From the Hansen website  http://www.hansenproducts.co.nz/products/tank/SFTFT-install.htm    RECOMMENDED HOLE SAW SIZES15mm = 22mm or 7/8" Hole Saw20mm = 27mm or 1 1/16" Hole Saw25mm = 34mm or 1 3/8" Hole Saw32mm = 42mm or 1 5/8" Hole Saw40mm = 48mm or 1 7/8" Hole Saw50mm = 60mm or 2 3/8" Hole Saw80mm = 95mm or 3 3/4" Hole Saw100mm = 121mm or 4 3/4" Hole Saw MINIMUM RECOMMENDED HOLE SIZESHole Size for 15mm = 22mmHole Size for 20mm = 27mmHole Size for 25mm = 34mmHole Size for 32mm = 42mmHole Size