How many bags of fast-setting concrete per post?

Q: How many bags of fast-setting concrete per post?

About 3 bags, including a waste allowance. 0.89 cu ft (concrete per post). 0.89 cu ft (concrete, whole job). 0.03 cu yd (same volume in cubic yards). 0.15 cu ft (volume the posts displace). 0.27 cu ft (gravel for the drainage bases).

About 3 bags, with a waste allowance included.

0.89 cu ft — concrete per post
0.89 cu ft — concrete, whole job
0.03 cu yd — same volume in cubic yards
0.15 cu ft — volume the posts displace
0.27 cu ft — gravel for the drainage bases

Change the numbers to match your project

Hole diameter Three times the post width, per Quikrete’s post-setting guide. A 4x4 wants 10–12 in; a 6x6 wants 12–18 in. Hole depth One-third to one-half of the above-ground post height, plus 6 in for the gravel base — and never shallower than your local frost depth. Post Wood is listed at its dressed size, not the nominal name. A 4x4 is 3.5 in on a side. Gravel in the bottom Quikrete’s guide calls for 6 in of gravel or crushed stone under the post so water drains away from the end grain. Concrete does not fill this. Set to 0 if you are skipping it. Concrete stops below grade The same guide stops the concrete 3–4 in down so you can backfill with soil. Set to 0 if you are instead crowning the concrete at grade to shed water. How many posts Extra for waste % An augered hole is never a clean cylinder. 10% covers ragged sides; use 15% in loose sand or gravel where the hole bells out. Bag Fast-setting mix yields the same 0.0075 cu ft per pound as general mix — but it is sold in a 50 lb bag, so you need more bags for the same hole.

Bags of mix, whole job 3 bags Rounded up. Buy one spare per ten posts, not per post.

Concrete per post 0.89 cu ft

Concrete, whole job 0.89 cu ft

Same volume in cubic yards 0.03 cu yd

Volume the posts displace 0.15 cu ft Already subtracted above. A plain hole-volume figure leaves this in.

Gravel for the drainage bases 0.27 cu ft

Water is the half of this order you have to carry yourself.

QUIKRETE's post-setting walkthrough for the dry method is specific about the quantity: "Pour about a gallon of water per 50 lb bag into the hole and allow the water to saturate the concrete mix." Three bags a post across a twenty-post fence is sixty gallons — twelve five-gallon buckets, or a hose that reaches. Nobody sells you that at the till, and it is the thing that stops the job.

That gallon is also roughly double what the same sack drinks in a mixer. The No. 1004 sheet gives "approximately 4 pt (1.9 L) of potable water" per sack and adds that "maximum water is expected to be below 5 pt (2.3 L)". Those are not measuring the same thing — one is water in a drum, the other is water poured into a hole whose soil takes a share — so do not carry either one across to the other method.

"Sets in 20 to 40 minutes" is a setting time, not a strength. That number is ASTM C191, the Vicat needle. Strength is ASTM C39, and the same sheet prints it separately: 1,000 psi at 24 hours, 2,500 psi at 7 days, 4,000 psi at 28 days. QUIKRETE's own instruction is to "Wait about 4 hours to begin constructing your fence or applying heavy weight to your post." The fast mix buys you the bracing, not the fence.

Check the sack against the bag list above. No. 1004 is sold as 50 lb and 60 lb in the United States and as 25 kg and 30 kg in Canada only, and the sheet yields those at 0.41 and 0.5 cu ft. A 30 kg sack holds a third more than the 50 lb bag this page assumes, so the same hole comes out at fewer bags — and a part-used sack of fast-setting mix does not keep once it has met water.

Open the full concrete calculator for post holes for other sizes and the full explanation →

Last reviewed 2026-07-31