What a 20x24 slab is for
A 20x24 slab is a two-car garage with 4 more feet of depth than a 20x20: room for a pickup, or for a workbench along the back wall in front of a car. It also suits a large patio or a pole barn for a small tractor.
Concrete for a 20x24 slab at 4, 5 and 6 inches
Every inch of thickness on a 20x24 slab is 40 cubic feet, or 1.48 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 | 160 | 5.93 | 6.52 cu yd | 294 | 392 |
| 5 in | 200 | 7.41 | 8.15 cu yd | 367 | 489 |
| 6 in | 240 | 8.89 | 9.78 cu yd | 440 | 587 |
The 4 inch row by hand: 24 × 20 × 0.333 = 160 cubic feet, and 160 ÷ 27 = 5.93 cubic yards. With 10 percent extra that is 176 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 294 of one or 392 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 20x24 slab
From bags, it is 294 bags of 80 lb mix at 4 inches, 23,520 lb to lift and mix, about $1,673 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 6.52 cubic yards, about $717 to $978 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 less than one truck load. HomeAdvisor says an order under a full truckload can carry a short-load fee of $40 to $60 per cubic yard, and suppliers often have a minimum order.
By these prices a 20x24 slab costs about $695 to $956 less from a truck than from bags, and it saves lifting 23,520 lb of mix. A short-load fee or a minimum order comes out of that gap, so get a delivered price for 6.52 cubic yards and compare it with $1,673. It is a bag job only if that quote comes in higher or a truck cannot reach the slab. 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 | 294 | 23,520 lb | $1,673 | 6.52 cu yd, less than one truck load | $717 to $978 |
| 5 in | 367 | 29,360 lb | $2,088 | 8.15 cu yd, about one truck load | $897 to $1,223 |
| 6 in | 440 | 35,200 lb | $2,504 | 9.78 cu yd, about one truck load | $1,076 to $1,467 |
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 20x24 slab
About 6.5 cubic yards at 4 inches, which is under one full truck, so ask whether a short-load fee applies. At 6 inches it is 9.78 cubic yards, a full truck. With a 20 foot width, use a screed rail down the middle and a crew of three or four. If the truck cannot back up to the forms, rent a power buggy or plan on many wheelbarrow trips, because the concrete has to be placed and struck off before it starts to set.
Gravel base and slope
A 4 inch gravel base under a 20x24 slab is 160 cubic feet, or 5.93 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 120 cubic feet, or 4.44 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 20 foot side, that is 5 inches from one edge to the other. Running along the 24 foot side, it is 6 inches. Set the forms to the slope, and the screed follows them.
As a garage floor
Dubuque, Iowa, publishes a simple foundation for a detached garage of up to 720 square feet: a floor at least 4 inches thick on a grade beam, a thickened edge at least 12 inches deep and 12 inches wide under every wall, with 6x6 welded wire or #4 bars no more than 2 feet apart both ways. A 20x24 garage is 480 square feet. Your town may want something else, especially where the ground freezes deep, so take this as an example and ask first.
The grade beam is the outer foot of the slab, made 8 inches deeper than the floor. Around a 20x24 slab that is 84 square feet of edge, 56 cubic feet, or 2.07 cubic yards more concrete. With 10 percent extra, that adds 2.28 cubic yards, for an order of about 8.8 at 4 inches. Dig the edge deeper if the grade falls away, because Dubuque measures the 12 inches from the ground outside.
Rebar or wire mesh
With bars 24 inches on center both ways, held 3 inches back from each edge, a 20x24 slab takes 11 bars 23 feet 6 inches long and 13 bars 19 feet 6 inches long. That is 512 feet of bar. With 10 percent extra, buy 29 bars 20 feet long. The bars that run the 24 foot way are 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, 6 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 20x24 slab into 3 panels along the 24 foot side and 3 along the 20 foot side. That makes 9 panels, each 8 feet by 6 feet 8 inches, with 88 feet of joint to cut.
Before you pour a 20x24 slab
Decide which way the doors face before you form the slab. With 16 foot doors on the 20 foot side, you get the 24 foot depth for long vehicles. With the doors on the 24 foot side, two 9 foot doors fit with more wall between and beside them, but the bays are only 20 feet deep.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 20x24 slab?
A 20 by 24 foot slab 4 inches thick holds 160 cubic feet of concrete, which is 5.93 cubic yards. With 10 percent extra, order 6.52 cubic yards of ready mix. At 5 inches thick it holds 7.41 cubic yards and at 6 inches 8.89.
How many 80 lb bags of concrete do I need for a 20x24 slab?
294 bags at 4 inches thick, with 10 percent extra. They weigh 23,520 lb in all and cost about $1,673 at the store price checked in October 2026. At 5 inches thick it is 367 bags and at 6 inches 440.
How thick should a 20x24 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 20x24 slab each extra inch adds 1.48 cubic yards, so going from 4 to 6 inches takes the ready mix order from 6.52 to 9.78 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
How many people does it take to pour a 20x24 slab?
Three or four at least: two on the screed, one raking concrete ahead of it, and one starting the edges. Add people with wheelbarrows if the truck cannot reach the forms.
Will a pickup fit in a 20x24 garage?
With the 24 foot side as the depth, a full-size pickup 19 to 20 feet long fits with about 4 feet to spare, less the thickness of the walls. With the 20 foot side as the depth, it does not fit with the door closed.
How much concrete for a 20x24 garage slab with a thickened edge?
About 8.8 cubic yards with 10 percent extra: 6.52 for the 4 inch floor and 2.28 for a grade beam 12 inches deep and 12 inches wide around the edge, the kind Dubuque, Iowa, allows for a detached garage of up to 720 square feet.
Tools for this job
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- 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.