Clothing for Madrid in January

You can expect high temperatures in the 50's and lows in the 30's at night.
Jeans are fine for men also.

Jeans are heavy and take up a lot of space and depending on where you are from, you may be on the warm side on some days. Maybe throw a pair of lighter weight chinos in the mix? Might want a short sleeved short. I was in Barcelona at the same time, and we actually got a little warm walking around on the streets when it was high 50s/low 60s. And especially at this time of year, layers are nice because it can be warm in museums.

Madrid is likely to be pretty cool at that time of year; it's not as warm as Barcelona in the winter.

Go to wunderground.com, enter Madrid, click on History > Monthly, and choose January 2018 from the pull-down boxes. You'll have actual day-by-day weather data. Do the same for January 2017, January 2016, etc. That will give you a good idea of the range of temperatures to expect. Don't forget that most of the time you're outdoors, the temperature will be below the reported daily high. Also, Spaniards eat dinner extremely late, so you may be walking back to your hotel at midnight.

" Also, Spaniards eat dinner extremely late, so you may be walking back to your hotel at midnight."

This is a great point. In many cities, the temperature at 11 PM or midnight is only relevant to those seeking "nightlife"; in Madrid, it's relevant to those eating dinner.

As for exactly what clothes to bring, a lot depends on what you're used to. If you're from Hawaii, you'll need much heavier clothes than if you're from Minnesota. So, do look at the actual weather for the past few years for your dates, and then check again just a few days before your trip, and adjust your wardrobe accordingly.

Always best to go to the horse's mouth...
http://www.aemet.es/en/eltiempo/prediccion/municipios/madrid-id28079

and if you're a bit nerdy with stats...
http://www.aemet.es/en/serviciosclimaticos/datosclimatologicos/valoresclimatologicos?l=3195&k=mad

Enjoy!