The number printed small on the back of the bag is the only one that matters: an 80 lb bag of concrete yields 0.60 cubic feet of mixed concrete — about the volume of a basketball and a half — and every project estimate flows from that figure. Underestimate and you’re racing back to the store mid-pour with a cold joint forming; overestimate by double (the classic rookie move) and you’ve hauled 800 wasted pounds. This is the complete bag-math reference: yields, bags per yard, coverage tables, water ratios, and the honest crossover point where you should stop lifting bags and call the ready-mix plant.
The Core Numbers
- Yield per 80 lb bag: 0.60 cu ft (Quikrete and Sakrete both spec 0.60; call it 0.6 and round bags up).
- Bags per cubic yard: 45. A yard is 27 cu ft; 27 ÷ 0.60 = 45 bags — that’s 3,600 lbs of material to move twice (cart to truck, truck to mixer).
- 60 lb bag: 0.45 cu ft, 60 bags per yard. 50 lb bag: 0.375 cu ft, 72 per yard. 90 lb bag: 0.675 cu ft, 40 per yard.
- Water per 80 lb bag: about 3 quarts (6 pints minimum, up to ~3.7 quarts max per Quikrete’s spec — start at 2.8 quarts and add sparingly).
- Strength: standard bag mix hits 4,000 PSI at 28 days when mixed to spec; high-early formulas (Quikrete 5000) reach 5,000 PSI and take foot traffic in 10-12 hours.
- Price: $6 to $9 per 80 lb bag at big-box stores in 2026 ($4.50-$6 for basic gray in some regions, more for 5000-series and crack-resistant blends).
Slab Coverage Table
Slab volume = length × width × depth (in feet). Divide by 0.6 for bags, then add 10 percent for waste, spillage, and uneven subgrade:
| Slab Size | 4″ Thick | 6″ Thick |
|---|---|---|
| 3 × 3 ft (stoop pad) | 5 bags | 8 bags |
| 4 × 4 ft | 9 bags | 14 bags |
| 4 × 8 ft (walkway section) | 18 bags | 27 bags |
| 6 × 6 ft | 20 bags | 30 bags |
| 8 × 10 ft (shed slab) | 45 bags | 67 bags |
| 10 × 10 ft | 56 bags | 84 bags |
| 12 × 20 ft (single-car pad) | 134 bags | 200 bags |
Quick mental shortcut: at 4 inches thick, one 80 lb bag covers about 1.8 sq ft; at 6 inches, about 1.2 sq ft. The 8×10 shed slab row is where most people’s eyebrows go up — 45 bags is a literal ton and a half of material, and it’s the point where this table starts arguing for a concrete truck.
Post Holes and Footings
For a cylindrical hole, volume in cubic feet ≈ (diameter in inches ÷ 12)² × 0.785 × depth in feet. Standard fence practice — hole diameter 3× the post width, depth 1/3 of post height plus 6 inches of gravel:
| Hole Size | Concrete Needed | 80 lb Bags |
|---|---|---|
| 8″ dia × 2 ft (4×4 post) | 0.70 cu ft* | 1-1.5 |
| 10″ dia × 2 ft | 1.09 cu ft* | 2 |
| 12″ dia × 2.5 ft (6×6 post) | 1.96 cu ft* | 3-3.5 |
| 12″ dia × 3 ft (gate/deck pier) | 2.36 cu ft* | 4 |
*Before subtracting the post’s own volume — a 4×4 displaces about 0.09 cu ft per foot, which is why the 8-inch hole often finishes with 1 bag plus a partial. For setting posts, fast-setting mix (Quikrete Red bag) pours dry into the hole followed by a gallon of water per 50 lb bag — no mixing, sets in 20-40 minutes, brace and walk away. Footings: a 16 × 16 × 8-inch deck footing runs 1.2 cu ft, exactly 2 bags.
Water Ratio and Mixing Effort
Water is the strength dial, and more is weaker: every extra quart past spec costs real PSI and adds shrinkage cracking. Aim for a mix like thick oatmeal that holds a trowel-cut edge — not pancake batter that self-levels. Mixing effort is the hidden cost of bag concrete. By hand in a wheelbarrow or mortar tub, an 80 lb bag takes 3 to 5 strenuous minutes; most people fade badly after 10 bags. A rented drum mixer ($50 to $80/day) handles 2 bags per batch and makes 20-40 bag days feasible — one person mixes while another places and screeds. Never mix more than you can place in about 30-45 minutes (less in heat), and in hot weather use cold water and stage bags in shade.
80 vs 60 vs 50 Lb Bags
Same concrete, different ergonomics. The 80 delivers the lowest cost per cubic foot and the fewest bags to open — the value pick for anyone comfortable lifting it. The 60 lb bag costs roughly 10-15 percent more per cubic foot but saves shoulders and suits repeated lifting into a mixer at chest height; it’s the right call for most homeowners over a long day. The 50 lb bag prices worst per cubic foot and exists for light-duty users, small patches, and carrying up stairs. Fast-setting and 5000-PSI formulations typically come in 50s and 80s. Whatever the size, buy 10 percent extra — unopened bags return for full refund, but a mid-pour shortage doesn’t.
Which Bag Formula for Which Job
The yellow standard bag isn’t the only choice on the pallet, and matching formula to task saves both money and callbacks. Standard gravel mix (Quikrete Concrete Mix, Sakrete High Strength) is the default for slabs, footings, and anything over 2 inches thick — 4,000 PSI, cheapest per cubic foot. Fast-setting mix (the red bag) trades working time for speed: sets in 20 to 40 minutes, perfect for posts and mailboxes, wrong for anything you need to screed and finish. Quikrete 5000 (or Sakrete 5000 Plus) adds strength and early cure — worth the extra $1 to $2 per bag for driveways, garage aprons, and cold-weather pours where you want foot traffic in 12 hours. Crack-resistant mix carries synthetic fibers in the bag for slabs poured without wire; sand/topping mix has no gravel and handles repairs and overlays under 2 inches where coarse aggregate would poke through; and countertop mix is a finer, pricier blend engineered for casting. Two technique notes carry more weight than any formula choice. First, don’t hand-mix half bags casually — dry ingredients segregate in the bag during transport, with cement fines settling, so partial bags can batch inconsistently; mix whole bags or dry-blend the full bag before splitting. Second, temperature rules apply to bags exactly as to truck concrete: below 40°F, hydration stalls and you need warm water and blankets; above 90°F, work in the morning and keep the cure wet, because a small slab has enormous surface area relative to its mass and dries out faster than any driveway pour.
When Ready-Mix Wins
The crossover math: a cubic yard as bags costs 45 × $7 = ~$315 in material alone, plus mixer rental, plus 3,600 lbs of handling, plus batch-to-batch inconsistency and cold-joint risk. Ready-mix delivers a consistent, plant-batched yard for $150 to $200, though short-load fees of $75 to $150 apply under 4-5 yards, and trucks want site access. The working rules:
- Under about 0.5 yard (22 bags): bags win — post sets, stoops, repairs, small pads.
- 0.5 to 1 yard (23-45 bags): judgment zone. Bags if pours can be split into independent sections; consider a U-cart (haul-it-yourself trailer, ~$100-$150/yard) if your truck can tow it.
- Over 1 yard, single monolithic pour: call the plant. A 10 × 12 patio at 4 inches is 1.5 yards — 67 bags of misery versus one truck and a $300 ticket. Any slab that must be poured continuously (driveways, garage floors) is truck territory regardless of your enthusiasm.
Run the numbers before the first bag goes in the cart: 0.6 cubic feet per 80 lb bag of concrete, 45 to the yard, 10 percent spare — and the honesty to call a truck when the count passes fifty.
