how many messages an hour a 1% duty cycle leaves you, which is the number that quietly kills naive LoRaWAN designs.
Low data rate optimisation is off for these settings.
How it works
Symbol time
One symbol lasts 2^SF / BW seconds, so SF12 at 125 kHz is a slow 32.8 ms per symbol. Low-data-rate optimisation kicks in automatically when a symbol runs past 16 ms (SF11 and SF12 at 125 kHz).
Packet time
Airtime is the preamble plus the payload symbols, where the payload symbol count depends on SF, coding rate, header mode, CRC and the low-data-rate flag. Longer payloads and higher SF stack up fast.
FAQ
Why does duty cycle matter?
Sub-GHz bands cap how long you can transmit, commonly 1% (36 seconds per hour). An SF12 packet near 1.5 seconds means only about 20 to 30 messages an hour before you are out of legal airtime.
Which settings cut airtime the most?
Lower the spreading factor, shrink the payload, and widen the bandwidth. Each drops the time on air, at the cost of range or robustness.
