A 20 × 20-foot rectangular slab at 4 inches of concrete thickness has 4.938272 cubic yards of net volume. With example 10% and 15% quantity allowances, the totals are 5.432099 and 5.679012 cubic yards. If your supplier accepts quarter-yard increments, those round up to 5.5 and 5.75 cubic yards. Confirm the allowance and ordering increment rather than treating either as a universal rule.
This guide calculates a uniform rectangular slab, not its structural thickness, reinforcement, base, joints or load rating. Use inside form dimensions and actual concrete thickness. The concrete calculator uses the same rectangular quantity model; thickened edges or other shapes need separate treatment.
The formula
Net cubic yards = length (ft) × width (ft) × concrete thickness (in) / 324.
With allowance = net cubic yards × (1 + allowance percent / 100). The 324 divisor is 12 inches per foot × 27 cubic feet per cubic yard. Keep full precision through allowance math, then round only to the confirmed delivery increment.
20 × 20 volume and allowance chart
Calculated thickness scenarios, not thickness recommendations. Net volume excludes any thickened edge, beam or base. Allowances are examples; no delivery rounding has been applied in this table.
| Concrete thickness | Net cu yd | With 10% | With 15% |
|---|---|---|---|
| 3 in | 3.703704 | 4.074074 | 4.259259 |
| 4 in | 4.938272 | 5.432099 | 5.679012 |
| 5 in | 6.172840 | 6.790123 | 7.098765 |
| 6 in | 7.407407 | 8.148148 | 8.518519 |
| 8 in | 9.876543 | 10.864198 | 11.358025 |
Other rectangular sizes: net cubic yards
No allowance or delivery rounding. These dimensions compare quantities only; they do not approve a garage, driveway or patio design.
| Inside length × width | 4 in concrete | 5 in concrete | 6 in concrete |
|---|---|---|---|
| 10 × 10 ft | 1.234568 | 1.543210 | 1.851852 |
| 12 × 12 ft | 1.777778 | 2.222222 | 2.666667 |
| 12 × 20 ft | 2.962963 | 3.703704 | 4.444444 |
| 20 × 20 ft | 4.938272 | 6.172840 | 7.407407 |
| 24 × 24 ft | 7.111111 | 8.888889 | 10.666667 |
| 24 × 30 ft | 8.888889 | 11.111111 | 13.333333 |
Coverage per cubic yard
Square feet per cubic yard = 324 / thickness in inches. Exact coverage is 81 sq ft at 4 inches, 64.8 sq ft at 5 inches and 54 sq ft at 6 inches. Schmitz’s table rounds the 5-inch figure to 65.1 That is a convenient display approximation, not the exact value to use for a precision takeoff.
Three checked examples
1. 20 × 20 ft at 4 inches
Net volume = 20 × 20 × 4 / 324 = 4.938272 cu yd. With 10% example allowance: 5.432099 cu yd; with 15%: 5.679012 cu yd. If quarter-yard ordering applies, those round up to 5.5 and 5.75 cu yd. Confirm supplier increments and site quantity first. These are volume examples, not suitable-thickness advice.
2. 24 × 24 ft at 6 inches
Net volume = 24 × 24 × 6 / 324 = 10.666667 cu yd. With 10% example allowance: 11.733333 cu yd; with 15%: 12.266667 cu yd. If quarter-yard ordering applies, those round up to 11.75 and 12.5 cu yd. Confirm supplier increments and site quantity first. These are volume examples, not suitable-thickness advice.
3. 60 × 18 ft at 5 inches
Net volume = 60 × 18 × 5 / 324 = 16.666667 cu yd. With 10% example allowance: 18.333333 cu yd; with 15%: 19.166667 cu yd. If quarter-yard ordering applies, those round up to 18.5 and 19.25 cu yd. Confirm supplier increments and site quantity first. These are volume examples, not suitable-thickness advice.
The supplier source has an allowance inconsistency
Schmitz’s page says most contractors order 10% to 15% above computed volume and round up to the nearest quarter yard, but its worked example then multiplies by 105%, which is only a 5% allowance.1 We do not combine those into one consistent recommendation. The tables here explicitly calculate 10% and 15% as separate scenarios using exact geometry. Ask the actual supplier and installer what allowance and increment fit the job.
An added quantity allowance is not proof of a compaction factor and does not repair an incorrect thickness or missing edge volume. Uneven base, form movement and handling can change needs, but a generic percentage is not a measured takeoff. Delivery minimums, short-load fees, access and placement schedule also need checking.
How many bags is a 20 × 20 slab?
At 4 inches, net volume is 133.333333 cu ft. The cited QUIKRETE Concrete Mix sheet gives about 0.60 cu ft per 80-lb bag.2 Net bags: 133.333333 / 0.60 = 222.222222, rounded up to 223 bags. With example 10% allowance: 146.666667 / 0.60 = 244.444444, rounded to 245 bags. Check the exact product yield.
Those counts do not establish whether bag mixing is a practical or safe placement plan. Compare delivered ready-mix and bags using full quotes, transport, mixing equipment, labor, working time and placement requirements. No universal truck threshold or automatic cost winner is stated. See the slab bag chart for other yields.
Concrete thickness is not base or excavation depth
Gravel under the slab is a separate quantity. Do not add base thickness to the concrete thickness input. The cited QUIKRETE patio project describes a particular compacted base and concrete section.3 That example is not a specification for every patio or driveway. Confirm the actual section, soil preparation, frost requirements, reinforcement and loads before estimating.
For a thickened edge, calculate only the additional volume beyond the uniform slab already counted, or split the whole shape into non-overlapping regions. Do not add a full beam volume that overlaps the slab and count the same concrete twice. The tube-volume guide covers separate cylindrical quantities, not structural sizing.
FAQ
How many yards for a 20 × 20 slab?
Net volumes are 4.938272 cu yd at 4 inches, 6.172840 at 5 inches and 7.407407 at 6 inches. Add only the separately chosen quantity allowance and round to the confirmed order increment. Footprint alone is not enough.
Is one yard 65 sq ft at 5 inches?
Exact geometry gives 64.8 sq ft. The cited supplier rounds its table to 65. Use unrounded geometry when computing the total.
Should I order 10% or 15% extra?
This page compares those scenarios rather than choosing one for your site. Verify the measured section, missing shapes, supplier guidance and placement plan. The cited supplier page itself has conflicting percentage wording and example math.
Does this select a driveway or garage thickness?
No. The charts calculate material for chosen thicknesses. Local requirements, use, soil, reinforcement and loads need a separate check.
Related tools
Concrete calculator | Slab bag chart | Tube-volume guide
Once you have the yardage, see how it turns into a budget with national cost references and itemized bid checks. See the concrete cost guide.
Sources
- Schmitz Ready Mix: Concrete Calculator (rounded coverage and supplier allowance/ordering guidance; wording and example percentages differ)
- QUIKRETE: Concrete Mix sheet (approximate bag yield and product directions)
- QUIKRETE: Patios (a product project example, not universal slab design)