Concrete Slab Calculator
Estimate cubic yards of concrete for a rectangular slab, patio, driveway, or footing. Enter length, width, and thickness with an optional waste allowance for spillage and over-excavation.
Assumptions
- Slab is a uniform rectangular prism with no steps or thickened edges.
- Thickness is consistent across the entire pour area.
- Default waste factor is 10% for typical residential pours.
- 1 cubic yard = 27 cubic feet.
- Does not include gravel base, rebar, or pump truck fees.
Volume = length × width × thickness (converted to cubic yards). The result adds your waste percentage: ordered volume = base volume × (1 + waste% ÷ 100).10 ft × 10 ft patio, 4 in thick
- Area = 10 × 10 = 100 ft². Thickness 4 in = 0.333 ft. Volume = 100 × 0.333 = 33.3 ft³ = 1.23 yd³. With 10% waste: 1.23 × 1.10 ≈ 1.36 yd³ to order.
How do you calculate concrete for a slab in cubic yards?
Multiply length, width, and thickness in consistent units, then convert. Thickness is almost always given in inches, so divide by 12 to get feet: a 4 in slab is 0.333 ft. Volume in cubic feet equals length times width times thickness in feet, and cubic yards equals that figure divided by 27, because one cubic yard contains 27 cubic feet. A 12 ft by 12 ft patio at 4 in works out to 144 × 0.333 = 48.0 ft³, or 1.78 yd³. A useful shortcut: at 4 in every square foot needs 0.333 ft³, and one cubic yard of concrete covers about 81 ft². The calculator then applies your waste percentage, so ordered volume = base volume × (1 + waste divided by 100).
Measure the forms, not the finished room. Outdoors, run the tape along the inside face of the formwork at grade and record the dimension the concrete will actually occupy. Subtract only areas that receive no concrete, such as a planter cut into the middle of a patio. Remember that the formula treats the slab as a uniform rectangular prism: it does not know about steps, thickened perimeter beams, footings, or a subgrade that dips half an inch across part of the pour. Each of those quietly adds volume. The result also excludes gravel base, reinforcing steel, vapor retarder, and pump or short-load fees. Treat it as a planning estimate, not a structural design.
How thick does a concrete slab need to be?
IRC Section R506.1 requires concrete slab-on-ground floors to be designed and built in accordance with Section R506 or ACI 332, and to be a minimum of 3-1/2 in thick [1]. Section R506.2.2 calls for a 4 in base course of clean graded sand, gravel, crushed stone, crushed concrete, or slag passing a 2 in sieve where the slab sits below grade, and R506.2.3 requires a 6 mil polyethylene vapor retarder with joints lapped not less than 6 in between the slab and that base [1]. Bagged-mix makers set their own floor: Quikrete Concrete Mix is designed for pouring concrete 2 in thick or more [2], and Sakrete flatwork instructions tell you to excavate deep enough for gravel plus a minimum of 4 in of concrete [3].
Thickness scales volume linearly, so the decision costs real money. Moving a 400 ft² pad from 4 in to 5 in raises the base volume from 4.94 yd³ to 6.17 yd³, a 25 percent jump before any waste is added. That is why patios and walkways usually stay at 4 in while driveways, garage floors, and pads under heavy equipment are commonly specified thicker. Those thicker cases are engineering decisions rather than estimating decisions: vehicle loads, frost depth, expansive soils under IRC Section R403.1.8, and local code amendments all matter, and any slab carrying walls or columns needs a designer. Use this calculator to price the thickness you have been given, not to choose it.
How much concrete do I need for a 12x12 patio?
Work the numbers once, in order. Area is 12 × 12 = 144 ft². Thickness of 4 in becomes 0.333 ft, so base volume is 144 × 0.333 = 48.0 ft³, and 48.0 divided by 27 is 1.78 yd³. Apply the default 10 percent allowance: 1.78 × 1.10 = 1.96 yd³, which you would order as 2 yd³. Now add the thickened edge the calculator ignores. A 48 ft perimeter with an extra 4 in of depth over an 8 in wide strip is 0.333 × 0.667 = 0.222 ft² of cross section, or 0.222 × 48 = 10.7 ft³, which is 0.40 yd³. The named total becomes about 2.36 yd³, so order 2.5 yd³.
Compare that with bags before you commit. At 0.60 ft³ per 80 lb bag, 1.96 yd³ is 52.9 ft³ and needs 89 bags, roughly 7,100 lb of material to buy, carry, and mix in one continuous session. Raising the same patio to 5 in lifts base volume to 2.22 yd³, or 2.44 yd³ with waste. Notice that every intermediate figure stayed unrounded until the final step, because rounding thickness or area early is the most common way a slab estimate drifts by a quarter yard. Ready-mix suppliers usually sell in quarter-yard or half-yard increments and charge for short loads, so confirm the increment before you round.
Is it cheaper to order ready-mix or mix bags for a slab?
There is no universal break-even point, but the arithmetic is simple. One cubic yard is 27 ft³, so it takes 45 bags of 80 lb mix at 0.60 ft³ each, 60 bags of 60 lb at 0.45 ft³, or 90 bags of 40 lb at 0.30 ft³ [2]. Below roughly one cubic yard, bags usually win because ready-mix carries a minimum load and short-load surcharges. Above two or three cubic yards the truck almost always wins on both price and labor. Compare the delivered ready-mix quote, including short-load and waiting-time charges, against whole-bag count times current bag price plus mixer rental. Those are arithmetic scenarios, not market quotes.
Quality and timing matter as much as price. Quikrete and Sakrete both list 4,000 psi at 28 days for their concrete mixes tested to ASTM C39, with a 2 in to 3 in slump range [2][3], so bagged material is not inherently weak, but consistency depends on whoever is adding the water. Both manufacturers also tell you to fill forms to full depth and work continuously from one end to the other, because partial-depth placement creates cold joints [2][3]. Ninety bags mixed by two people over an afternoon is a genuine cold-joint risk. This calculator estimates volume only; it does not price material, select a mix design, or schedule labor.
How much extra concrete should you order for waste?
The 10 percent default suits most residential flatwork. Use about 5 percent for a small, tightly formed rectangle on a screeded, well-compacted subgrade where you can measure everything twice. Use 12 to 15 percent for curves, multiple levels, sloped sites, or ground that has been over-excavated. The loss is real rather than theoretical: subgrade dips, forms that bow outward under the weight of fresh concrete, spillage from the chute and wheelbarrow, material clinging to the mixer drum, and test cylinders all take volume out of the load. Over-excavating by just half an inch across 400 ft² is 16.7 ft³ on its own, or 0.62 yd³.
Running short is far more expensive than running long. A pour that stops while someone drives for more material leaves a cold joint that is both a structural and a cosmetic defect, and a second small delivery usually carries its own minimum charge. Add the allowance, then round up to the increment your supplier actually sells. Keep the allowance separate from thickness in your notes so that nobody later confuses a 10 percent buffer with a thicker slab. Finally, remember that the allowance covers estimating uncertainty only. It does not compensate for an unverified thickness, a missing base course, or reinforcement that was never designed.
How to measure
Measure the longest straight length and width of the pour area at grade. For thickness, use the specified slab depth from your plan — typical patios are 3.5–4 in and driveways 4–6 in. Deduct only areas that will not receive concrete.
How many cubic yards does a slab need at each thickness?
| Slab area | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 50 ft² | 0.62 yd³ | 0.77 yd³ | 0.93 yd³ |
| 100 ft² | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 144 ft² (12 × 12) | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 200 ft² | 2.47 yd³ | 3.09 yd³ | 3.70 yd³ |
| 400 ft² (20 × 20) | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 600 ft² | 7.41 yd³ | 9.26 yd³ | 11.11 yd³ |
Base volume before any waste allowance. Area times thickness in feet, divided by 27.
What slab thicknesses do codes and manufacturers reference?
| Element | Thickness | ft³ per ft² of area | Basis |
|---|---|---|---|
| Slab-on-ground floor, code minimum | 3-1/2 in | 0.29 | IRC Section R506.1 minimum thickness |
| Minimum depth for bagged concrete mix | 2 in | 0.17 | Quikrete Concrete Mix is designed for pours 2 in or more |
| Residential patio or walkway | 4 in | 0.33 | Sakrete flatwork instructions call for at least 4 in of concrete |
| Base course under a below-grade slab | 4 in of gravel | not concrete | IRC Section R506.2.2 graded material passing a 2 in sieve |
| Driveway or parking pad | 5 in | 0.42 | Common residential practice; confirm locally |
| Slab under vehicle or equipment loads | 6 in | 0.50 | Engineering decision, not an estimating default |
Thickness is a code and design input. This calculator converts it to volume; it does not select it.
How much does a waste allowance add to a concrete order?
| Base volume | Plus 5 percent | Plus 10 percent | Plus 15 percent |
|---|---|---|---|
| 1.00 yd³ | 1.05 yd³ | 1.10 yd³ | 1.15 yd³ |
| 1.23 yd³ | 1.29 yd³ | 1.35 yd³ | 1.41 yd³ |
| 1.78 yd³ | 1.87 yd³ | 1.96 yd³ | 2.05 yd³ |
| 2.47 yd³ | 2.59 yd³ | 2.72 yd³ | 2.84 yd³ |
| 4.94 yd³ | 5.19 yd³ | 5.43 yd³ | 5.68 yd³ |
| 7.41 yd³ | 7.78 yd³ | 8.15 yd³ | 8.52 yd³ |
Round the final figure up to the increment your supplier sells, commonly a quarter or half yard.
Common mistakes
- Measuring thickness in feet when the field is in inches (4 in ≠ 4 ft).
- Forgetting to add waste for spillage, uneven subgrade, or partial loads.
- Using interior room dimensions instead of the actual form or edge dimensions outdoors.
Frequently asked questions
How many cubic yards of concrete do I need for a 10x10 slab 4 inches thick?
How much concrete do I need for a 20x20 slab?
How thick should a concrete patio slab be?
How many bags of concrete are in a cubic yard?
How many square feet does a yard of concrete cover?
Does a 4 inch concrete slab need rebar or wire mesh?
How far apart should control joints be in a concrete slab?
Sources and references
Last updated:
- 2015 International Residential Code, Section R506 Concrete Floors (On Ground) — International Code Council, excerpt published by the City of Richmond, Kentucky Building Inspection Division (2015)
- Concrete Mix product no. 1101 data sheet: bag yields, mixing water, strength, and joint spacing — The Quikrete Companies (2022)
- High Strength Concrete Mix technical data sheet: yields, water, flatwork and joint instructions — Sakrete of North America (2016)
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