What a 12x20 slab is for
A 12x20 slab is the floor of a small one-car garage or carport, or of a large shed or backyard workshop. At 240 square feet, a shed this size is over the model code's 200 square foot limit, so it needs a building permit.
Concrete for a 12x20 slab at 4, 5 and 6 inches
Every inch of thickness on a 12x20 slab is 20 cubic feet, or 0.74 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 | 80 | 2.96 | 3.26 cu yd | 147 | 196 |
| 5 in | 100 | 3.7 | 4.07 cu yd | 184 | 245 |
| 6 in | 120 | 4.44 | 4.89 cu yd | 220 | 294 |
The 4 inch row by hand: 20 × 12 × 0.333 = 80 cubic feet, and 80 ÷ 27 = 2.96 cubic yards. With 10 percent extra that is 88 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 147 of one or 196 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 12x20 slab
From bags, it is 147 bags of 80 lb mix at 4 inches, 11,760 lb to lift and mix, about $836 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 3.26 cubic yards, about $359 to $489 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 12x20 slab costs about $347 to $477 less from a truck than from bags, and it saves lifting 11,760 lb of mix. A short-load fee or a minimum order comes out of that gap, so get a delivered price for 3.26 cubic yards and compare it with $836. 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 | 147 | 11,760 lb | $836 | 3.26 cu yd, less than one truck load | $359 to $489 |
| 5 in | 184 | 14,720 lb | $1,047 | 4.07 cu yd, less than one truck load | $448 to $611 |
| 6 in | 220 | 17,600 lb | $1,252 | 4.89 cu yd, less than one truck load | $538 to $734 |
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 12x20 slab
At about 3.3 cubic yards, this is past what most people want to mix by hand: 147 bags weigh almost six tons. A truck can usually pull alongside a slab this size and reach most of it with its chute, and the rest goes in by wheelbarrow. Have two or three helpers ready: NRMCA says concrete normally sets two to eight hours after mixing, and on a hot day it is the short end.
Gravel base and slope
A 4 inch gravel base under a 12x20 slab is 80 cubic feet, or 2.96 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 60 cubic feet, or 2.22 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 12 foot side, that is 3 inches from one edge to the other. Running along the 20 foot side, it is 5 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 12x20 garage is 240 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 12x20 slab that is 60 square feet of edge, 40 cubic feet, or 1.48 cubic yards more concrete. With 10 percent extra, that adds 1.63 cubic yards, for an order of about 4.89 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 12x20 slab takes 7 bars 19 feet 6 inches long and 11 bars 11 feet 6 inches long. That is 263 feet of bar. With 10 percent extra, buy 15 bars 20 feet long. Every bar is shorter than the 20 foot length sold, so none has to be joined.
With wire mesh instead, 3 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 12x20 slab into 3 panels along the 20 foot side and 2 along the 12 foot side. That makes 6 panels, each 6 feet 8 inches by 6 feet, with 44 feet of joint to cut.
Before you pour a 12x20 slab
As a garage, check what you plan to park. A midsize sedan is about 16 feet long, but many full-size pickups are 19 to 20 feet, and inside the walls a 20 foot slab is a few inches shorter than 20 feet. Garage door openings for one car are usually 8 or 9 feet wide, which leaves only about 3 feet of wall on the 12 foot end.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 12x20 slab?
A 12 by 20 foot slab 4 inches thick holds 80 cubic feet of concrete, which is 2.96 cubic yards. With 10 percent extra, order 3.26 cubic yards of ready mix. At 5 inches thick it holds 3.7 cubic yards and at 6 inches 4.44.
How many 80 lb bags of concrete do I need for a 12x20 slab?
147 bags at 4 inches thick, with 10 percent extra. They weigh 11,760 lb in all and cost about $836 at the store price checked in October 2026. At 5 inches thick it is 184 bags and at 6 inches 220.
How thick should a 12x20 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 12x20 slab each extra inch adds 0.74 cubic yards, so going from 4 to 6 inches takes the ready mix order from 3.26 to 4.89 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
Is 12x20 big enough for a one-car garage?
For a sedan or a small SUV, yes, with a little room to walk around. A full-size pickup is 19 to 20 feet long, which leaves almost nothing between the bumpers and the walls. That is why many one-car garages are built longer than 20 feet.
Can a 12x20 slab hold a car?
A 4 inch slab is the least Dubuque, Iowa, allows for a small garage floor, so it is the usual starting point for one car. Ask your building department, because some towns want more, and a thickened edge under the walls.
How much concrete for a 12x20 garage slab with a thickened edge?
About 4.89 cubic yards with 10 percent extra: 3.26 for the 4 inch floor and 1.63 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.