What a 16x20 slab is for
A 16x20 slab is the floor of a roomy one-car garage with space for a workbench and storage along one side, a backyard studio, or a large patio. As a patio, its 320 square feet hold a table for six or eight and a lounge area side by side.
Concrete for a 16x20 slab at 4, 5 and 6 inches
Every inch of thickness on a 16x20 slab is 26.7 cubic feet, or 0.99 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 | 106.7 | 3.95 | 4.35 cu yd | 196 | 261 |
| 5 in | 133.3 | 4.94 | 5.43 cu yd | 245 | 326 |
| 6 in | 160 | 5.93 | 6.52 cu yd | 294 | 392 |
The 4 inch row by hand: 20 × 16 × 0.333 = 106.7 cubic feet, and 106.7 ÷ 27 = 3.95 cubic yards. With 10 percent extra that is 117.3 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 196 of one or 261 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 16x20 slab
From bags, it is 196 bags of 80 lb mix at 4 inches, 15,680 lb to lift and mix, about $1,115 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 4.35 cubic yards, about $478 to $653 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 16x20 slab costs about $462 to $637 less from a truck than from bags, and it saves lifting 15,680 lb of mix. A short-load fee or a minimum order comes out of that gap, so get a delivered price for 4.35 cubic yards and compare it with $1,115. 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 | 196 | 15,680 lb | $1,115 | 4.35 cu yd, less than one truck load | $478 to $653 |
| 5 in | 245 | 19,600 lb | $1,394 | 5.43 cu yd, less than one truck load | $597 to $815 |
| 6 in | 294 | 23,520 lb | $1,673 | 6.52 cu yd, less than one truck load | $717 to $978 |
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 16x20 slab
At about 4.35 cubic yards, it is a truck job, about half of a full truck. Sixteen feet is too wide for most people to screed with one board, so set a temporary screed rail down the middle, strike off each half, then pull the rail and fill its trough. Two people can bull float it from the edges with extension handles.
Gravel base and slope
A 4 inch gravel base under a 16x20 slab is 106.7 cubic feet, or 3.95 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 80 cubic feet, or 2.96 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 16 foot side, that is 4 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 16x20 garage is 320 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 16x20 slab that is 68 square feet of edge, 45.3 cubic feet, or 1.68 cubic yards more concrete. With 10 percent extra, that adds 1.85 cubic yards, for an order of about 6.2 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 16x20 slab takes 9 bars 19 feet 6 inches long and 11 bars 15 feet 6 inches long. That is 346 feet of bar. With 10 percent extra, buy 20 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 16x20 slab into 3 panels along the 20 foot side and 2 along the 16 foot side. That makes 6 panels, each 6 feet 8 inches by 8 feet, with 52 feet of joint to cut.
Before you pour a 16x20 slab
This is the size where the slab design starts to matter as much as the concrete. A 16 foot span of concrete will crack if the base under it is soft in one spot, so compact the gravel in thin lifts and do not pour over fresh fill. As a garage, the extra 4 feet of width over a 12x20 is what lets you open a car door and walk past a workbench at the same time.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 16x20 slab?
A 16 by 20 foot slab 4 inches thick holds 106.7 cubic feet of concrete, which is 3.95 cubic yards. With 10 percent extra, order 4.35 cubic yards of ready mix. At 5 inches thick it holds 4.94 cubic yards and at 6 inches 5.93.
How many 80 lb bags of concrete do I need for a 16x20 slab?
196 bags at 4 inches thick, with 10 percent extra. They weigh 15,680 lb in all and cost about $1,115 at the store price checked in October 2026. At 5 inches thick it is 245 bags and at 6 inches 294.
How thick should a 16x20 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 16x20 slab each extra inch adds 0.99 cubic yards, so going from 4 to 6 inches takes the ready mix order from 4.35 to 6.52 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
Do I need a pump for a 16x20 slab?
Not if the truck can back up beside it, because its chute reaches most of a slab this size and wheelbarrows do the rest. If the slab is behind the house or up a slope, a pump or a rented power buggy saves the crew, since NRMCA notes a loaded truck may not cross loose dirt or a residential curb.
Is a 16x20 garage big enough for one car and a workshop?
Usually. A car about 6 feet wide in a 16 foot garage leaves about 10 feet of width, enough for a bench and a path past it. Measure your car with its doors open before you decide.
How much concrete for a 16x20 garage slab with a thickened edge?
About 6.2 cubic yards with 10 percent extra: 4.35 for the 4 inch floor and 1.85 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.