What a 30x40 slab is for
A 30x40 slab is a big shop floor: the inside of a pole barn, a three-car garage with a workshop, or a garage for an RV or a boat on a trailer.
Concrete for a 30x40 slab at 4, 5 and 6 inches
Every inch of thickness on a 30x40 slab is 100 cubic feet, or 3.7 cubic yards. The ready mix order and the bags in this table include 10 percent extra. Change the thickness or the extra in the calculator above.
| Thickness | Cubic feet | Cubic yards | Ready mix to order | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 400 | 14.81 | 16.3 cu yd | 734 | 978 |
| 5 in | 500 | 18.52 | 20.37 cu yd | 917 | 1,223 |
| 6 in | 600 | 22.22 | 24.44 cu yd | 1,100 | 1,467 |
The 4 inch row by hand: 40 × 30 × 0.333 = 400 cubic feet, and 400 ÷ 27 = 14.81 cubic yards. With 10 percent extra that is 440 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 734 of one or 978 of the other.
Quikrete says a sidewalk should be at least 4 inches thick and small slabs normally run 4 to 6 inches, depending on the load. The concrete slab calculator page covers what the building code asks for.
Bags or a truck for a 30x40 slab
From bags, it is 734 bags of 80 lb mix at 4 inches, 58,720 lb to lift and mix, about $4,176 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 16.3 cubic yards, about $1,793 to $2,445 at HomeAdvisor's $110 to $150 per cubic yard delivered. The National Ready Mixed Concrete Association says a truck mixer carries 8 to 12 cubic yards, so this is 2 to 3 truck loads. The association says to tell the producer the rate of delivery you need, so each load arrives when you are ready for it.
This is a truck job. 734 bags would be 58,720 lb to mix by hand, and they cost about $1,731 to $2,383 more than the ready mix. Check the route to the slab first. The association notes that a loaded truck weighs about 80,000 lb and may not be able to cross loose dirt or a residential curb.
| Thickness | 80 lb bags | Bags weigh | Bags cost | Ready mix | Ready mix cost |
|---|---|---|---|---|---|
| 4 in | 734 | 58,720 lb | $4,176 | 16.3 cu yd, 2 to 3 truck loads | $1,793 to $2,445 |
| 5 in | 917 | 73,360 lb | $5,218 | 20.37 cu yd, 2 to 3 truck loads | $2,241 to $3,056 |
| 6 in | 1,100 | 88,000 lb | $6,259 | 24.44 cu yd, 3 to 4 truck loads | $2,688 to $3,666 |
Prices are from the sources below, checked in October 2026; multiply the table by your own to compare. Gravel, steel, forms, finishing and labor are extra.
Pouring a 30x40 slab
At 4 inches, 16.3 cubic yards, which takes two trucks; at 6 inches, 24.4 cubic yards, which takes two or three. Many crews pump a slab this size, or pour it in two halves on different days with a construction joint between them. In a pole barn, the floor is often poured after the building is up, so the trucks have to reach it through the doors or the slab is pumped in.
Gravel base and slope
A 4 inch gravel base under a 30x40 slab is 400 cubic feet, or 14.81 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 300 cubic feet, or 11.11 cubic yards. Those are volumes of the finished layer. Gravel settles when it is compacted, so order a little more.
Outside, Quikrete says to slope the slab away from buildings and gives 1/4 inch per foot as the usual fall. Running across the 30 foot side, that is 7 1/2 inches from one edge to the other. Running along the 40 foot side, it is 10 inches. Set the forms to the slope, and the screed follows them.
Rebar or wire mesh
With bars 24 inches on center both ways, held 3 inches back from each edge, a 30x40 slab takes 16 bars 39 feet 6 inches long and 21 bars 29 feet 6 inches long. That is 1,252 feet of bar. With 10 percent extra, buy 69 bars 20 feet long. Every bar is longer than the 20 foot length sold, so each one is made of pieces that overlap. Fridley, Minnesota, says to overlap joined bars by at least 40 bar diameters, which is 20 inches for a 1/2 inch #4 bar. The 10 percent extra is there for those overlaps and the offcuts.
With wire mesh instead, 16 sheets 8 feet by 12.5 feet, lapped 8 inches, cover it. The spacing and the sheet size are where the calculator starts. Your plans or your building department set the steel for your slab.
Control joints
Quikrete says control joints in a 4 inch slab go every 6 to 8 feet and about a quarter of the slab deep, which is 1 inch. Keeping them no more than 8 feet apart cuts a 30x40 slab into 5 panels along the 40 foot side and 4 along the 30 foot side. That makes 20 panels, each 8 feet by 7 feet 6 inches, with 240 feet of joint to cut.
Before you pour a 30x40 slab
This is past the 720 square foot limit of the floating garage slab in the Dubuque handout, so the foundation needs its own design, often frost footings or a post frame. Ask your building department before you set forms. A floor this big is also a full day of finishing, so book a crew that has done one.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 30x40 slab?
A 30 by 40 foot slab 4 inches thick holds 400 cubic feet of concrete, which is 14.81 cubic yards. With 10 percent extra, order 16.3 cubic yards of ready mix. At 5 inches thick it holds 18.52 cubic yards and at 6 inches 22.22.
How many 80 lb bags of concrete do I need for a 30x40 slab?
734 bags at 4 inches thick, with 10 percent extra. They weigh 58,720 lb in all and cost about $4,176 at the store price checked in October 2026. At 5 inches thick it is 917 bags and at 6 inches 1,100.
How thick should a 30x40 concrete slab be?
Quikrete calls for at least 4 inches for a sidewalk and 4 to 6 inches for small slabs, depending on the load. The International Residential Code minimum for a house floor slab is 3.5 inches. On a 30x40 slab each extra inch adds 3.7 cubic yards, so going from 4 to 6 inches takes the ready mix order from 16.3 to 24.44 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
Should a 30x40 shop floor be thicker than 4 inches?
It depends on what it carries. Quikrete gives 4 to 6 inches for small slabs, depending on the load. For trucks, a lift or heavy machinery, the floor needs to be designed for that load, so ask your building department or an engineer before you order.
How many concrete trucks for a 30x40 slab?
Two at 4 inches thick, because a truck carries 8 to 12 cubic yards and the order is 16.3. At 6 inches, the 24.4 cubic yard order takes two to three trucks, depending on their size.
Tools for this job
Each link opens an Amazon search for that item. As an Amazon Associate I earn from qualifying purchases.
- Form stakes. Hold the form boards in place while you pour.
- Concrete float. Smooths the surface after the concrete is leveled.
- Edger. Rounds the slab edges so they do not chip.
- Groover. Cuts control joints so cracks follow a straight line.
- Finishing trowel. Gives the slab its final smooth surface.
- Rubber boots. Keep wet concrete off your skin while you stand in the pour.
Sources
- Quikrete, Pouring and Finishing Concrete Slabs
- Quikrete, Concrete Sidewalks and Small Slabs project guide
- Quikrete Concrete Mix data sheet (yield per bag)
- National Ready Mixed Concrete Association, CIP 31: Ordering Ready Mixed Concrete
- International Residential Code 2021, Chapter 5: concrete floors on the ground (R506)
- International Residential Code 2021, Chapter 4: foundations (R403, concrete cover over steel)
- Wire Reinforcement Institute, Manual of Standard Practice: Structural Welded Wire Reinforcement
- City of Dubuque, Iowa, minimum requirements for a detached garage slab
- City of Fridley, Minnesota, Garage Slabs handout
- PJ's Rebar, stock rebar sizes and lengths
- International Residential Code 2021, Chapter 1: work exempt from permit (R105.2)
- McCoy's Building Supply: Quikrete Concrete Mix, 80 lb bag
- HomeAdvisor: concrete delivery cost, per cubic yard and short-load fees
Facts on this page were last checked against these sources in October 2026.