At 4.94 cubic yards, ready-mix is strongly recommended. Ready-mix saves time and produces more consistent results. Bags work fine for a patient DIYer — but mixing 62–82 bags by hand is a full day of hard work.
Quick Breakdown
Slab: 20 ft × 20 ft · 4-inch thickness · 10% waste factor
Thickness changes everything. A 6-inch slab requires 50% more concrete than a 4-inch slab. Use the full calculator to adjust for any dimensions.
Bags vs. Ready-Mix
Factor
Bags
Ready-Mix
Volume
~326 bags (60 lb) or ~245 bags (80 lb)
4.94 cubic yards
Practicality
✗ Physically demanding at this volume
✓ Fast, consistent, one delivery
Best suited for
Pours under 1 cu yd
Pours over 1 cu yd
Verdict
Possible but not recommended
Better choice for this pour
How This Is Calculated
The formula is: Volume = Length × Width × Thickness. Convert thickness from inches to feet first, then divide by 27 to get cubic yards.
20 ft × 20 ft slab, 4 in thick — step by step
Step 1 — Volume20 × 20 × (4 ÷ 12) ft = 133.33 cu ft
Step 2 — Cubic Yards133.33 ÷ 27 = 4.94 cu yd
Step 3 — Add 10% Waste133.33 × 1.10 = 146.67 cu ft
Step 4 — 60 lb Bags146.67 ÷ 0.45 = ~326 bags
Step 5 — 80 lb Bags146.67 ÷ 0.60 = ~245 bags
Practical Tips for a 20×20 Slab
4 inches is the minimum. Going thinner risks cracking under normal foot traffic.
Always add a waste factor. 10% is the industry standard. Uneven ground and spillage always consume extra material.
Compact your sub-base first. 4–6 inches of gravel under the slab prevents settling and cracking.
Don't thin the mix with extra water. It makes mixing easier but weakens the concrete significantly.
Cure for at least 7 days. Keep the slab moist after pouring. This dramatically increases final strength.
Round up, not down. You cannot pour half a slab and come back — you'll get a visible cold joint.
Assumptions
◦Slab dimensions: 20 ft × 20 ft at 4 inches thick. Changing thickness changes all totals.
◦Concrete yield: 60 lb bags = 0.45 cu ft; 80 lb bags = 0.60 cu ft. Standard manufacturer figures.
◦Waste factor: 10% applied. Increase to 15% for rough terrain or irregular shapes.
◦Results are estimates only: Confirm quantities with your concrete supplier before ordering.
🔢 Need a Different Size?
Enter any dimensions — length, width, thickness — and get an instant bag count, cubic yard estimate, and cost breakdown.
A 20×20 ft slab at 4 inches thick requires 133.33 cubic feet (4.94 cubic yards) of concrete. With a 10% waste factor, that's about 326 bags of 60 lb mix or 245 bags of 80 lb mix.
At 4.94 cubic yards, ready-mix is strongly recommended. Mixing 245–326 bags by hand is impractical, time-consuming, and far more labor-intensive than ordering a single ready-mix delivery.
For a patio or shed base, 4 inches is the standard minimum. For a driveway, use 5–6 inches. At 6 inches thick, a 20×20 slab needs about 7.41 cubic yards — 50% more concrete than at 4 inches.
One cubic yard equals 27 cubic feet. At standard yields, you need about 60 bags of 60 lb mix or 45 bags of 80 lb mix per cubic yard — before adding a waste factor.
Concrete reaches initial set in 24–48 hours (safe to walk on lightly). You can drive on a driveway slab after about 7 days. Full structural strength takes 28 days. Keep the slab moist for at least 7 days after pouring — this dramatically increases long-term strength.