How many bags of concrete do I need? (per yard, 10×10 slab, fence posts)
45 bags of 80 lb (or 60 of 60 lb) make a cubic yard, a 10×10 slab at 4 in takes 56 bags of 80 lb, and a 4×4 fence post takes 3 — counts for every bag size plus waste and yield rules.
Short answer: it takes 45 bags of 80 lb concrete mix (or 60 bags of 60 lb) to make one cubic yard. A 10′ × 10′ slab at 4" thick is 33.3 ft³ and needs 56 bags of 80 lb — order 65 with waste — or 74 bags of 60 lb (order 85). A standard 4×4 fence post in a 12" hole takes 3 bags of 80 lb per post. Every other bag size and hole size follows the same two-step math below: cubic feet needed ÷ yield per bag, rounded up, plus 10%.
Bags per cubic yard (every bag size)
One cubic yard is 27 cubic feet of placed concrete. Divide 27 by the yield printed on the bag and you get the count. These are the standard Quikrete/Sakrete concrete-mix yields:
1 cubic yard = 27 ft³
Bag size Yield per bag Bags per yard
40 lb 0.30 ft³ 90
50 lb 0.375 ft³ 72
60 lb 0.45 ft³ 60
80 lb 0.60 ft³ 45
90 lb 0.675 ft³ 40
Bags needed = cubic feet required / yield per bag
= (cubic yards × 27) / yield per bag
Always round UP. Then add 10% waste.Two numbers worth memorizing: 45 × 80 lb bags = 1 yard and 60 × 60 lb bags = 1 yard. Everything on a bag-sized job is a fraction of those. That's also 3,600 lb of dry mix per yard — which is why the threshold above exists.
Worked example: 10×10 slab, 4 inches
The most-asked version of this question. Thickness in feet first: 4" = 0.333 ft.
Volume: 10 × 10 × 0.333 = 33.3 ft³ = 1.23 cu yd 60 lb bags (0.45 ft³ yield): 33.3 / 0.45 = 74 bags + 10% waste = 82 → order 85 80 lb bags (0.60 ft³ yield): 33.3 / 0.60 = 56 bags + 10% waste = 62 → order 65
At ~1.2 yards, a 10×10 is right at the bag-vs-truck threshold. Bags are defensible here — but 62 bags of 80 lb is 5,000 lb of mix and a full day at the mixer, so pour in sections and don't outrun your screed. Anything bigger, call ready-mix: see what the math looks like one size up in bags for a 20×15 slab at 4" — same formula, hopeless as a bag job.
Fence posts: the 3× rule
Post holes are sized off the post, not guessed: hole diameter is 3× the post width, and depth is 1/3 of the above-ground height — minimum 24" for a 6 ft fence, and always below the frost line. Concrete per hole is the hole volume minus the post that displaces it.
Sizing the hole:
Hole diameter = 3 × post width
Hole depth = 1/3 of above-ground height
(min 24" for a 6 ft fence; go below frost line)
Concrete per hole = π × r² × depth − post volume
Worked example — 4×4 post (3.5" actual), 12" dia hole, 24" deep:
Hole: π × 0.5² × 2 = 1.571 ft³
Post: (3.5/12)² × 2 = 0.170 ft³
Net: 1.571 − 0.170 = 1.40 ft³
Bags per post:
60 lb: 1.40 / 0.45 = 3.1 bags
80 lb: 1.40 / 0.60 = 2.3 bags → buy 3 per post
10-post fence run: ~30 bags of 80 lbThe 2.3-bags answer rounds up to 3 bags of 80 lb per post in practice — you can't split a bag between holes cleanly, and a post set in a half-filled hole is a callback. For a typical 10-post run, show up with ~30 bags of 80 lb.
Fast-setting vs regular mix
Fast-setting mix (the red bag) yields about the same per pound — a 50 lb fast-set bag gives ~0.375 ft³ — but it changes the workflow for posts: per the manufacturer's instructions, it can be poured dry into the hole and watered in place. That is a manufacturer-sanctioned exception for fast-set products only. Plain concrete mix gets mixed properly before it goes in the hole — dry-pouring standard mix leaves unhydrated pockets and a post that wobbles in year two. Fast-set typically sets in 20–40 minutes, so on a long fence run it also means you can brace, plumb, and move on instead of babysitting wet posts.
Don't get shorted: yield notes
- Bag yield assumes the spec water ratio. A soupy mix does not give you more concrete — it gives you the same volume at lower strength. If your count keeps coming up short, check the water before blaming the bag.
- 10% waste is the standard, not padding. Uneven subgrade, spillage, and over-dig eat it every time. Rounded up, always.
- Coming up short mid-pour = cold joint. The seam between today's concrete and the emergency-store-run batch is a permanent weak plane. Extra bags return; a cold joint doesn't.
- Real-world yield runs slightly under label yield. Hand-measured water and shrinkage shave a few percent — another reason the +10% is non-negotiable on bag jobs.
Count it on your phone
The BuilderCalc app does all of this in the field: slab, post, and footing presets, a bag-size toggle (40/50/60/80/90 lb), adjustable waste %, and feet-inch math so you can punch in 10′ 6" without converting. 100% offline. Free on the App Store and Google Play.
Related
- Concrete yardage — slab, footing, and pier formulas
- Bags for a 20×15 slab at 4" — why big pours want a truck
- Deck joist span and spacing
- All free construction calculators
- BuilderCalc — construction calculator on iOS + Android
Note: Structural slabs and footings may require an engineered mix design and inspection — local code and the engineer of record override these defaults. Bag yields per current Quikrete/Sakrete published data; reviewed 2026-07.