Concrete Slab Calculator
Short answer: a 10 × 10 ft slab at 4 inches thick needs 1.23 cubic yards (about 56 × 80 lb bags). Pick a common size below or enter your own measurements.
Calculate your slab
Common sizes (feet):
Slab thickness, strength and reinforcement
Thickness and PSI are chosen together: PSI (pounds per square inch) is the concrete's compressive strength at 28 days. A stronger mix does not change the volume — the same forms hold the same yards — it changes what the slab can carry. The table below is typical residential practice reported by industry sources, not a code table. Your local building code governs — check it before pouring, especially for driveways and anything carrying vehicles.
| Use | Typical thickness | Typical PSI | Reinforcement |
|---|---|---|---|
| Sidewalk / walkway | 4 inches (10 cm) | 3,000 | Mesh or fiber optional |
| Patio, shed base, AC pad | 4 inches (10 cm) | 3,000–4,000 | Mesh recommended |
| Driveway, passenger cars | 5 inches (13 cm) | 4,000 | Mesh or rebar grid |
| Driveway, trucks / RVs | 6 inches (15 cm) | 4,000+ | Rebar grid |
| Garage floor | 5–6 inches (13–15 cm) | 4,000 | Rebar grid |
| Heavy equipment pad | 6–8 inches (15–20 cm) | 5,000 | Heavy rebar grid, engineered design |
Control joints — choose where the cracks go
Concrete shrinks as it cures, so it cracks. Control joints are shallow, planned lines that give the crack a straight place to form, instead of wandering across the slab. Industry guidance from NRMCA (Concrete in Practice CIP 6, Joints in Concrete Slabs on Grade) sets the rules below.
| Slab thickness | Joint spacing (24–36 × thickness) | Plan on |
|---|---|---|
| 3.5 inches | 7 to 10.5 ft | About 8 ft |
| 4 inches | 8 to 12 ft | About 8–10 ft |
| 5 inches | 10 to 15 ft | About 10–12 ft |
| 6 inches | 12 to 18 ft, capped at 15 ft | About 12–15 ft |
- Maximum spacing: never more than 15 ft, even for thick slabs.
- Panel shape: keep panels square or nearly so — length no more than 1.5 times the width, and avoid L-shaped panels.
- Joint depth: at least one-quarter of the slab thickness, and at least 1 inch deep.
- Timing: saw-cut joints within 4–12 hours after finishing, or tool them while finishing. Cut too late, and the slab may already have cracked on its own.
- Isolation joints: separate the slab from walls, steps, footings and existing slabs so each can move on its own.
The sidewalk calculator counts joints along a walkway at the spacing you enter, using these rules as its guide.
Curing — and when you can use the slab
Curing is not drying. Concrete gains strength through hydration, a chemical reaction with water, so keeping it moist in the first days builds strength; letting it dry out too fast causes surface cracking and weaker concrete.
| After the pour | About how strong | Safe for |
|---|---|---|
| 24–48 hours | Still soft on the surface | Light foot traffic only — do not drag tools or wheelbarrows |
| 7 days | About 70% of design strength | Cars and light vehicles on driveways |
| 14 days | About 85% of design strength | Furniture, planters, light equipment |
| 28 days | Full design strength | Heavy loads, sealing and polishing |
Keep new concrete moist for at least 7 days — cover it with plastic sheeting, wet burlap, or a curing compound, and do not sprinkle water on the surface to make finishing easier. Protect it from freezing in the first 48 hours, and do not use de-icing salt on it in its first winter. Cold, wet weather slows curing, so wait longer before loading the slab.
Pour checklist
A repeatable order of work that prevents the most common DIY slab mistakes. Do every step before the truck arrives — once concrete starts setting, you cannot pause it.
- Excavate to depth. Slab thickness plus base depth plus form height. For a 4 inch slab on a 4 inch base, dig about 10 inches below final grade.
- Compact the subgrade. Remove sod and loose soil, then tamp or roll the native soil firm. Do not skip this on clay or recently disturbed soil.
- Lay and compact the base. Usually 4 inches of crushed gravel, compacted in 2 inch layers. Dampen it without leaving puddles.
- Set the forms. Stake every 4 ft, check level, and slope the slab about 1/4 inch per foot away from buildings.
- Place the vapor barrier and reinforcement. 6 mil poly sheeting under interior slabs; raise mesh or rebar on chairs so it sits inside the slab, not on the ground.
- Pour and screed. Pour from the far end working back, screed level with the forms, and work continuously so you do not leave a cold joint.
- Float, edge and joint. Float once the water sheen disappears, edge the perimeter, and cut or tool control joints at the spacing in the table above.
- Finish and cure. Broom finish for traction outdoors, or trowel for smooth interior work. Cover and keep moist for at least 7 days, as in the curing table above.
Shopping list — worked example: a 10 × 10 ft patio at 4 inches
Quantities below are for this one example only. Use the calculators for any other size — the concrete and gravel volumes follow the same math, and reinforcement follows your plans and local code.
| Item | For this 10 × 10 ft patio |
|---|---|
| Concrete | 1.23 cubic yards net (1.36 with 10% waste), or 62 × 80 lb bags from the order volume |
| Gravel base (4 inches, compacted) | 1.23 cubic yards net (1.36 with 10% waste), about 3,670 lb of gravel |
| Wire mesh (or rebar) | 2 sheets, 5 × 10 ft each (6 × 6 grid) — overlap the joint by one grid square and trim to fit, so check your layout and allow a spare sheet on larger slabs. Or use rebar: see the Rebar Calculator. |
| Form boards and stakes | Perimeter 40 ft: five 8 ft boards (or four 10 ft boards), 2 × 4 for a 4 inch slab, plus about 10 stakes — one every 4 ft — and screws or nails |
| Vapor barrier (interior slabs) | About 110 sq ft of 6 mil poly sheeting — the 100 sq ft slab area plus overlap at seams and edges |
| Sealer or curing compound | About 1 gallon for one coat — coverage runs roughly 200–350 sq ft per gallon depending on the product and the surface, so check the label; allow a second gallon if two coats are recommended |
| Tools | Screed board, bull float, hand float, edger, joint tool, broom, shovels, wheelbarrow or mixer, tamper or plate compactor, level, tape measure, string line |
Pouring in hot or cold weather
The guidance below follows published industry practice (NRMCA and Quikrete). Weather changes timing, not the volume — but it decides whether the slab cures strong or cracks.
- Hot, dry or windy: pour early in the day, before the hottest part; dampen the subgrade and forms without leaving puddles; have enough helpers to place and finish quickly; start curing as soon as finishing is done; do not sprinkle water on the surface to make finishing easier.
- Cold: protect fresh concrete from freezing for the first 48 hours with plastic sheeting and insulating blankets; never pour onto frozen ground, snow or ice; allow longer before loading the slab, because cold slows curing.
- Rain: if rain is forecast in the first few hours after the pour, cover the slab — rain that soon can weaken the surface. Light rain after 12–24 hours is usually fine if the slab is protected.
Ordering ready-mix
Suppliers often round a ready-mix order up to the next quarter cubic yard — for example, a calculated 2.61 cubic yards becomes a 2.75 yard order — and charge a delivery or short-load fee on small loads. Ask about both before you order: the price fields on this calculator accept your quoted price per yard, and any delivery fee should be added to the total separately. The cost guide lists the other fees to ask about.
Common slab sizes
| Slab | At 4 in | At 6 in |
|---|---|---|
| 10 × 10 ft | 1.23 cubic yards · 56 × 80 lb bags | 1.85 cubic yards · 84 × 80 lb bags |
| 12 × 12 ft | 1.78 cubic yards · 80 × 80 lb bags | 2.67 cubic yards · 120 × 80 lb bags |
| 20 × 20 ft | 4.94 cubic yards · 223 × 80 lb bags | 7.41 cubic yards · 334 × 80 lb bags |
| 24 × 24 ft | 7.11 cubic yards · 320 × 80 lb bags | 10.67 cubic yards · 480 × 80 lb bags |
Volumes are net (before waste). Bag counts use the 0.60 cu ft yield per 80 lb bag, rounded up.
Frequently asked questions
How many cubic yards for a 10×10 slab?
At 4 inches thick: 1.23 cubic yards net (1.36 with 10% waste), or about 56 × 80 lb bags net (62 with waste).
How thick should my slab be?
4 inches for patios, walkways and shed floors; 5–6 inches for driveways and garage floors carrying vehicles. Check your local code.
How many bags for a 12×12 slab?
At 4 inches thick: 1.78 cubic yards net, about 80 × 80 lb bags (88 with 10% waste).
How far apart should control joints be?
24 to 36 times the slab thickness, capped at 15 ft — about 8 to 12 ft for a 4-inch slab. Cut joints at least one-quarter of the thickness deep, and at least 1 inch deep.
How long before I can drive on a new slab?
Light foot traffic in 24–48 hours, cars after about 7 days, and full strength at 28 days. Keep the concrete moist for at least 7 days.
What else do I need to buy for a slab?
Besides concrete: a gravel base, reinforcement (mesh or rebar), form boards and stakes, and — for interior slabs — 6 mil poly sheeting, plus sealer or curing compound. The slab page lists quantities for a 10 × 10 ft patio example.