Boeing is predicting demand for 2,300 freighters over the next two decades, with little more than a third of them new-build aircraft.
It says the global freighter fleet will rise from 1,730 in 2012 to 2,810 aircraft in 2032.
But the 2,300 deliveries will comprise just 850 new-build aircraft with the remaining 1,450 being sourced from conversions.
While the airframer's latest 20-year commercial outlook predicts demand for 940 small freighters, up to 45t capacity, none will be new-build aircraft.
Three-quarters of the new-build freighters, 640 aircraft, will be in the high-capacity sector, 80t and above, with another 130 converted.
Boeing's forecast says the other 210 new-build freighters will be medium-sized aircraft, such as the 767 and Airbus A330. This sector will also attract 380 conversions.
It predicts that the market value of the new-build freighters will total $240 billion, with the highest-capacity aircraft - such as the Boeing 747-8F - accounting for 80% of this figure.