How Much Concrete Do I Need for Fence Posts?

The hole size rules

Quikrete's post-setting sheet says the hole diameter should be 3 times the post diameter and the depth one third of the post length, with 6 inches of gravel below the post. For structural posts it says the bottom should be at least 6 inches below the frost line. Millard Lumber also gives about one third of the total post length buried. Frost depth varies by region and we did not find a national table, so ask your local building office.

Bags per hole, with the maths shown

Quikrete lists the yield of its fast-setting mix as 0.375 cubic feet for a 50 lb bag and 0.45 cubic feet for a 60 lb bag, and about 1 gallon of water per 50 lb bag. The bags needed per hole are the hole volume minus the post volume, divided by the yield.

Worked example: an 8 foot (96 inch) post, so 32 inches buried plus 6 inches of gravel. A 4 by 4 measures about 3.5 inches square as lumber is sold. The hole is 3 x 3.5 = 10.5 inches across. Concrete fills 32 inches of depth.

Hole volume: pi x 5.25 x 5.25 x 32 = 2,771 cubic inches. Post volume: 3.5 x 3.5 x 32 = 392 cubic inches. Concrete needed: 2,379 cubic inches, or 1.38 cubic feet.

Bags: 1.38 / 0.375 = 3.7 bags of 50 lb, or 1.38 / 0.45 = 3.1 bags of 60 lb. Buy 4 bags either way.

Check against the manufacturer: its data sheet gives two 50 lb bags for a 3 inch square post in a 9 inch hole 30 inches deep. Our arithmetic gives 2.0 bags if the bottom 6 inches of that hole is gravel, which agrees with the sheet.

Worked example: a 48 foot fence

Posts 8 feet apart, as Millard Lumber's 6 to 8 foot range allows, give 48 / 8 = 6 sections and 7 posts. At 3.7 bags of 50 lb each, that is 7 x 3.7 = 25.9, so buy 26 bags, plus a few spare. With 6 foot spacing you would need 9 posts instead.

Remember that a corner or gate post carries more load, so many installers dig those wider. We did not find a figure for this and do not give one.

How many pickets

The number of pickets per foot is 12 divided by the picket width plus any gap, both in inches. A picket sold as 1 by 6 is about 5.5 inches wide, so with no gap it is 12 / 5.5 = 2.18 per foot. A 3.5 inch picket with a 0.5 inch gap is 12 / 4 = 3.0 per foot. Check the real width of what you buy.

For the 48 foot fence with 5.5 inch pickets and no gap: 48 x 2.18 = 105 pickets, or about 115 with 10 percent waste.

Common mistakes

• Digging holes too narrow: three times the post width leaves room to pour and tamp.
• Leaving out the gravel base, which Quikrete specifies for drainage.
• Buying exactly the calculated bags. Round up per post, not just the total.
• Ignoring frost depth in cold regions.

How much concrete for common slab sizes

Each row uses a 4 in thickness and 10% extra. Change any number in the concrete calculator.

Frequently asked questions

Sources: Quikrete: Setting posts project sheet, Quikrete: Fast-Setting Concrete Mix data sheet, Millard Lumber: How to build wood fences. Read 7 November 2026.