LoRa airtime calculator.

How do I calculate LoRa time on air?

Semtech's formula is: symbol time Tsym = 2^SF / BW; preamble time (Npreamble + 4.25) x Tsym; and payload symbols 8 + max(ceil((8 PL - 4 SF + 28 + 16 CRC - 20 H) / (4 (SF - 2 DE))) x (CR + 4), 0), where PL is the payload in bytes, CRC is 1 when the payload CRC is on, H is 1 with no header, CR is 1 to 4 for coding rate 4/5 to 4/8, and DE is 1 when low data rate optimisation is on (needed when the symbol lasts more than 16 ms). Time on air is the preamble plus the payload symbols times Tsym.

Worked example. A 20 byte payload at SF7, 125 kHz, 4/5, 8 preamble symbols, explicit header and CRC takes 56.6 ms. The same packet at SF12 takes 1318.9 ms, about 23 times longer, because each step up in spreading factor doubles the symbol time.

For LoRaWAN, the PHY payload is the application data plus the 13 byte MAC overhead for an uplink without options, so enter the full PHY payload size. The duty cycle you enter is your own assumption: the allowed percentage varies by region and sub-band, so look up your regulator's current rule. The calculation was cross-checked against published worked results (SF12, 125 kHz, 64 bytes = 2793.472 ms).