For a 4” post, your hole diameter should be 12”. For a standard 48” high installation, your hole depth should be 18” deep. To fill a 12” hole to a depth of 18” (minus the annular space of the post), you will need 1.04 cubic feet of concrete. Each 50 pound bag yields approximately 0.375 cubic feet, so you will need just a little under 3 bags.
QUIKRETE Fast-Setting Concrete Mix is suitable for all applications requiring a rapid setting time and 4,000 psi once cured. For structural applications, we recommend pre-mixing the product with water prior to placement.
Pilings can be reinforced with QUIKRETE® Fast-Setting Concrete Mix if the pilings are excavated to the frost line, a building tube form is placed around the piling, and the piling is back-filled. Fast-Setting Concrete Mix can then be placed in the building form around the piling.
Quikrete has a variety of mix types. You may want to consider something more specific to the job. https://www.quikrete.com/ProductLines/Alphabetical.asp We've set posts with this type and it works great. It specifically identifies setting posts. If you were doing a driveway or something you are driving on a lot, you may want to consider something like the QUIKRETE® Crack Resistant Concrete Mix (No. 1006-80) is a 4000psi (27.5 MPa) blend of properly proportioned stone or gravel, sand, Portland cement, special synthetic fibers and other ingredients approved for use in concrete. The synthetic fibers eliminate the need for wire mesh in slab-on-grade construction. Exceeds the compressive strength requirements of ASTM C387. The manufacturer states it is used for: Use for: Structural and non-structural concrete over 2" (51mm) thick. Applications such as driveways, sidewalks, floor slabs, curbs, steps, ramps, walkways, poured walls and footings, tool shed bases, appliance and equipment bases.
Each 50-pound bag of QUIRKETE® Fast-Setting Concrete Mix yields approximately 0.375 cubic feet. For a volume of a 1-1/2 cubic yards, you will need 40-1/2 cubic feet of concrete or 108 bags, of 50-pounds each. Yields will vary with waste, uneven subgrade, etc.
There are typically 63 bags of QUIKRETE® Fast-Setting Concrete Mix per pallet. Please contact your local The Home Depot store for confirmation and for pricing.
I don't know what a steel face line post is but one bag per hole should be good enough.
The number of bags required will depend on several dimensions – the hole depth and diameter, as well as the post size. We offer an online calculator at https://www.quikrete.com/calculator/main.asp#fast_setting or you may contact our customer service representatives online by LiveChat at www.quikrete.com or toll free at 800-282-5828 for assistance with your calculation.
QUIKRETE® Fast-Setting Concrete Mix will continue to cure and out-gas for 28 days after placement. Please consult the coating manufacturer for their recommendation regarding placement over fresh concrete.
Would recommend to use when above freezing