Concrete Yardage Chart: Slab Volume, Bags, and Truck Loads

The yardage formula

Convert every dimension to feet before multiplying. Thickness is the one that gets missed: 4 inches is 4 ÷ 12 = 0.333 ft, not 4. Then divide the result by 27, the number of cubic feet in a cubic yard (3 × 3 × 3).

Multiplying 27 by 12 gives a shortcut that works at any slab size: one cubic yard covers 324 ÷ thickness (in inches) square feet. At 4 inches that is 81 sq ft, and at 6 inches it is 54 sq ft. If your result is far from those figures, a conversion went wrong.

Slab volume chart

Volume grows in direct proportion to both area and thickness, so the chart can be extended without another table. The 5-inch column is always 1.25 times the 4-inch column, and the 6-inch column is 1.5 times it. Two 20 x 20 ft slabs at 4 inches need 2 × 4.94 = 9.88 cubic yards.

For an L-shaped or otherwise irregular slab, split the plan into rectangles, look up or calculate each one, and add the results. Subtract openings only when they are large and fully formed out, such as a floor penetration or a sump pit. Small blockouts for posts rarely change the order by a measurable amount.

Thickness conversions

Lumber sizes cause more thickness errors than arithmetic does. A nominal 2 x 4 laid on edge is 3.5 inches tall, and a 2 x 6 is 5.5 inches tall. A slab screeded to the top of 2 x 4 forms over a level subgrade is 3.5 inches thick, which is the IRC R506.1 minimum for slab-on-ground floors. Local amendments can change that minimum.

The difference matters for the order. A 3.5-inch slab needs 1.08 cubic yards per 100 sq ft. A 4-inch slab needs 1.23, which is 14% more. Pick the design thickness first, then choose form lumber that produces it, and price the order from the thickness you will actually form.

Bag counts for 60 lb and 80 lb mixes

Bag labels list a yield: about 0.45 cubic feet for a 60 lb bag and about 0.60 cubic feet for an 80 lb bag. The calculator uses those two figures. Yields differ slightly between brands, so read the label on the product you buy.

One cubic yard is 27 cubic feet. That takes 60 bags of 60 lb or 45 bags of 80 lb, which is 3,600 lb of dry mix either way.

Bags suit repairs, post holes, steps, and small pads. On a slab of several yards, hand mixing stops being practical. A 20 x 24 ft slab at 5 inches needs 334 bags of 80 lb, or 26,720 lb of dry mix, and mixing that many batches takes hours. The first batch can start to set before the last one is placed, which can leave visible seams and cold joints.

Footings and round piers

The calculator treats a footing as a rectangular prism: length times width times depth. A 100 ft run of footing 16 inches wide and 8 inches deep is 100 × 1.333 × 0.667 = 88.89 cubic feet, or 3.29 cubic yards.

Step-downs, thickened slab edges, and pad sections at columns are separate prisms. Calculate each one and add it to the run. A footing poured into an unformed trench takes the shape of the trench, and sandy or soft soil sloughs into it. Measure the finished trench width and depth at several points, and use the averages instead of the drawn dimensions.

A round pier is a cylinder: π × (diameter ÷ 2)² × depth. A 12-inch pier is 1 ft across, so a 4 ft deep pier holds π × 0.5² × 4 = 3.14 cubic feet. The engine reads the pier depth from its length field.

Worked example: 20 x 24 ft slab, 5 inches thick

1. Area: 20 × 24 = 480 sq ft.
2. Volume: 480 × 5 ÷ 12 = 200 cubic feet.
3. Net cubic yards: 200 ÷ 27 = 7.41.
4. Add 10%: 7.41 × 1.10 = 8.15 cubic yards to order.
5. Trucks: 8.15 cubic yards fits in 1 load at the 10 cubic yard planning capacity.

Ten percent of a 5-inch slab is half an inch of average extra depth. It is a margin for normal subgrade variation. It does not cover a systematic error. Suppose grading leaves the subgrade 3/4 inch low on average, so the slab averages 5.75 inches. The volume becomes 480 × 5.75 ÷ 12 ÷ 27 = 8.52 cubic yards, which is 0.37 cubic yards more than the 8.15 ordered. Each quarter inch of average depth over 480 sq ft is 10 cubic feet, or 0.37 cubic yards.

Ordering ready-mix

Give the plant the volume in cubic yards, the compressive strength from the plans or local code, the slump you want, the maximum aggregate size, and whether the concrete needs air entrainment for freeze-thaw exposure. Add the placement method (chute, wheelbarrow, or pump), the site address, and a delivery window.

The calculator assumes a mixer truck holds 10 cubic yards. That is a planning figure. Actual capacity depends on the truck and on local weight limits, so ask the plant what it will send. A full 10 cubic yard load weighs about 40,500 lb at 4,050 lb per cubic yard, so check the route across any driveway, septic lid, or culvert before the truck arrives.

Many plants charge an extra fee when an order, or the last load of an order, falls below a set quantity. The threshold and the fee differ from plant to plant. Ask for both before you settle on a number, because a small change in the order can move it across the threshold.

Time matters once the truck is loaded. ASTM C94 sets a default limit of 1.5 hours or 300 drum revolutions after batching, whichever comes first, unless the buyer and the supplier agree otherwise. Have the forms, reinforcement, and finishing crew ready before the truck leaves the plant. See the material waste guide for how allowances compare across materials, and the rebar weight chart for reinforcement quantities.

Why pours run over or short

• Low subgrade. Over-excavated spots, wheel ruts, and fill that settles all add depth. An average of half an inch extra uses the whole 10% allowance on a 5-inch slab. Fill low spots with compacted base, not concrete.
• Nominal lumber. Forms made from 2 x 4s give 3.5 inches, not 4. Forms that bow outward under the weight of wet concrete add volume along their whole length. Stake and brace them so the formed depth matches the order.
• Rain before the pour. Standing water in the forms must be removed, and a saturated subgrade ruts under wheelbarrows and washes at the edges. Re-grade and re-compact softened areas, then re-measure depth before you confirm the quantity. Rain during a pour shortens the usable window, so check the C94 time limit above.
• Water added to fix slump. Water raises slump, but it does not make more useful concrete. It lowers strength and raises shrinkage cracking. If the mix is too stiff to place, ask the plant about a water-reducing admixture.
• A missing division. Forgetting the ÷ 12 on thickness makes a slab look 12 times too large, and forgetting ÷ 27 makes it look 27 times too large. Check the result against the 324 ÷ thickness shortcut.
• Ordering exactly the net volume. Concrete cannot be added later once the last truck is gone. A late top-up load leaves a cold joint. Order a margin, and plan a use for leftover concrete, such as a small pad or stepping stones in a form set aside for that purpose.

Reinforcement, control joints, and sub-base thickness change the design and sometimes the volume. Follow the plans and the engineer of record, and check the calculator again whenever the formed dimensions change.

Frequently asked questions