Bags of concrete per post hole

How to measure

Measure the hole across the top for the diameter and from the bottom to ground level for the depth. Post hole diggers and augers usually make holes 6, 8, 10, or 12 inches wide. A common rule is a hole about three times the width of the post, so a 4x4 gets a 10 to 12 inch hole and a 6x6 gets a 12 to 18 inch hole.

For depth, many builders aim for about one third to one half of the post height above ground, and at least below your frost line. A 6 foot fence often ends up with holes 2 to 3 feet deep. Your local code and frost depth come first.

The formula

The hole is a cylinder. Concrete only fills the part above the gravel, and the post takes up some of that space:

• Hole area = 3.1416 x (diameter / 2) x (diameter / 2), in square inches
• Concrete = (hole area minus post area) x (depth minus gravel), divided by 1,728 for cubic feet
• Bags = cubic feet divided by the bag yield

A 4x4 is really 3.5 inches square, so it covers 12.25 square inches. A 10 inch hole that is 36 inches deep with 6 inches of gravel holds about 1.15 cubic feet of concrete around a 4x4. That is about 3.1 bags of 50 lb fast-setting mix or just under 2 bags of 80 lb mix.

Bag yields

• 50 lb fast-setting post mix: about 0.375 cubic feet
• 60 lb concrete mix: about 0.45 cubic feet
• 80 lb concrete mix: about 0.60 cubic feet

Check the label on the bag you buy, since yields vary a little by brand. Totals are rounded up to whole bags across all holes, so leftover from one hole helps fill the next.

Tips

• A few inches of gravel at the bottom helps water drain away from wood posts.
• Slope the top of the concrete away from the post so rain runs off.
• Buy one or two extra bags. Holes are rarely perfectly round, and returning a bag is easier than a second trip mid-job.
• Pouring a pad or a footing too? Try the footing calculator or the slab calculator.