About this tool
LoRa reaches long distances because it transmits very slowly. How long a packet occupies the channel – its time on air or airtime – depends mainly on the spreading factor, the bandwidth and the packet size. The LoRa airtime calculator uses the formula from the Semtech data sheet, for plain LoRa, LoRaWAN (+13 bytes header) and Meshtastic (+16 bytes header).
In the EU 868 MHz band devices may transmit only 1 % of the time in most sub-bands. The calculator shows how many messages per hour this allows and the minimum interval between them, and for LoRaWAN also the fair use limit of The Things Network (30 s of airtime per day). A table compares all spreading factors with airtime and estimated sensitivity.
The advanced parameters let you set the coding rate, preamble, duty cycle, header, CRC and low data rate optimisation and choose any bandwidth from 7.8 to 500 kHz. Use the estimated sensitivity in the Link Budget Calculator to estimate the range. Everything is calculated in your browser.
How it works
- Choose the protocol, spreading factor and bandwidth.
- Enter the size of the application payload in bytes – the calculator adds the protocol header itself.
- Read time on air, messages allowed per hour and minimum interval; turn on the advanced parameters for special cases.
Frequently asked questions
How is LoRa airtime calculated?
One symbol lasts 2^SF / bandwidth, so 1.024 ms at SF7 and 125 kHz and 32.8 ms at SF12. The preamble takes (preamble length + 4.25) symbols, followed by 8 symbols and further blocks depending on payload, coding rate, header and CRC. 10 bytes of LoRaWAN payload (23 bytes on air) thus take about 62 ms at SF7 and almost 1.5 s at SF12.
How many messages may I send per hour?
With a 1 % duty cycle a device may transmit 36 s per hour. At 62 ms per message that is 583 messages per hour, at SF12 with 1.48 s only 24 – at most one every 2.5 minutes. The Things Network’s fair use policy additionally allows only 30 s of airtime per day and device.
What does a higher spreading factor give me?
Each SF step doubles the symbol time and improves the sensitivity by about 2.5 dB. At 125 kHz SF12 hears signals down to about −137 dBm, SF7 only to about −124.5 dBm. The price is airtime: packets take up to 25 times longer, occupy the channel longer and use more energy.
What is the low data rate optimisation?
With very long symbols (over 16 ms) the transmission becomes sensitive to frequency drift. The low data rate optimisation then uses fewer bits per symbol, which lengthens the airtime a little. LoRaWAN turns it on for SF11 and SF12 at 125 kHz; the calculator does this automatically, and the advanced parameters let you force it on or off.