How Many Bags of Concrete for 100 Square Feet?
At a standard 4 inches thick, 100 sq ft needs 56 bags of 80 lb concrete (or 74 bags of 60 lb). That is 33.3 cubic feet, or 1.23 cubic yards — the same volume as a 10×10 slab.
slab Dimensions
Typical finished slab: $6–12 / sq.ft (labor + materials)
Total Material Needed
Recommended For This Job
Quick Reference: 100 Sq Ft by Thickness
| Thickness | Cubic Feet | Cubic Yards | 80 lb Bags | 60 lb Bags |
|---|---|---|---|---|
| 3 inches (walkway) | 25 | 0.93 | 44 | 59 |
| 4 inches (patio, standard) | 33.3 | 1.23 | 56 | 74 |
| 5 inches | 41.7 | 1.54 | 70 | 93 |
| 6 inches (driveway) | 50 | 1.85 | 88 | 117 |
| 8 inches (heavy-duty) | 66.7 | 2.47 | 117 | 156 |
* Includes 5% waste factor. Bag yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft.
Common 100 Sq Ft Layouts
100 square feet is a very common project size because it lines up with standard patio, shed, and walkway dimensions. The bag count is the same for all of these shapes — what matters is area × thickness.
- 10 × 10 (square patio): Most popular size for a small backyard patio or seating area
- 8 × 12.5 (narrow pad): Good for a garden shed or a single-vehicle parking pad
- 5 × 20 (long walkway): Classic sidewalk or back-door-to-garage path
- 4 × 25 (narrow path): Common for side-yard access paths
- L-shape 6×12 + 4×7: AC pad + walkway combo, 100 sq ft total
Which Shape Should Your 100 Sq Ft Be?
The bag count is the same for any 100 sq ft pour at the same thickness — but the forming difficulty, edge length, and final usefulness are very different. Use this decision tree to pick the shape before you start digging.
1. Will the slab sit against the house or another wall?
Yes → Go narrow (5×20 or 4×25 strip). The wall acts as one form, halving the lumber cost. Standard back-door-to-garage walkway.
No → You have full freedom — pick by use case below.
2. Will it be the main outdoor seating area?
Yes → Choose 10×10 square. Easiest to form (4 equal pieces of 2×4), best for patio furniture (a 6-person table needs at least 8 ft clear).
No → Skip to the access question.
3. Do you need to walk between two destinations (door → garage, deck → shed)?
Yes → 4×25 or 5×20 long strip. Width follows the rule: 36 inches for single-file foot traffic, 48–60 inches for two abreast or for moving furniture.
No → Skip to the equipment question.
4. Combining two purposes (AC pad + walkway, shed base + door pad)?
Yes → L-shape (e.g., 6×12 + 4×7 = 100 sq ft). Pour as one slab to avoid a control joint between the two rectangles. Adds ~30% to forming time vs a single rectangle.
No → Default to 10×10 — it is the most common choice for a reason.
5. Want a curved or freeform edge?
Yes → Plan for 8×12 (96 sq ft) rectangular core + curved bump-out. Use 1/4-inch hardboard or thin plywood for the curve forms. Add 10% more bags as buffer (62 instead of 56) — curve geometry is hard to predict exactly.
No → Stick to rectangular for first-time DIY. The labor difference is real.
Forming an L-Shape, Strip, or Curve (Non-Square 100 Sq Ft)
A square 10×10 form needs 4 pieces of 10-ft 2×4. Anything else takes more lumber and more layout time. Here is the practical guide for the three non-square layouts most homeowners actually pour.
Long Strip (5×20 or 4×25)
- Two long sides (20 ft or 25 ft) typically need two 2×4s spliced with a backing block — buy 10-ft lengths, not 16-ft (cheaper, easier to transport).
- Stake every 3 ft, not 4 ft — long forms bow outward under the weight of wet concrete more than square forms do.
- Cut a control joint every 5 ft (4 joints across a 20-ft strip) within 24 hours of pouring. Long strips crack predictably without joints.
- Pour from one end to the other; never start in the middle. The leading wet edge needs to stay open until you reach the next end.
L-Shape (e.g., 6×12 + 4×7)
- Form perimeter is longer than a 10×10 (38 ft vs 40 ft — actually similar). Inside corner gets the most stress, brace it with a kicker stake at 45°.
- Pour the long leg first, then the short leg, working from the inside corner outward. The inside corner is the last spot to receive concrete.
- Always pour as one continuous slab — do not split into two separate rectangles. A cold joint at the L-corner cracks within 1–2 winters.
- Place a single rebar across the inside corner (diagonal, 4 ft long, 30° from each leg). Prevents stress cracks at the re-entrant angle.
Curved Edge (rectangle + arc)
- Form the curve with 1/4-inch tempered hardboard or 1/8-inch luan plywood. Anything thicker than 1/4 inch will not bend smoothly.
- Drive stakes every 12 inches along the curve — half the spacing of straight forms. Curved hardboard wants to spring back outward.
- Pre-soak the hardboard with a garden hose for 30 minutes before bending it. Wet hardboard bends without cracking.
- Add 5–10% extra concrete (3–6 more bags) to your estimate. Curves are notoriously hard to calculate exactly, and running short on a curve is uglier than running short on a square.
Bags vs Ready-Mix for 100 Sq Ft
100 sq ft at 4 inches is almost exactly 1 cubic yard — right at the decision boundary. Here is how to think about it:
- Bags win if: You have a weekend, can rent a mixer (or have 2 people), and your local ready-mix supplier charges a short-load fee.
- Ready-mix wins if: You want the pour done in 30 minutes, the slab needs to be monolithic (no cold joints), or you can order 3+ cu yd together with a neighbor.
- Contractor wins if: The slab is visible/decorative, requires rebar grids, or you are not comfortable finishing (screeding, floating, edging).
Frequently Asked Questions
How many 80 lb bags of concrete for 100 square feet?
At 4 inches thick, 100 sq ft needs 56 bags of 80 lb concrete (with 5% waste included). This is identical to a 10×10 slab.
How many cubic yards of concrete for 100 square feet?
1.23 cubic yards at 4 inches thick. 1.85 cubic yards at 6 inches (driveway). 2.47 cubic yards at 8 inches (heavy-duty).
Is it cheaper to use bags or ready-mix for 100 sq ft?
100 sq ft is right at the crossover. Bags cost ~$370 in materials but take a full day of labor. Ready-mix has a 3 cu yd minimum, so 1.23 cu yd incurs a $60–$150 short-load fee.
Does it matter if 100 sq ft is 10×10 or 5×20?
No. Concrete volume is area × thickness. Any shape totaling 100 sq ft at the same thickness uses the same number of bags. Only the forming labor differs — square pours are easier.