# Time On Air calculation

**URL:** <https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728>\
**Category:** Uncategorized\
**Created:** [August 10, 2018, 12:24pm UTC](https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728 "2018-08-10T12:24:09Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![bantob](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/bantob/32/15971_2.png) [@bantob](https://www.thethingsnetwork.org/forum/u/bantob)\
**Post date:** [August 10, 2018, 12:24pm UTC](https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728/1 "2018-08-10T12:24:09Z")

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Hi,  
do you know where i can find an explanation for the calculation of Time On Air?

thanks

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**Author:** ![arjanvanb](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/arjanvanb/32/46626_2.png) [@arjanvanb](https://www.thethingsnetwork.org/forum/u/arjanvanb)\
**Post date:** [August 10, 2018, 1:53pm UTC](https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728/2 "2018-08-10T13:53:45Z")

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See [Spreadsheet for LoRa airtime calculation](https://www.thethingsnetwork.org/forum/t/spreadsheet-for-lora-airtime-calculation/1190) and [Semtech’s documentation](https://www.semtech.com/uploads/documents/LoraDesignGuide_STD.pdf).

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**Author:** ![bantob](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/bantob/32/15971_2.png) [@bantob](https://www.thethingsnetwork.org/forum/u/bantob)\
**Post date:** [August 10, 2018, 3:20pm UTC](https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728/3 "2018-08-10T15:20:15Z")

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thanks lot.

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**Author:** ![bantob](https://www.thethingsnetwork.org/forum/user_avatar/www.thethingsnetwork.org/bantob/32/15971_2.png) [@bantob](https://www.thethingsnetwork.org/forum/u/bantob)\
**Post date:** [August 12, 2018, 7:29pm UTC](https://www.thethingsnetwork.org/forum/t/time-on-air-calculation/16728/4 "2018-08-12T19:29:14Z")

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According to Semtech formula:

Tsym = 2^SF / BW

Why is SF in the formula?  
Does SF (spreding factor) represent the number of changing frequency happening to send the symbol?  
Honestly i thought SF = 7,… 12 were just number agreed by the standard to distinguish the modes.

thanks.