Fence Post Concrete Calculator

How much concrete does a fence post need?

Each hole is a cylinder with the post standing in it, so the concrete is the hole’s volume minus the post’s, over the depth that isn’t gravel. For a round hole that’s π × (hole diameter ÷ 2)² × concrete depth, less the post’s cross-section × the same depth. The calculator does this for square or round posts and multiplies by the number of posts.

Measure the post. Lumber is sold by nominal size, so a 4×4 is 3.5 in square and a 6×6 is 5.5 in.

1. Concrete depth: 30 in − 6 in of gravel = 24 in, or 2 ft.
2. Hole: π × (5 ÷ 12)² × 2 ft = 1.091 cu ft.
3. Post: (3.5 ÷ 12)² × 2 ft = 0.170 cu ft.
4. Concrete per post: 1.091 − 0.170 = 0.921 cu ft.
5. 12 posts: 0.921 × 12 = 11.05 cu ft (0.41 cubic yards).
6. Adding 10% gives 12.15 cu ft: 33 bags of 50 lb fast-setting mix (12.15 ÷ 0.375), or 21 bags of 80 lb mix.

Buy for the whole fence, then pour hole by hole. Each hole needs 2.7 bags of fast-setting mix with the extra, so rounding every hole up to 3 bags would mean 36 bags; buying for all 12 at once takes 33, because the part bag from one hole goes into the next.

The 33 bags weigh 1,650 lb, inside the 2,360 lb payload of the 2026 Ram 1500 Tradesman the calculator uses as its example truck, so they’re one trip. For your own truck, enter the payload from the sticker on the driver’s door jamb.

How wide and deep to dig the hole

Quikrete and Sakrete both say to dig the hole about 3 times as wide as the post. That’s 10½ in for a 4×4 and 16½ in for a 6×6. The calculator’s example uses a 10 in hole, a little narrower than the rule; at 10½ in the same 12 posts take 12.39 cubic feet before extra instead of 11.05.

For depth, Quikrete’s data sheets and Sakrete’s technical data sheets say one third of the post’s overall length. That puts a third of the post in the ground and two thirds above it, so a fence 6 ft high needs 9 ft posts set 36 in deep, and an 8 ft post set the same way stands 5 ft 4 in tall. Sakrete’s project guide gives a shallower rule, a hole one third of the height above ground, which for a 6 ft fence is 24 in.

Both rules are general guidance, and two local checks come before either of them. Ask your building department for the frost depth where you live and for any fence rules, such as height limits. Then call 811 before you dig so the utility companies can mark buried lines.

How many bags of concrete per fence post?

These are the calculator’s results for one post, with 4–6 in of gravel in the bottom of the hole and 10% extra. Multiply by your number of posts, or enter the count in the calculator so it rounds the total instead of each hole.

Per post, with 6 in of gravel in the bottom of the hole and 10% extra.

Sakrete publishes its own chart for a 10 in hole: 1¼ bags of its 50 lb fast-setting mix for a hole 12 in deep, 2 bags for 18 in, 2½ for 24 in and 3 for 30 in. The calculator’s figures for a 4×4 in a 10 in hole, with no gravel and no extra, are 1.21, 1.82, 2.42 and 3.03 bags of Sakrete’s 0.38 cu ft bag, close to the chart at every depth.

A square post needs a little room around it. The calculator checks that the post’s diagonal fits inside the hole, which is 4.95 in for a 4×4 and 7.78 in for a 6×6, and asks for a wider hole if it doesn’t.

Gravel at the bottom of the hole

Quikrete’s post-setting guide calls for 6 in of gravel or crushed stone in the bottom of the hole for drainage. Sakrete’s fast-setting data sheet says 4 to 6 in of gravel, compacted. Quikrete’s standard Concrete Mix data sheet puts 6 in of dry concrete mix in the bottom of the hole instead.

The calculator’s gravel box has a 6 in chip and takes the gravel out of the concrete depth. Leave the box empty if you skip the gravel, and the whole hole is counted as concrete.

You won’t need much gravel. Six inches in a 10 in hole is 0.27 cu ft, so the 12-post fence takes 3.27 cu ft of gravel, or 7 half-cubic-foot bags of Quikrete all-purpose gravel. For a bigger job, the gravel calculator handles bulk orders.

Sonotube calculator: deck and pergola footings

Switch to the Column tab for round forms such as Sonotubes. Sonoco’s Builder’s Tube brochure lists 6, 8, 10 and 12 in diameters (the chips in the calculator) and 4, 8 and 12 ft lengths. It notes that the diameters are nominal, so measure the tube if the exact size matters. Enter the full height of concrete in the tube, including the part below ground, and the number of tubes.

Tube	Concrete per foot (Sonoco)	4 ft tube, with 10% extra	80 lb bags	50 lb fast-setting bags
6 in	0.20 cu ft	0.86 cu ft	2	3
8 in	0.35 cu ft	1.54 cu ft	3	5
10 in	0.55 cu ft	2.40 cu ft	4	7
12 in	0.80 cu ft	3.46 cu ft	6	10

The calculator works from π × radius² × height, so its volumes differ from Sonoco’s rounded per-foot figures in the second decimal: a 10 in tube 4 ft tall is π × (5 ÷ 12)² × 4 = 2.18 cu ft before extra, against 2.2 from the chart. The tube’s diameter, its depth and any footing pad under it come from your deck or pergola plans and the frost depth your building department gives you. The concrete calculator adds the slab, footing and steps tabs if the project needs more than tubes.

Fast-setting vs standard concrete mix

Both kinds are sold for setting posts. Quikrete’s standard Concrete Mix data sheet has its own post-setting method, and its Fast-Setting Concrete Mix data sheet gives a post-setting example too. Sakrete’s fast-setting data sheet includes the bag chart above.

The two differ in bag size and count. Quikrete’s 50 lb fast-setting bag yields 0.38 cu ft, and Sakrete’s yields 0.38 cu ft. An 80 lb bag of standard mix yields 0.6 cu ft. Per cubic foot of concrete, the dry mix weighs the same either way, about 133 lb, so the 12-post fence is 1,650 lb of fast-setting bags or 1,680 lb of 80 lb bags. The fast-setting bags are lighter to carry one at a time, and there are more of them: 33 against 21.

The calculator leads with 50 lb fast-setting bags and also shows 80 lb and 60 lb counts, so you can price both at your store. Follow the directions on the bag you buy, since mixing and setting instructions differ between products.

Questions people ask

How many bags of concrete do I need per fence post?

For a 4×4 post in a 10 in hole with 24 in of concrete above 6 in of gravel, about 1 cubic foot with 10% extra: 3 bags of 50 lb fast-setting mix (2.7 rounded up) or 2 bags of 80 lb mix. Wider or deeper holes take more; the table on this page covers common sizes.

How much concrete for a 4x4 post?

About 0.46 cubic feet for every foot of concrete depth in a 10 in hole, once the post's own volume is taken out. With 24 in of concrete that's 0.92 cubic feet per post, before any extra. A 4×4 is 3.5 in square, so measure yours and enter the actual size.

How much concrete for a 6x6 post?

A 6×6 (5.5 in actual) in a 16 in hole, 36 in deep with 6 in of gravel, takes 2.97 cubic feet of concrete, or 3.26 with 10% extra. That's 9 bags of 50 lb fast-setting mix or 6 bags of 80 lb mix per post.

How deep should a fence post hole be?

Quikrete's and Sakrete's data sheets say one third of the post's overall length, so a post with 6 ft above ground is 9 ft long and goes 36 in deep. Sakrete's project guide gives a shallower rule, one third of the height above ground. Your local frost depth and code come first: ask the building department, and call 811 before you dig.

How wide should a fence post hole be?

About three times the width of the post, according to both Quikrete and Sakrete. That's 10½ in for a 4×4 (3.5 in actual) and 16½ in for a 6×6 (5.5 in actual). A wider hole takes more concrete: at 10½ in, a 4×4 with 24 in of concrete needs 1.03 cubic feet instead of 0.92.

How much concrete does a Sonotube take?

Sonoco's chart for its Builder's Tube lists 0.20 cubic feet per foot of height for a 6 in tube, 0.35 for 8 in, 0.55 for 10 in and 0.80 for 12 in. A 10 in tube 4 ft tall holds 2.18 cubic feet, which with 10% extra is 4 bags of 80 lb mix or 7 bags of 50 lb fast-setting mix.

Should I put gravel in the bottom of a fence post hole?

Quikrete's post-setting guide calls for 6 in of gravel or crushed stone in the bottom for drainage, and Sakrete's fast-setting data sheet says 4–6 in, compacted. Quikrete's standard Concrete Mix data sheet puts 6 in of dry concrete mix in the bottom instead. Leave the calculator's gravel box empty if you skip the gravel.

Sources

1. The Quikrete Companies, LLC, Setting Posts in Concrete. Accessed Oct 19, 2026.
2. Sakrete, Setting Posts and Poles (project guide). Accessed Oct 19, 2026.
3. SAKRETE of North America LLC, Sakrete Fast Setting Concrete Mix Technical Data Sheet (Rev. 05/18). Accessed Oct 19, 2026.
4. The Quikrete Companies, LLC, QUIKRETE Fast-Setting Concrete Mix (No. 1004) Data Sheet, revised November 2025. Accessed Oct 19, 2026.
5. The Quikrete Companies, LLC, QUIKRETE Concrete Mix (No. 1101) Data Sheet, revised October 2022. Accessed Oct 19, 2026.
6. Federal Highway Administration, U.S. Department of Transportation, Appendix C: Proposed Revisions to the AASHTO Standard Specifications for Highway Bridges (FHWA-HRT-05-057). Accessed Oct 19, 2026.
7. National Ready Mixed Concrete Association (NRMCA), CIP 36 - Structural Lightweight Concrete. Accessed Oct 19, 2026.
8. FEMA National US&R Response System, US&R Structural Collapse Technician, Module 1C Part 3 - Hazard Identification & Building Monitoring. Accessed Oct 19, 2026.
9. U.S. Navy (Naval Education and Training), Builder Advanced (NAVEDTRA 14045A), Chapter 3 Concrete Construction, 5.3.1. Accessed Oct 19, 2026.
10. Sonoco, Sonotube Builder's Tube concrete forms brochure ((c)2016). Accessed Oct 19, 2026.
11. Ram Trucks (Stellantis), 2026 Ram 1500 Specs (Tradesman 4x2 Quad Cab 6'4" box). Accessed Oct 19, 2026.
12. T.H. Kinsella, Inc. (Kinsella Quarries), Fayetteville, NY, Project Calculator - Materials & Pricing Estimates. Accessed Oct 19, 2026.
13. Federal Highway Administration, Strength Characterization of Open-Graded Aggregates for Structural Backfills (FHWA-HRT-15-034), Chapter 3, Table 7. Accessed Oct 19, 2026.
14. Graniterock, Southside 1" x #4 Round Rock (Tech Note). Accessed Oct 19, 2026.
15. Do it Best (retailer), Quikrete 0.5 Cu. Ft. 50 Lb. All-Purpose Gravel (retailer listing). Accessed Oct 19, 2026.