A back of the envelope calculation can be made by adding major inputs: Like amount of steel produced/used, autos/vehicles, electricity produced/consumed, cement produced/consumed, electronics, pharmaceuticals etc., If you add up the top 100 or 500 items you get a fairly good picture. If Pakistan is a 3 trillion economy, its per capita production/consumption of steel, cement, electricity, vehicles, petroleum, electronics, etc., will be in line with other economies with similar per capita GDP. Nominally, 3 trillion divided by 243 million gives $12,000 per capita.