Concrete Calculator

What this sheet does that a plain volume calculator does not

Most concrete calculators multiply three numbers and stop. This sheet is built around the parts of the job that actually change the order:

• Several sections in one order. An L-shaped patio, a pad plus a landing, a driveway with an apron: enter each rectangle and the yardage is summed into a single order quantity.
• Thickened edge (turndown). A monolithic slab with a turned-down perimeter is not a rectangular prism; the extra concrete is calculated from the perimeter, not guessed at.
• Bags and ready-mix side by side, with the bag count and the weight you would handle next to the ready-mix order.
• Short-load fee on its own line. Enter your producer's minimum load and fee, and the sheet shows the fee whenever the order falls under that minimum.
• A takeoff you can take with you, exported as CSV or printed, with the assumptions attached.
• Dimensioned drawings, a plan and a section, so a wrong entry is visible before it is ordered.

How to calculate concrete for a slab

1. Get the area. Length times width, in feet. Add up every section you are pouring on the same day.
2. Convert the thickness to feet. Inches divided by 12. A 4 inch slab is 0.3333 ft, 6 inches is 0.5 ft.
3. Multiply. Area times thickness gives cubic feet.
4. Divide by 27. A cubic yard is 3 ft by 3 ft by 3 ft.

Area            A  =  L x W                       (sum every section)
Net volume      V  =  A x T                       (T in feet)
Turndown        Ve =  P x We x (De - T)           (optional thickened edge)
Cubic yards     Y  =  (V + Ve) / 27
Order quantity  Yo =  ceil( Y x (1 + waste) / 0.25 ) x 0.25
Bags            B  =  ceil( (V + Ve) x (1 + waste) / bag yield )
Truck loads     N  =  ceil( Y x (1 + waste) / 10 )
Weight          M  =  (V + Ve) x (1 + waste) x unit weight
Joints          J  =  24 to 36 x T

At 4 inches thick, one cubic yard covers 81 square feet; at 6 inches, 54 square feet. Weight is not a constant: normal-weight concrete runs about 140 to 155 lb per cubic foot according to FHWA, rising with compressive strength. This sheet uses 150 lb/ft³ for hauling estimates, editable under Advanced, as is the 10 cu yd truck capacity.

Worked example: a 20 x 12 patio at 4 inches

1. Area: 20 ft x 12 ft = 240 sq ft.
2. Thickness: 4 in / 12 = 0.3333 ft.
3. Volume: 240 x 0.3333 = 80 cu ft.
4. Cubic yards: 80 / 27 = 2.96 cu yd net.
5. Waste at 10 percent: 2.96 x 1.10 = 3.26 cu yd.
6. Order quantity: rounded up to the next quarter yard, this sheet's rounding step (confirm your plant's) = 3.5 cu yd.
7. Weight at the default 150 lb/ft³: 88 x 150 = 13,200 lb.
8. Contraction joints: 24 to 36 x 4 in = 8 to 12 ft apart.
9. If you bagged it instead: 88 / 0.45 = 196 bags of 60 lb mix. That is 11,760 lb of bags to buy, haul and mix. Order the truck.

How thick should the slab be?

One number here is code, not practice: IRC 2021 R506.1 sets a 3.5-inch minimum for slab-on-ground floors of dwellings; local adoption governs, and it says nothing about patios or driveways. Everything in the table below is ordinary practice, attributed to nobody: a sanity check, not a substitute for a drawing or a permit.

One inch of extra thickness on a 1,200 sq ft slab is 100 cu ft, or 3.7 cubic yards, which is why nobody rounds thickness up "to be safe" on a large pour.

Bags: yields and counts

Bagged mix is sold by dry weight but used by volume, so the number that matters is the yield: how much mixed concrete one bag makes. These are the yields printed on the QUIKRETE Concrete Mix No. 1101 data sheet; yields vary slightly by manufacturer, so check the bag you are actually buying.

The data sheet gives these yields as approximate. The calculator rounds up to the whole bag, because you cannot buy 0.4 of one; the waste allowance above is the only margin it adds.

Bags or ready-mix?

The sheet cannot source a volume at which bagging stops making sense, so it prints what each choice means instead. At the data sheet's yield, one cubic yard is 60 bags of 60 lb mix: 3,600 lb of dry material to buy, load, unload and mix in batches.

Set that against a delivered quote: ask your plant for its minimum load and any short-load fee, enter them under Advanced, and compare the two totals for your own pour.

Short-load fees and the rest of the ticket

If your producer quotes a short-load fee, enter its minimum load and the fee under Advanced: when your order falls below that minimum the fee appears as its own line on the takeoff and in the cost. Other line items that are not in the per-yard price and belong in your estimate:

• Fuel or environmental surcharge: ask whether one applies and how it is charged.
• Standby time: ask how long the truck may stay on site before it is billed.
• Admixtures your mix design calls for (accelerator, retarder, fiber, air entrainment).
• Pump or conveyor when the truck cannot reach the forms, quoted separately by the pumping company.

The price field applies your delivered price per cubic yard to the rounded order quantity, so the line it produces is the concrete only; everything above is yours to add.

Thickened edge (turndown)

A monolithic slab with a turned-down perimeter, common for detached garages, shops and sheds in areas with a shallow frost line, is poured in one shot: a field thickness plus a deeper strip around the outside. On the 20 x 12 example, a 12 inch deep by 12 inch wide turndown adds 64 linear feet of edge x 1 ft x (12 in − 4 in) = 42.7 cu ft, which is 1.58 cu yd — a 53 percent increase over the 2.96 cu yd field slab. Switch it on in the advanced panel and both the drawing and the order quantity update.

Gravel base

IRC 2021 R506.2.2 calls for a 4-inch base course of clean graded sand, gravel, crushed stone, crushed concrete or crushed slag passing a 2-inch sieve where the slab is below grade, with an exception for well-drained Group I sand-gravel soils. R506.2.3 then calls for a minimum 10-mil vapor retarder meeting ASTM E1745 Class A, joints lapped not less than 6 inches, between the slab and the base course. Local adoption governs.

Set the base depth under Advanced and it becomes its own takeoff line. Stone is placed compacted but sold loose, so the sheet orders the compacted volume divided by a 0.89 load factor for dry gravel — about 12 percent more than the hole. To price and order that base by the ton, carry these dimensions to sheet C-101, the gravel calculator: it opens on this slab's length, width and base depth and gives short tons, metric tonnes and truck loads with the density source printed next to the number.

Control joints

A slab cracks; joints decide where. NRMCA CIP 6 puts maximum contraction joint spacing at 24 to 36 times the slab thickness, about 10 feet for a 4-inch slab, capped at 15 feet. The sheet prints that range as Control-joint spacing next to the yardage: 8 to 12 ft at 4 inches, 10 to 15 ft at 5 inches, 12 to 15 ft at 6 inches. Cut or tool them within the first day, a quarter of the slab depth deep, and keep panels close to square.

Waste allowance

Ten percent is the default because the subgrade is never perfectly flat, the forms bow slightly, and some concrete stays in the chute. A small, well-prepared pad on a gravel base can work at 5 percent; uneven ground or long thin forms, 15 percent is not generous. The allowance is yours to set, 0 to 25 percent: being short on a pour is far more expensive than being over.

Frequently asked

How do I calculate concrete for a slab?

Multiply length by width to get square feet, multiply by the thickness in feet, then divide by 27. A 20 by 12 ft slab at 4 inches is 240 sq ft x 0.3333 ft = 80 cu ft = 2.96 cu yd. Add 10 percent for waste and you order 3.5 cu yd: this sheet rounds up to the next quarter yard, so ask your plant which increment it sells in.

How many bags of concrete do I need for a 4x8 slab?

At 4 inches thick a 4 by 8 ft pad is 10.67 cu ft, or 11.73 cu ft with 10 percent waste. That is 20 bags of 80 lb, 27 bags of 60 lb, or 40 bags of 40 lb mix. Bagging a pad this size is reasonable; much beyond it, price a delivery.

How much is 1 yard of concrete right now?

There is no national price, and we will not print a fake one. Ready-mix is quoted per cubic yard delivered and it moves with your plant, your mix design, your haul distance and the season. Call two plants, ask for the delivered price per yard, minimum load, short-load fee and fuel surcharge, then put those numbers above.

How much does a 20x20 slab cost?

A 20 by 20 ft slab at 4 inches is 400 sq ft, 4.94 cu yd net, 5.5 cu yd ordered with waste. The concrete is only part of the bill: base gravel, forms, reinforcement, the pour crew, finishing, control joints and any pump or wheelbarrow access all price separately. Multiply your quoted yard price by 5.5 and treat that as the material line only.

Limits of this sheet

S-101 calculates rectangular slabs and the turndown around them. It does not size reinforcement, check bearing capacity, design a footing, or tell you whether 4 inches is enough for your loads. Its unit weight (150 lb/ft³) and truck capacity (10 cu yd) are editable assumptions, not measured values. Circular pads, tapered sections and openings are not deducted; break the pour into rectangles or calculate those separately.