The "Whitebelt" consists of lands between the outer edge of the approved urban settlement areas surrounding the Greater Toronto Area and Hamilton and the Greenbelt Plan area. This area is known as the "Whitebelt" because it is shown as white on Growth Plan maps. These lands are currently undeveloped, but are not protected from urban development in the future.
The Neptis Foundation identified and mapped the "Whitebelt" through a review of municipal Official Plans, Official Plan Amendments and other planning documents before and after the introduction of the Growth Plan conformity amendments.
Inventory of Whitebelt lands by municipality, Table 3.13, and land supply in GGH, Table 3.14: https://www.neptis.org/publications/story-so-far/chapters/note-about-whitebelt
R. Tomalty and B. Komorowski (2011) Inside and Out: Sustaining Ontario's Greenbelt, estimated a total of 58,696 hectares in the Whitebelt; since then, 11,000 hectares (19% of the land area) have been removed for development: http://www.greenbelt.ca/inside_and_out_sustaining_ontario_s_greenbelt2011
The David Suzuki Foundation report (2013), Nature on the Edge: Natural Capital and Ontario's Growing Golden Horseshoe, calculates 94,472 hectares of land in the Whitebelt; this number includes approved vacant greenfield lands already designated for urban expansion: http://www.davidsuzuki.org/publications/reports/2013/nature-on-the-edge-natural-capital-and-ontarios-growing-golden-horseshoe/