How to calculate concrete for a slab
Concrete is ordered by volume. The math is length times width times thickness, with everything in the same unit, plus a little extra. Here it is with the values the calculator opens with: a 10 x 10 slab, 4 inches thick, 10% extra and 80 lb bags.
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1
Convert the thickness to feet
Divide inches by 12. A 4 inch slab is 0.333 ft thick.
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2
Multiply length, width and thickness
10 ft x 10 ft x 0.333 ft = 33.3 cubic feet.
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3
Add extra for waste
Uneven ground, bowed forms and spills eat concrete. 33.3 x 1.10 = 36.7 cubic feet. Running short mid-pour leaves a weak cold joint, so always round up.
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4
Divide by 27 for cubic yards
There are 27 cubic feet in a cubic yard, so 36.7 ÷ 27 = 1.36 cubic yards. That is the number a ready-mix plant wants.
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5
Or divide by the bag yield
An 80 lb bag makes 0.60 cubic feet. 36.7 ÷ 0.60 = 61.1, so buy 62 bags. That is 4,960 lb, about 2.5 tons: more than most pickups can carry in one trip.
At the default prices, 62 bags cost $402, plus $65 for a day of mixer rental, or $467. A ready-mix truck delivering 1.5 yards costs $423 with the short-load and delivery fees, so the truck wins by $44. The full bag-route shopping list, with forms, stakes, 1.5 tons of gravel base, an expansion joint and finishing tools, comes to $634.
Footings, posts and round columns
A footing is just a long, narrow, deep slab: length x width x depth. Pick "Footing" and the calculator starts at 10 ft long, 2 ft wide and 12 inches deep. That is 20 cubic feet, 22 with 10% extra, so 0.81 cubic yards or 37 bags of 80 lb mix.
A round hole or form tube uses the area of a circle: π x radius² x depth, with the radius in feet. The "Column" default is 12 inches across and 4 ft deep. The radius is 0.5 ft, so 3.1416 x 0.25 x 4 = 3.14 cubic feet, or 3.46 with waste. That is 6 bags of 80 lb mix per column. For a fence post, the post itself takes up some room: a 4x4 (really 3.5 x 3.5 inches) in a 10 inch hole 2 ft deep displaces about 15% of the hole. The calculator does not subtract it, which gives you a little spare.
How much does one bag make?
The yield is printed on every bag. Bigger bags cost less per yard, and fast-setting mix costs the most. Fast-set is the 50 lb post mix you pour dry into the hole.
| Bag | Yield, cu ft | Bags per yard | Cost per yard |
|---|---|---|---|
| 80 lb | 0.60 | 45 | $292 |
| 60 lb | 0.45 | 60 | $329 |
| 40 lb | 0.30 | 90 | $412 |
| 50 lb fast-set | 0.375 | 72 | $574 |
Costs use the calculator's bag prices ($6.48, $5.48 and $4.58) and about $7.97 for fast-setting mix. Lighter bags are easier to lift and mix, which matters more than price for a few post holes.
How thick, and how far a yard goes
| Thickness | 1 yd covers | Yd per 100 sq ft | Typical use |
|---|---|---|---|
| 4 in | 81 sq ft | 1.23 | Patios, walkways, shed floors |
| 5 in | 65 sq ft | 1.54 | Driveways, garage floors |
| 6 in | 54 sq ft | 1.85 | RV pads, heavy trucks |
These figures have no waste added. Each extra inch adds 25% to a 4 inch slab, so check your forms with a tape before ordering, not after.
Concrete for common slab sizes
4 inches thick with 10% extra. Tap a size to load it into the calculator.
| Slab | Cubic yards | 80 lb bags |
|---|---|---|
| 10 x 10 | 1.36 | 62 |
| 10 x 12 | 1.63 | 74 |
| 12 x 12 | 1.96 | 89 |
| 12 x 20 | 3.26 | 147 |
| 20 x 20 | 5.43 | 245 |
| 24 x 24 garage | 7.82 | 353 |
| 30 x 40 | 16.30 | 734 |
Bags or a ready-mix truck?
The calculator prices both. Bags cost the bag price times the bag count, plus a day of mixer rental once the job tops 0.75 cubic yards. The truck price rounds your order up to the next quarter yard, then adds a $100 short-load fee under 4 yards and $75 for delivery. With those defaults, bags win up to about 1 cubic yard (an 8 x 8 slab saves $16), and the truck wins from about a 9 x 9 slab up. At 20 x 20 the truck saves $670 and about 10 hours of mixing.
Fees vary a lot from plant to plant. Some charge a flat short-load fee, others add about $50 per yard on any load under a full truck, and many have a minimum order. Call two plants, enter the real numbers, and ask how close the truck can park: if the chute can't reach the forms, you will be moving it all by wheelbarrow against the clock. A wheelbarrow load is only 2 to 3 cubic feet, so each yard is 9 to 14 trips.
What it costs: DIY vs. hiring a contractor
A contractor pours a plain slab for about $6 to $12 per square foot, including base prep, forms, concrete, finishing and labor. Labor is roughly $3 to $5 of that. Removing an old slab adds about $3 to $8 per square foot.
| Slab, 4 in | DIY materials | Contractor, installed |
|---|---|---|
| 10 x 10 | $590–$634 | $600–$1,200 |
| 20 x 20 | about $1,430 | $2,400–$4,800 |
DIY figures are this calculator's totals with gravel base, by truck and by bags, at default prices. On a small slab, doing it yourself saves little, and a pro's finish is hard to match on a first try. The savings grow with size, but so does the risk: a 5-yard pour sets up fast, and you need three or four people with rakes, a screed board and a bull float ready before the truck arrives. Decorative work, such as stamping or color, runs higher and is best left to a contractor.
Tips from the trade
- Build on a compacted base. Strip the topsoil, then lay about 4 inches of gravel and compact it. Soft spots under a slab turn into cracks.
- Mind the form height. A 2x4 is only 3½ inches tall. For a true 4 inch slab, dig the base half an inch deeper inside the forms. Use 2x6 forms for 5 or 6 inch slabs.
- Don't add extra water. Soupy concrete is easier to place but weaker and cracks more. Use the water amount on the bag, or ask the plant for a higher slump instead of hosing the truck.
- Cut control joints. Space them in feet at 2 to 3 times the slab thickness in inches, so every 8 to 12 ft on a 4 inch slab, and about a quarter of the slab deep. Keep the panels close to square.
- Cure it slowly. Keep the slab damp or covered for about the first week. Concrete that dries too fast never reaches its full strength.
- Watch the weather. Never pour on frozen ground. On hot, windy days the surface sets before you can finish it, so start early.
- Protect your skin. Wet concrete is caustic and can burn. Wear gloves, eye protection, long sleeves and boots, and rinse splashes off right away.
Common guidance is to keep foot traffic off for a day or two and cars off for about a week. Concrete reaches its rated strength at 28 days. Driveways, footings and anything that holds up a structure must follow your local building code, and many towns require a permit and an inspection before the pour.
Common questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 slab 4 inches thick needs about 1.23 cubic yards. With 10% extra for waste, that is 1.36 cubic yards, or 62 bags of 80 lb premix.
How much does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches, before any extra for waste.
Is it cheaper to use bags or order ready-mix concrete?
Bags are usually cheaper for jobs under about 1 cubic yard. Above that, a ready-mix truck often costs less once you count mixer rental, and past 2 to 3 yards it is cheaper and far less work, even with a short-load fee.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and sheds. Driveways and garage floors are usually 5 to 6 inches, and slabs carrying heavy trucks or RVs need 6 inches or more.
Do I need rebar in a concrete slab?
A patio or walkway does not always need it, but rebar or wire mesh helps hold cracks together, especially on clay soil. Driveways, garage floors and anything structural should have it, and your local code may require it.
How many bags of concrete do I need for a fence post?
A 4x4 post in a 10 inch hole 2 feet deep takes about 0.9 cubic feet of concrete. That is one and a half 80 lb bags or two to three 50 lb bags of fast-setting mix. Use the Column option with your hole size for an exact count.
Sources and assumptions
Volumes are plain geometry. Everything else comes from what bag makers, suppliers and the concrete trade publish. Prices are typical US prices checked in October 2026, and both bag and ready-mix prices can be edited.
- Bag yields (0.60 cu ft for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb) are the figures printed on premixed concrete bags; see the Quikrete Concrete Mix product page and its project calculator. Fast-setting mix yields about 0.375 cu ft per 50 lb bag, per the Ace Hardware listing for Quikrete Fast-Setting.
- Bag prices: defaults of $6.48, $5.48 and $4.58 for 80, 60 and 40 lb bags are typical big-box shelf prices. A Menards check in October 2026 showed $4.85 for a 60 lb bag and $7.97 for 50 lb fast-setting mix.
- Waste: 10 percent by default, the common allowance for uneven subgrade and spillage. Raise it for rough ground or thick slabs.
- Ready-mix: $165 per cubic yard by default, a $100 short-load fee under 4 yards and $75 for delivery. HomeGuide's concrete price guide (January 2026) reports $119 to $147 per yard on a full truck plus about $53 per yard on short loads, and ConcreteNetwork's price guide gives similar ranges. Call two local plants for a real quote.
- Installed cost ($6 to $12 per sq ft, $3 to $5 of it labor, slab sizes, removal) comes from HomeGuide, concrete slab cost (February 2026).
- Joints and curing: joint spacing follows the rule in ConcreteNetwork's control joint guide; curing time follows the Portland Cement Association's curing page, which cites ACI's seven-day minimum for ordinary cement.
- Shopping list defaults: mixer rental $65 a day, 2x4x8 form boards $4.50, stakes $0.90, gravel base at 1.4 tons per cubic yard and $45 a ton, #4 rebar $12 per 20 ft stick. Driveways, footings and anything structural must meet your local building code.