Quick answer: work out the volume in cubic yards (length × width × thickness in feet ÷ 27), add about 10%, and round up to the next half or whole yard your supplier sells. A 20 × 20 ft slab that is 5 inches thick is 6.17 yd³, which is 6.79 with 10% extra, so you would order 7 cubic yards. The same pour in 80 lb bags would take about 306 bags, roughly 12 tons, which is why most big jobs go to a truck.
Start with the Concrete Calculator. This guide covers what happens after the number: how much to add, what to ask the supplier, and how to prepare for the delivery.
Step 1: Calculate the volume
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
For a 20 × 20 ft slab, 5 in thick:
20 × 20 × 5 ÷ 12 = 166.7 ft³, and 166.7 ÷ 27 = 6.17 yd³.
For footings, columns and steps, calculate each piece separately and add them. Our cubic yard guide covers circles, triangles and L-shapes.
Step 2: Add extra
Ground is uneven, forms bulge, and trucks leave some concrete in the chute. A 10% allowance is a common starting point: 6.17 × 1.10 = 6.79 yd³. Round up to 7 yards. Ask your supplier how they round, since many sell in fixed increments. Running short mid-pour is worse than having a little left over.
Step 3: Compare with bags
Using QUIKRETE’s yield of 0.60 ft³ per 80 lb bag:
- 166.7 ft³ ÷ 0.60 = 278 bags with no allowance.
- With 10% extra: 183.3 ÷ 0.60 = 306 bags.
- Weight: 306 × 80 lb = 24,480 lb, about 12.2 tons.
Moving, mixing and placing that much by hand is a very big job, and concrete has to be placed before it sets. That’s the reason to order from a ready-mix supplier for anything beyond a small slab. For small jobs, bags win. As a rough decision point, one cubic yard is 45 bags of 80 lb mix; most people consider ready-mix once a project passes a yard or two. Check your supplier’s pricing for small loads, because many charge extra for short loads (we don’t quote figures, since they vary by region and supplier).
Step 4: Ask the supplier
Before the truck is scheduled, ask:
- What mix do I need? Strength, air entrainment for freeze-thaw climates, and slump suit the job. Ask your local supplier or contractor what they recommend for your use and climate.
- What are the minimum load, short-load and delivery charges?
- How long can the truck wait? Suppliers typically allow a limited unloading time, and extra time can cost extra.
- Can the truck reach the site? Concrete trucks are very heavy and can damage driveways, lawns and septic systems. Ask about the truck’s size and weight.
- Do I need a pump or wheelbarrows? If the truck can’t reach the forms, plan a pump or helpers.
Step 5: Prepare before it arrives
- Forms set, level and braced, with the base compacted.
- Reinforcement (if any) in place and on chairs.
- Tools out: screed board, bull float, edger, groover, hoses, wheelbarrows.
- Helpers lined up. Concrete sets on its own schedule, not yours.
- Weather checked. Very hot, cold or rainy conditions need extra care. Follow the supplier’s advice.
- Plan for curing. QUIKRETE recommends keeping bagged concrete moist for about 5 days in warm weather (70 °F or higher) or 7 days at 50 to 70 °F, and protecting from freezing for the first 48 hours. Ask your supplier about curing for your mix.
Never add water at the site without asking
Extra water makes concrete easier to place but weakens it. If the mix is too stiff, ask the driver or supplier what is acceptable.
Common mistakes
- Ordering the exact number. There is always some variation.
- Forgetting footings and thickened edges. They add volume.
- Not checking access. A heavy truck on a soft yard is a problem.
- Not having enough people. Placing, screeding and finishing is a team effort.
- Skipping the cure. Concrete gains strength as it cures.
Frequently asked questions
How many yards of concrete do I need for a 10×10 slab at 4 inches?
10 × 10 × 4 ÷ 12 ÷ 27 = 1.23 yd³, or about 1.36 with 10% extra. Check whether your supplier has a minimum load.
How much does a cubic yard of concrete cover?
At 4 inches thick, one yard covers about 81 ft². At 6 inches, 54 ft².
How many bags equal a yard of concrete?
45 bags of 80 lb mix (27 ÷ 0.60), 60 bags of 60 lb mix, or 90 bags of 40 lb mix.
How long does ready-mix take to harden?
It starts setting within hours, but strength keeps developing over days and weeks. Follow your supplier’s guidance on when you can walk on it and when you can drive on it.
Is ready-mix cheaper than bags?
For larger pours, usually per cubic yard, but compare real quotes, including delivery and short-load charges.
Sources
- QUIKRETE, >Concrete Mix Product No. 1101 data sheet: bag yields, curing.
- NIST, >Guide to the SI, Appendix B.8: conversions.
Planning guidance only. Mix design and placement should follow your supplier’s and local code’s requirements.