# Add values from MQTT
**URL:** https://discourse.nodered.org/t/add-values-from-mqtt/12974
**Category:** General
**Created:** [6 July 2019 08:42 UTC](https://discourse.nodered.org/t/add-values-from-mqtt/12974 "2019-07-06T08:42:54Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@crd](https://discourse.nodered.org/u/crd)
#### Post date: [6 July 2019 08:42 UTC](https://discourse.nodered.org/t/add-values-from-mqtt/12974/1 "2019-07-06T08:42:54Z")
Hi,
I want to do some calculations with the values i receive from MQTT.
The "convert functions" converts the string, coming from the MQTT, to a number.
msg.payload = Number(msg.payload);
return msg;
What i want to do is : subtract the number that is coming from MQTT "rem/rechts" with the number that is comming from MQTT "rem/links" this result must be devided by MQTT "rem/links" and multiplied by 100 =\> the result is the difference between MQTT "rem/rechts" and "rem/links" in %.
thanks for your help 😉
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### Author:  [@knolleary](https://discourse.nodered.org/u/knolleary)
#### Post date: [6 July 2019 08:52 UTC](https://discourse.nodered.org/t/add-values-from-mqtt/12974/2 "2019-07-06T08:52:02Z")
To do that calculation you need both values in a single message that you then pass to your Function node. This recipe in the cookbook shows how you can do that: [https://cookbook.nodered.org/basic/join-streams](https://cookbook.nodered.org/basic/join-streams)
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### Author:  [@crd](https://discourse.nodered.org/u/crd)
#### Post date: [23 July 2019 10:08 UTC](https://discourse.nodered.org/t/add-values-from-mqtt/12974/3 "2019-07-23T10:08:48Z")
Thx knolleary for your answer, ...
I tried your suggestion to merge (join) both values, but i could't manage to get any message out of the function.
I did found a solution =\> I used variables instead to store the values.
Thanks and greets 😉