I just bought a new soldering iron and it has a digital display and is adjustable......it adjusts and shows everything in degree's F. What is the formula to convert this to watts? Everything I read about what temps to use to solder different things says to use 40watts, 60watts and so on.

If you look at your instructions it will give you the current draw in watts for your iron....the temp. reading is unrelated.

I could be wrong but I don't believe you can.

Dan.

It gave me a max amp draw of 100 watts and is adjustable from 250 to 850 degrees. So would that be adjustable from 25 to 100 watts. If you are useing a 40 watt iron what temp is it working at? If I had my old iron I would just turn it on adjust the watts and use my temp gun to get an idea.

Watts have nothing to do with temperature. A 100W iron can work at any temperature. The power tells you how fast it gets to the temperature and how big things you can solder.

Wattage can affect temperature but there is no real relationship.

Watts is power
Amps is current

It takes more power to bring a larger soldering head up to a specific temp.

Ryan

Thanks for the help guys. I have little soldering experience and have only seen different wattages, I've never had one that allowed me to adjust temp in exact degrees. If I'm doing wire to battery soldering..what temp should I use. If I'm soldering motor to esc what temp should I use?

A solder iron with adjustable temp is really not needed. The good ones are really expensive too. A cheap one from radio shack works just fine for day to day use.
With regular lead/tin solder you should use somewhere around 600F or a bit above.

For soldering wire or anything where current passes through, use Rosin Core Solder Only.

Ryan

Thanks again guys. I know this isn't the best iron out there but, fgured it would do what I need it to do, which is basic rc wiring.

Converting watts to temperature (deg C) is a thermodynamics problem, it can be done, but you are not giving enough information to solve the problem. You need to know the power dissipation per degree (temp) of your iron. Once you know that, you can (in most cases) assume a linear relationship in the absence of convection cooling or forced air cooling.

The temperature is based more on the type of solder being used than anything else. Use at least 100-200F over the melting point of your solder. At least 600F for lead-based solder, higher if it is RoHS-compliant (lead-free). Wattage is based more on what you are soldering (how much metal), etc. For dean's plugs, 60W is more is best.