Direct concrete answer
How much concrete for a 20x20 driveway (6 inches thick)?
A 20 ft wide by 20 ft long by 6 inch driveway slab requires 7.41 cubic yards of concrete, or 334 bags of 80-lb premix before field waste. With 10% overage, order about 8.2 cubic yards.
Quick answer
| Project dimensions | 20 ft wide x 20 ft long x 6 in |
|---|---|
| Area or count | 400 sq ft |
| Volume | 200.00 cubic feet |
| Volume | 7.41 cubic yards |
| Metric volume | 5.66 cubic meters |
| 80-lb bags needed | 334 bags |
| 60-lb bags needed | 445 bags |
| 50-lb bags needed | 534 bags |
| Ready-mix order with 10% overage | 8.2 cubic yards |
| Ready-mix planning cost at $150/yd3 | $1,222 |
| Bagged planning cost at $5/80-lb bag | $1,670 |
Calculation steps
- Convert depth to feet: 6 inches / 12 = 0.500 ft.
- Calculate cubic feet: 20 ft x 20 ft x 0.500 ft = 200.00 ft3.
- Convert to cubic yards: 200.00 ft3 / 27 = 7.41 yd3.
- Add waste for ordering: 7.41 yd3 x 1.10 = 8.2 yd3. Round according to the supplier's ordering increment.
This is the exact math behind the answer, but the field order should reflect the actual formed dimensions, the base condition, and the supplier's minimum order policy. Measure the driveway as separate rectangles if it flares at the street or widens near the garage. This answer uses a clean 20 ft by 20 ft rectangle.
Number of bags by size
Bag counts use common premix yields and round up to whole bags. Always check the yield printed on the exact product before buying because specialty mixes can differ.
| Bag size | Common yield | Calculation | Bags to buy |
|---|---|---|---|
| 80-lb bag | 0.6 ft3 | 200.00 / 0.6 | 334 |
| 60-lb bag | 0.45 ft3 | 200.00 / 0.45 | 445 |
| 50-lb bag | 0.375 ft3 | 200.00 / 0.375 | 534 |
Cost snapshot
At a planning price of $150 per cubic yard, the waste-adjusted ready-mix concrete is about $1,222 before short-load, delivery, pump, labor, forms, base stone, reinforcement, tax, or finish upgrades. The 80-lb bag material estimate is about $1,670 before mixer rental or labor.
Cost is where small concrete jobs surprise people. A short-load fee can make the delivered price look high, while bagged concrete can look cheap until the crew has to mix 334 heavy bags fast enough to place and finish one continuous surface. Use the material number as a quote starting point, not as the final installed price.
Ready-mix vs bagged concrete
Driveways are usually ready-mix projects because vehicle slabs need consistent placement, finishing, and curing across the full surface.
As a rule of thumb, ready-mix becomes easier once a project is near or above one cubic yard. This page's base volume is 7.41 cubic yards, so the practical choice depends on access, crew size, weather, finish timing, and whether the supplier charges a short-load fee. If the pour must be continuous, the truck often reduces risk even when the invoice includes a delivery charge.
Recommended PSI and reinforcement
| Use | Typical planning PSI | Notes |
|---|---|---|
| Patio or walkway | 3,000 PSI | Light foot traffic on compacted base. |
| Garage floor | 3,500 PSI | Verify vapor barrier, joints, and reinforcement. |
| Driveway | 3,500 to 4,000 PSI | Use higher strength for trucks, RVs, or weak soil. |
| Footings | 3,000 to 4,000 PSI | Plan and local code control final mix. |
Use 3,500 to 4,000 PSI for passenger vehicles and consider 4,000 PSI or higher for trucks, RVs, trailers, or weak subgrade.
Driveway reinforcement depends on soil, base, vehicle loads, and local practice. #4 rebar at 12 to 18 inches on center or welded wire is common planning language, not a final design.
Project-specific notes
This answer is for a thicker two-car driveway pad. The geometry is straightforward, but the site details decide whether the estimate is clean in the field. Before ordering, compare the calculated dimensions against the actual form layout, the base depth, and the planned finish elevation.
- The 6 inch thickness increases durability planning but also raises the yardage by half compared with a 4 inch slab.
- This size can still trigger short-load pricing, so ask the supplier how they price partial truck loads.
- If the project includes thickened edges, a landing, a flare, a step, or a separate footing, calculate that concrete separately and add it to the base result.
Common mistakes
- Averaging a flared driveway into one width. Split the flare, lane, and parking pad into separate rectangles.
- Choosing 4 inches for heavy vehicles without checking base, reinforcement, and local driveway practice.
- Comparing bag cost to ready-mix cost without including mixer rental, labor, short-load fees, and finish timing.
The expensive mistake is running short during placement. The second most expensive mistake is ordering more concrete than the site can place before it starts setting. The best order is not the smallest number; it is the number that fits the measured work, the crew, the truck access, and the supplier's rounding policy.
What to say when ordering
I am estimating a 20 ft wide by 20 ft long by 6 inch driveway slab. My calculated volume is 7.41 cubic yards, and I want to plan around 8.2 cubic yards with 10% overage. Can you confirm the order size, mix strength, short-load fee, delivery charge, chute reach, and whether this job needs a pump or different placement method?
Related calculations
Sources and methodology
BuilderCalc uses standard geometric volume formulas, the 27 cubic feet per cubic yard conversion, common premix bag yields, and planning cost ranges that should be verified with local suppliers.