Fence Post Concrete Calculator: Bags per Hole

Quikrete's guide says to dig each post hole 3 times the width of the post. For a 4x4 post (about 3.5 in) that is a 10.5 in hole; at 30 in deep with 6 in of gravel and the top 3 in left for soil, each hole takes 0.90 cu ft: about 2.4 bags of 50 lb Quikrete Fast-Setting. Ten posts take 25 bags.

Set up for: fence or mailbox posts · Different project?

The maker's post-hole method

Quikrete's other documents word the depth differently: “Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height.” (Fast-Setting data sheet) and “the depth of the post hole should be one-half of the above-ground post height.” (online calculator). The calculator uses the depth you type and shows the guide's range when you enter the post's height above ground.

By post size: 4x4 posts · 6x6 posts

FAQ

How many bags of concrete per fence post?

It depends on the hole, not the post alone. With Quikrete's rule (hole 3× the post width) a 4x4 at 30 in deep with a 6 in gravel base takes about 2.4 bags of 50 lb fast-setting mix. The calculator works it out for your post and depth.

How deep should a fence post hole be?

Quikrete's guide words it as: “Dig the post hole , making it three times the width of the post and at a depth equal to 1/3 to 1⁄2 of the above-ground length of the post, plus 6"”. Its data sheet and online calculator word it differently (“Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height.”; “the depth of the post hole should be one-half of the above-ground post height.”). Local rules and frost depth can require more; check your building department. We use the depth you enter.

Can you pour fast-setting concrete dry into the hole?

Quikrete's Fast-Setting data sheet describes pouring the dry mix into the hole and then adding water, “approximately 1 gal (3.8 L) of water for each 50 lb (22.6 kg) bag”, depending on soil conditions. Regular concrete mix is mixed with water first.

Sources

Volumes only. ConcreteMath calculates volumes, bag counts, weights and your own cost figures. It gives no structural or engineering advice and never sizes footings, piers, slabs or rebar: those come from your plans, an engineer or your local building department. Bag counts are estimates from each maker's published yield; actual use varies with hole shape, uneven ground, waste and mixing. Bulk weights are published ranges; your supplier's figure wins.