Concrete: How Much to Order, Mixing, Pouring and Finishing a Slab

How much concrete you need, with a worked example

Bags are rated by the cubic feet of wet concrete they make, and ready-mix is sold by the cubic yard. The volume of any slab is length times width times thickness, with all three in feet, so divide the thickness in inches by 12. One cubic yard is 27 cubic feet (3 ft by 3 ft by 3 ft).

Here is the arithmetic for a 10 ft by 10 ft patio slab poured 4 in thick. Four inches is 4 divided by 12, or 0.333 ft. The volume is 10 x 10 x 0.333 = 33.3 cubic feet, which is 33.3 divided by 27 = 1.23 cubic yards.

NRMCA, the ready-mix trade association, says to order 4 percent to 10 percent more than the volume calculated from the plan dimensions, to cover contingencies. For a small hand-mixed pour we suggest the 10 percent end, because running short mid-pour can leave a cold joint, while a leftover bag costs little.

A manufacturer's data sheet for a general-purpose mix gives these yields: about 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. Divide your volume by the yield for the bag size you plan to buy and round up. One cubic yard works out to 27 divided by 0.60 = 45 bags of 80 lb mix, which weighs 3,600 lb on its own, so a big slab is a serious hauling job.

• Slab volume: 10 ft x 10 ft x (4 in / 12) = 33.3 cubic feet
• 80 lb bags, no waste: 33.3 / 0.60 = 55.6, so 56 bags
• 80 lb bags with 10 percent extra: 36.7 / 0.60 = 61.1, so 62 bags (4,960 lb of dry mix)
• 60 lb bags with 10 percent extra: 36.7 / 0.45 = 81.5, so 82 bags
• 40 lb bags with 10 percent extra: 36.7 / 0.30 = 122.2, so 123 bags
• Ready-mix: 1.23 cubic yards, plus 4 to 10 percent = 1.28 to 1.36 cubic yards

Choosing the right product: bagged mix or ready-mix

Bagged mix is a pre-blended cement and aggregate that needs only water. The general-purpose type is rated for pours 2 in thick or more and is sold for slabs, patios, walks, steps, footings and post holes. The data sheet reports about 2,500 psi at 7 days and 4,000 psi at 28 days.

Ready-mix is made to order and arrives in a truck mixer that holds 8 to 12 cubic yards. NRMCA notes that concrete normally sets and hardens within two to eight hours after mixing, so you need the forms, helpers and tools ready before the truck arrives. Typical specified strength is 3,500 to 5,000 psi. NRMCA says concrete exposed to freezing in service should be air-entrained, so tell the producer the exposure.

The sources we read give no cutoff between bags and a truck. The limit is physical: a 1 cubic yard job is 45 bags of 80 lb mix, and each batch must be placed before it stiffens.

What to buy

Buy by category and check each item against the job.

• Concrete: one product type and bag size throughout, plus the 10 percent allowance, or a ready-mix order with strength, slump and air content stated
• Forms: straight, rigid lumber at least as tall as the slab is thick, plus stakes and bracing
• Base material: crushed stone or gravel for the subbase, and a hand or plate tamper that compacts it firm
• Reinforcement: welded wire mesh or a fiber-reinforced mix, plus small stones or chairs to hold mesh at the middle of the slab
• Mixing gear: a barrel or mortar mixer (allow at least 1 cubic foot of mixer capacity per 80 lb bag), or a tub or wheelbarrow and a hoe for hand mixing
• Placing and finishing tools: shovel, rake, a straight board for screeding, a darby or bull float, hand float, edger, groover, steel trowel and a broom for a non-slip surface
• Curing supplies: plastic sheeting at least 4 mil thick, clean burlap, or a spray-on curing compound
• Protection: impervious gloves such as nitrile, eye protection, a dust mask, rubber boots and kneeboards

Preparing the surface or site

Stake out the area with string and a line level, then remove sod and soil to the depth you need. The depth is the slab thickness plus the subbase, and you must allow for the width of the form lumber. For a 4 in slab on a 4 in subbase that is 8 in of excavation. A trade-association guide calls for a compacted subbase at least 4 in thick, for unstable material to be removed to a depth of at least 6 in, and says concrete must never be placed on a frozen subbase.

Tamp the subbase until it is firm. Nail the forms to stakes, backfill against them to hold them, and check the top with a level. The manufacturer's sidewalk guide recommends sloping a walk or slab away from buildings by about 1/4 in per foot, and says to check local codes and conditions.

Install expansion joint strip wherever the new concrete meets a foundation, wall or existing slab, and around posts. Support any wire mesh on small stones so it sits at the middle of the thickness. Just before the pour, dampen the subgrade without leaving puddles.

Step by step: pouring a slab

Read the bag first, because water and timing vary by product. These steps follow the manufacturer's general-purpose mix instructions.

• 1. Check the weather. You want ample moisture and moderate temperature and wind for curing, and no frozen ground (see the weather section).
• 2. Set up the mixer or tub near the forms with water, tools and helpers ready.
• 3. Add about three quarters of the expected mixing water to the mixer, then the mix. For each 80 lb bag start with about 6 pints (2.8 L) and stop around 9 pints at most. In a tub, work the mix with a shovel or hoe until it is stiff and moldable with no dry pockets and no standing puddles.
• 4. Dampen the subgrade, then shovel the concrete into the forms to the full depth, working it tight into corners and edges. Avoid partial-depth fills, which lead to cold joints.
• 5. Strike off the surface with a straight board, moving it back and forth with a saw-like motion. Fill low spots and repeat as needed.
• 6. Float immediately with a darby or bull float to level ridges and fill voids.
• 7. Run an edging tool along the forms to compact and round the slab edges.
• 8. Cut control joints 1 in deep with a groover, using a straight board as a guide. The manufacturer says every 6 ft to 8 ft. The trade-association guide says 2 to 2.5 times the slab thickness in inches, as feet, so 8 ft to 10 ft for a 4 in slab.
• 9. Wait until the surface sheen and bleed water have evaporated before troweling or brooming. The sidewalk guide says this is 30 minutes to an hour in most cases but can be several hours.
• 10. Trowel for a smooth finish, or pull a broom across for a non-slip texture. Redo the edges and joints after troweling.
• 11. Start curing as soon as the surface can take it (next section).
• 12. Remove the forms after the concrete has hardened enough, about 48 hours according to the manufacturer's guide.

Finishing, curing and drying times

Curing means keeping the concrete moist and warm enough for the cement to keep reacting. NRMCA says concrete in a dry environment can lose as much as 50 percent of its potential strength compared with moist-cured concrete, and recommends keeping it above 50 F. It describes the usual curing period as 3 to 7 days. The manufacturer of the bagged mix says 5 days in warm weather at 70 F or higher, or 7 days at 50 F to 70 F.

Pick one method and keep it up without gaps. A sprinkler or soaker hose must run continuously, because NRMCA says alternate wetting and drying is not acceptable. Damp burlap must stay damp. Plastic sheeting at least 4 mil thick should lie flat with taped, overlapping edges, weighted down and extending past the slab edge by at least twice its thickness. A spray-on curing compound goes on right after the water sheen disappears.

For how soon you can use the slab, the trade-association guide says normal conditions allow foot traffic after 48 hours, vehicles after at least 7 days, and heavy loads such as dumpsters or RVs after at least 28 days. The data sheet reports about 2,500 psi at 7 days and 4,000 psi at 28 days for the general-purpose bagged mix. Quick-setting and high-early-strength products list shorter times, so follow the label for those.

Common mistakes

• Adding water to make the mix easier to place. NRMCA says excessive water can cause segregation, delay setting, increase cracking and reduce strength and durability.
• Skipping curing, or curing for a day and then stopping.
• Pouring onto loose, soft or frozen ground, or skipping the compacted subbase.
• Stopping partway and leaving a cold joint.
• Leaving out control joints or cutting them too shallow (roughly a quarter of the slab thickness).
• Using a curing compound in late fall on a surface that will be salted with de-icers.

When to hire a professional

A patio, walk or pad is a reasonable DIY job. Other work is risky or governed by rules.

• Structural work: footings for decks, additions or house walls. The manufacturer's data sheet tells you to follow local building codes for load-bearing footings,.
• Anything in the public right-of-way, such as a driveway apron or sidewalk. The trade-association guide says this work may be subject to local municipal or county requirements, including thickness.
• Large areas. Concrete normally sets and hardens within two to eight hours, so a big slab needs a crew that can place and finish it before that window closes.
• Limited truck access. A loaded concrete truck weighs about 80,000 lb and may not be able to maneuver on loose dirt or over residential curbs and pathways, so a pump or another delivery method may be needed.
• Cutting or grinding hardened concrete, which makes silica dust that OSHA links to silicosis and lung cancer.
• Freezing, very hot or windy conditions, where the margin for error shrinks.

Cost and ordering tips

We do not quote prices. For bags, buy one product type and bag size, add the 10 percent allowance, and compare the number of trips and the weight you must move: 62 bags of 80 lb mix is 4,960 lb.

For ready-mix, NRMCA advises ordering well ahead, having the crew ready when the truck arrives, and avoiding small clean-up loads. State the strength, the slump, the air content if the slab will freeze, and where the concrete will go.

Ask the supplier about the minimum load and short-load charge, the delivery fee, how long the truck can wait, and whether it can reach the site. Check and sign the delivery ticket. Do not add water to the load yourself: the producer cannot be responsible for concrete modified at the job site. We found no trade source that publishes a standard minimum load, so your local plant must confirm it.

Care and maintenance

NRMCA says proper curing reduces crazing, dusting and scaling. Control joints give cracks a planned place to form, so a crack along a joint is expected.

The manufacturer warns against curing compounds in late fall on surfaces that will be treated with de-icers. We found no sourced resealing interval, so follow the sealer's label.

Footings, post holes and columns

Concrete in a hole follows the same volume rule. For a round hole, volume is 3.14 times the radius squared times the depth, with all values in feet. A 12 in diameter hole has a radius of 0.5 ft, so each foot of depth holds 0.785 cubic feet. A 2.5 ft column of concrete holds about 1.96 cubic feet, which is 3.3 bags of 80 lb mix, so buy 4.

For ordinary fence posts, the manufacturer says to dig the hole about three times the diameter of the post, make its depth one third of the overall post height, place 6 in of dry mix at the bottom, plumb the post, fill, and wait 24 hours before putting the post under strain.

Structural footings are different. Residential building codes set a floor of 12 in wide and 6 in thick for a footing, with the width based on the load-bearing value of the soil, and exterior footings must go at least 12 in below the undisturbed ground surface and below the frost depth where frost protection applies. Those are minimums, so confirm frost depth and adopted code with the building department.

Pouring in cold and hot weather

Cold: NRMCA says concrete should be kept above 50 F while curing and protected from freezing until it reaches 500 psi, using insulating materials. The manufacturer of the bagged mix says to protect it from freezing for the first 48 hours, with plastic sheeting and insulating blankets if temperatures will fall below 32 F, and not to apply a curing compound if rain or temperatures below 50 F are expected within 24 hours.

Hot, dry or windy: water leaves the surface fast, which the manufacturer says can cause finishing difficulties and shrinkage cracking. NRMCA adds that concrete cured hot gains early strength quickly but ends up weaker, and that reflective plastic suits warm weather. Pour in cooler hours when possible.

How much concrete for common slab sizes

Each row uses a 4 in thickness and 10% extra. Change any number in the concrete calculator.

Frequently asked questions

Sources: Manufacturer data sheet: general-purpose concrete mix (yield, water, curing, slabs, posts), Manufacturer project guide: concrete sidewalks and small slabs, NRMCA CIP 31: Ordering Ready Mixed Concrete, NRMCA CIP 11: Curing In-Place Concrete, Illinois Ready Mixed Concrete Association: residential flatwork guide, Washington State residential code, WAC 51-51-0403 (footings), International Residential Code 2021, Chapter 4 Foundations (as adopted in Connecticut), OSHA: Concrete and concrete products, controlling hazards. Read 7 November 2026.