What a 20x20 slab is for
A 20x20 slab is about the smallest common size for a two-car garage, and it makes a large patio or the floor of a big workshop. Two cars fit, but with the doors open the space between them is tight.
Concrete for a 20x20 slab at 4, 5 and 6 inches
Every inch of thickness on a 20x20 slab is 33.3 cubic feet, or 1.23 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 | 133.3 | 4.94 | 5.43 cu yd | 245 | 326 |
| 5 in | 166.7 | 6.17 | 6.79 cu yd | 306 | 408 |
| 6 in | 200 | 7.41 | 8.15 cu yd | 367 | 489 |
The 4 inch row by hand: 20 × 20 × 0.333 = 133.3 cubic feet, and 133.3 ÷ 27 = 4.94 cubic yards. With 10 percent extra that is 146.7 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 245 of one or 326 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 20x20 slab
From bags, it is 245 bags of 80 lb mix at 4 inches, 19,600 lb to lift and mix, about $1,394 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 5.43 cubic yards, about $597 to $815 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 20x20 slab costs about $579 to $797 less from a truck than from bags, and it saves lifting 19,600 lb of mix. A short-load fee or a minimum order comes out of that gap, so get a delivered price for 5.43 cubic yards and compare it with $1,394. 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 | 245 | 19,600 lb | $1,394 | 5.43 cu yd, less than one truck load | $597 to $815 |
| 5 in | 306 | 24,480 lb | $1,741 | 6.79 cu yd, less than one truck load | $747 to $1,019 |
| 6 in | 367 | 29,360 lb | $2,088 | 8.15 cu yd, about one truck load | $897 to $1,223 |
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 20x20 slab
At about 5.4 cubic yards at 4 inches, or 8.15 at 6 inches, this is one truck. A 20 foot width needs a screed rail down the middle or a wet screed, and a crew of three or four: two on the screed, one moving concrete with a come-along rake, and one starting the edges. Lay out the control joints before the truck comes; at 8 feet apart the slab cuts into three panels each way.
Gravel base and slope
A 4 inch gravel base under a 20x20 slab is 133.3 cubic feet, or 4.94 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 100 cubic feet, or 3.7 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. Either way across a 20x20 slab, that is 5 inches from one edge to the other. 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 20x20 garage is 400 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 20x20 slab that is 76 square feet of edge, 50.7 cubic feet, or 1.88 cubic yards more concrete. With 10 percent extra, that adds 2.06 cubic yards, for an order of about 7.49 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 20x20 slab takes 11 bars each way, all 19 feet 6 inches long. That is 429 feet of bar. With 10 percent extra, buy 24 bars 20 feet long. Every bar is shorter than the 20 foot length sold, so none has to be joined.
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 20x20 slab into 3 panels each way. That makes 9 panels, each 6 feet 8 inches square, with 80 feet of joint to cut.
Before you pour a 20x20 slab
The floating slab in the Dubuque, Iowa, handout has to sit on level, original soil that bears at least 2,000 pounds per square foot, not on fill. On a site that was built up or leveled with dirt from somewhere else, ask the building department what it wants before you form the slab.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 20x20 slab?
A 20 by 20 foot slab 4 inches thick holds 133.3 cubic feet of concrete, which is 4.94 cubic yards. With 10 percent extra, order 5.43 cubic yards of ready mix. At 5 inches thick it holds 6.17 cubic yards and at 6 inches 7.41.
How many 80 lb bags of concrete do I need for a 20x20 slab?
245 bags at 4 inches thick, with 10 percent extra. They weigh 19,600 lb in all and cost about $1,394 at the store price checked in October 2026. At 5 inches thick it is 306 bags and at 6 inches 367.
How thick should a 20x20 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 20x20 slab each extra inch adds 1.23 cubic yards, so going from 4 to 6 inches takes the ready mix order from 5.43 to 8.15 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
What door fits a 20x20 garage?
Either one 16 foot door or two 8 or 9 foot doors. Two 9 foot doors leave only 2 feet of wall to split between the two ends and the post in the middle, so most 20 foot wide garages use a single 16 foot door.
Is 20x20 big enough for a two-car garage?
For two cars, yes, but only just. Two midsize cars about 6 feet wide each leave about 8 feet of width for opening doors and walking, split around them. For pickups, storage or a workbench, many people build 24x24.
How much concrete for a 20x20 garage slab with a thickened edge?
About 7.49 cubic yards with 10 percent extra: 5.43 for the 4 inch floor and 2.06 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.