LoRa Airtime Calculator
General Guides
Every LoRa message spends a set amount of time transmitting, and that time drives your legal limits, how much your node congests the network, and your battery life. Set your parameters below to see it.
0 bytes of text. This counts the message text only; the real LoRa payload also carries protocol and encryption overhead, so treat this as a floor. Fills in the payload field below.
Sets which legal limit applies where you operate. Regions differ in kind, not just number.
How slowly and carefully the radio talks. Higher reaches further, but each step roughly doubles airtime.
Wider is faster but shorter range. 125 kHz is the common default.
How much error correction is added. More correction is more robust but slower.
Size of your actual message payload in bytes. Auto-filled when you type a message above; edit directly if you know the exact size.
Sync symbols sent before the data. 8 is standard.
On adds a header describing the packet. Leave on unless you know you need implicit mode.
Stabilizes slow high-SF transmissions. Auto enables it when needed (SF11 or SF12 at 125 kHz).
Adds an error-check to the payload. Normally on.
What time on air means
Time on air is how long the radio is physically transmitting for a single message. During those milliseconds the radio is powered up and putting a signal into the antenna, and nothing else on the channel can be heard while it does.
The surprise: spreading factor roughly doubles airtime each step
Spreading factor is the big lever, and it is not gentle. Each step up (SF7 to SF8 to SF9 and on) roughly doubles the airtime, because the radio spreads the same data over more time to be heard further. The same tiny 20 byte message runs about 57 ms at SF7 and over a second at SF12. Most people expect a small bump and get a 20x one. Change the spreading factor above and watch the headline number.
Why it matters
Legal. Some regions cap how much of each hour you may transmit (a duty cycle). Others cap the length of a single transmission (dwell time), or cap power instead. Longer airtime eats a duty-cycle budget faster and can bump a per-transmission limit. Which rule applies depends entirely on where you are, which is what the region selector above switches between. See Frequencies and Regions.
Network. A mesh is a shared channel where one node talks at a time, like a group sharing a single walkie-talkie frequency. Every millisecond you transmit is a millisecond nobody else can. Longer airtime means more congestion, more collisions, and more dropped messages when the mesh is busy.
Battery. Transmitting is the single biggest power draw a node has. Longer airtime per message means more battery spent per message, which adds up quickly on a node sending regular position or telemetry.
Airtime is the one number that ties those three together, which is why it is worth a look before you crank the spreading factor for more range.