Have question. Which airport is busier? Karachi or Islamabad?.
like other members answered your question
Karachi is busier. But it is not guaranteed that the Karachi Airport will remain busiest airport in Pakistan. There is no doubt Karachi is many times larger than Islamabad. But if you notice Islamabad Airport is expected to become a hub airport for almost half of Punjab and Khyber-Pakhtunkhwa
(Half-Punjab is larger than the entire Sindh province and the other half may be served by Allama Iqbal International Airport in Lahore). You can say roughly 70% of Pakistan's population lives in Punjab and Khyber-Pakhtunkhwa and only 2 major Airports are serving this entire region. Allama Iqbal International Airport in Lahore and BB INT Airport in Islamabad. There are quite a few International Airports in Punjab and K-P but they are designed to serve few airlines and flights per day. So I would say probably 10-15% of the flights would be diverted to other "smaller" Airports in both provinces.
What I see In the future is...
New Islamabad International Airport: would be the largest Airport in Pakistan serving most parts of Punjab and Khyber-Pakhtunkhwa including Islamabad, Rawalpindi, Gujar Khan, Jhelum, Abbotabad, Haripur, Mansehra, Mardan, Hasan Abdal, Attock and surrounding areas. Peshawar does not handle flights from the entire world so I guess Peshawar would also be partially served by this Airport.
Some of the cities like Gujrat are closer to Lahore but they also prefer travelling from Islamabad International Airport due to beautiful carpet roads and stunning scenery. So I expect many people would continue travelling from Islamabad International Airport instead of closer Airport of Lahore. Some cities I mentioned above will also take flights from Peshawar International Airport but its handling would remain limited.
2)
Jinnah International Airport: would be second largest Airport in Pakistan serving whole of Sindh and parts of Balochistan such as Karachi, Hyederabad, Sukkur, Larkana, Rahim Yar Khan, Dadu and Nawabshah etc.
3)
Allama Iqbal International Airport: would be the third largest Airport in Pakistan serving Lahore, Faisalabad, Gujranwala, Multan, Sargodha, Bhawalpur, Sialkot, Sheikhupura, Nankana Sahib, Jhang and surrounding areas.
4)
Gwadar International Airport: would be the fourth busiest Airport if it finally start attracting investment and even go to higher ranks if all Central Asian Countries are handling their cargo from over there. It will definitely share its passenger traffic alongside Quetta and both Airports will serve the entire province of Balochistan.
5)
Gilgit-Baltistan: would probably become the fifth busiest Airport in Pakistan serving the entire Azad Kashmir and Gilgit Baltistan region. Lots of expatriates belongs to this region so it may even surpass both Quetta and Gwadar Airport if no investment was made in Balochistan and it remained like that (Allah na karay)
Of course I am talking about International flights only otherwise they are already very busy for domestic flights. There is also another reason why I believe Islamabad International Airport would get more flights is because most of the Pakistani diaspora belongs to Punjab and they are closely connected to Islamabad. I believe most Pakistani expatriates are from Gujrat, Jhelum, Lahore, Sialkot, Azad Kashmir, Islamabad and surrounding cities so they would be using Islamabad Airport more often than others. Some of the flights are usually intended to go to Punjab but they have a stop-over or change-over at Karachi International Airport that is also reflecting in higher traffic in Jinnah International Airport, Karachi.
