Some of the questions I got after coming from Iran was: What do women have to wear in Iran? Basically women should dress in accordance with the Islamic dress code that is required to be worn by all females in Iran. I wear the Islamic dress regardless, so it was not too hard to follow the rules. I wore the same outfits I usually wear in London. My Iran visit was during September, so the weather was still warm, so I packed light outfits. Keep in mind that it starts getting cold after that and they have a proper winter, so make sure you pack according to the month you are visiting. Find the weather of your destination here.
The Islamic Dress Code for females and males in Iran:
The Islamic dress code for females in Iran means that the body and the hair need to be covered with loose clothing. Only the face and the hands are allowed. Sometimes the feet are fine, but some of the religious places I visited asked me to wear socks too, so keep some in case! Many Iranian women put the headscarf loosely on the hair and showing the front. That is acceptable in big cities and the capital. You will see it all over Tehran. Iranian fashion is also growing and there are so many creative manteau and scarves covered in traditional designs and calligraphy.
Males in Iran also need to keep the Islamic dress code in mind. T-shirts and shirts are commonly worn. They are usually matched with trousers or jeans. Shorts are not common and if a man plans to wear some, I would advice that it's below the knees.
You might also be interested in reading this: Travel Blogger in a headscarf Q&A
Iranian traditional Clothes:
There are two important components to the outfit of Iranian women: Manteau and Chador. The Mento dress is like a trench light coat that is buttoned in the front and comes down to the knees. The Chador is the loose fabric that is placed on the head to cover the full shape of the women. Mentos and trousers are the most common. Chadors are mostly worn in religious cities or by religious females. I would recommend darker colours when visiting religious cities like Qom & Mashhad because most people either wear black or darker outfits with a black scarf called maqnah. Though, colourful head scarves are accepted too.