Concrete Calculator
Free concrete calculator for slabs, circular pads, and footings. Estimate how much concrete you need in cubic yards, bags, and cost.
Last updated: September 2026 · Volume and bag-yield math reviewed (not a structural design, mix design, or PSI specification service)
Best for rectangular slabs, circular pads, and simple footings. This tool estimates cubic yards and bag counts from your dimensions and waste factor — not reinforcement design, mix PSI, or frost-depth engineering. Confirm thickness and strength with local code and your supplier before ordering.
Quick Answer: How Much Concrete Do I Need?
To find how much concrete you need, multiply length × width × thickness (in feet), divide by 27 for cubic yards, then add waste. Convert to bags using the bag yield if you are buying bagged mix.
Cubic yards ≈ (length × width × thickness in feet) ÷ 27 × (1 + waste%).
Bags ≈ ceil( cubic feet with waste ÷ bag yield ) — about 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb, 0.30 ft³ per 40 lb.
- Typical residential slabs: 4 inches thick (driveways often 5–6 in)
- Add about 10% waste for most pours
- Circular pads: π × (radius)² × thickness; footings: length × width × depth × count
Example: 10 × 10 ft slab at 4" with 10% waste → about 1.36 yd³ or ~62 bags of 80 lb. Use the calculator above for your exact dimensions.
How to Use This Concrete Calculator
- Select the shape: Slab / Rectangular, Circular, or Footing
- Enter the dimensions and choose the correct units (feet, meters, inches, or cm)
- Set the number of slabs/pads/footings if you're pouring more than one identical section
- Select a waste factor — 10% is recommended for most projects
- Choose bag size if you are buying bagged concrete (40 lb, 60 lb, or 80 lb)
- Optionally enter a price — either per cubic yard (ready-mix) or per bag — to see estimated cost
- Click Calculate Concrete to see volume, bags required, and cost. Press Enter in any field to calculate instantly.
Concrete Bag Yield Reference
Use this table to understand how many bags you need for a given volume. Yields follow Quikrete / Sakrete published values for standard pre-mix:
| Bag Size | Approximate Yield | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cubic feet | ~90 bags |
| 60 lb | 0.45 cubic feet | ~60 bags |
| 80 lb | 0.60 cubic feet | ~45 bags |
Always round up to the next whole bag. Add 5–10% waste before ordering.
How Much Concrete for a Slab? (Examples)
Use this as a quick 4 inch concrete slab calculator reference for popular residential sizes. Volumes include a recommended 10% waste factor. Bag counts are rounded up. Results are also shown in cubic yards for ready-mix orders.
| Slab Size | Area | Cubic Yards (with 10%) | 80 lb Bags | 60 lb Bags |
|---|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 0.22 yd³ | 10 bags | 14 bags |
| 8 × 8 ft | 64 sq ft | 0.87 yd³ | 40 bags | 53 bags |
| 10 × 10 ft | 100 sq ft | 1.36 yd³ | 62 bags | 82 bags |
| 12 × 12 ft | 144 sq ft | 1.96 yd³ | 88 bags | 118 bags |
| 10 × 20 ft | 200 sq ft | 2.72 yd³ | 123 bags | 163 bags |
| 20 × 20 ft | 400 sq ft | 5.43 yd³ | 245 bags | 326 bags |
Quick tip (4-inch slabs only): Divide the square footage by 81 to get approximate cubic yards before waste. Example: 100 sq ft ÷ 81 ≈ 1.23 yd³ (then add 10%). If you still need the floor area, use our square footage calculator first.
Circular example: A 10 ft diameter pad at 4 inches thick needs roughly 0.97 yd³ before waste (about 1.07 yd³ with 10% waste) — approximately 49 bags of 80 lb concrete. Use the Circular tab above for exact numbers.
Footing example: Four footings, each 2 × 2 ft × 12 in deep → 4 × (2 × 2 × 1) = 16 ft³ ≈ 0.59 yd³ before waste (about 0.65 yd³ with 10% waste) — roughly 30 bags of 80 lb concrete. Use the Footings tab and set the count.
Formulas Used
Rectangular / Slab: Volume = Length × Width × Thickness × Number of Slabs
Circular: Volume = π × (Diameter ÷ 2)² × Thickness × Number of Pads
Footing: Volume = Length × Width × Depth × Number of Footings
Cubic Yards = Cubic Feet ÷ 27
A waste factor (usually 5–10%) is added so you do not run short on the job.
Slabs, Circular Pads & Footings
Knowing the right amount of concrete is critical. Ordering too little means delays and cold joints. Ordering too much wastes money. This calculator helps you get an accurate estimate for three of the most common concrete projects: slabs, circular pads, and footings.
Concrete Slab Calculator
Slabs are used for patios, garage floors, sidewalks, and house foundations. Most residential slabs are poured 4 inches thick. For driveways and garage floors that will carry vehicles, 5–6 inches is recommended. Many slabs sit on a compacted base — estimate that layer with our gravel calculator before you pour. Enter length, width, and thickness to get the exact volume. If you're pouring several identical slabs (stepping stones, for example), use the "Number of Slabs" field to total them in one calculation. After the slab is poured and cured, you can estimate finishing materials with our paint calculator or flooring calculator.
Circular Concrete Calculator
Useful for round patios, fire pit bases, or circular pads under tanks and equipment. Measure the full diameter across the circle and the desired thickness. The calculator uses the standard πr² formula to find the volume, then converts it to cubic yards and bags. For the area of the circle before pouring, use the square footage calculator.
Concrete Footing Calculator
Footings support posts, decks, and foundations. Enter the length, width, and depth of one footing, then multiply by the number of footings. Typical deck footings are 12–48 inches deep depending on local frost lines and soil conditions. See our fence calculator if you're pricing out post footings for a fence line.
Looking for more material estimators? Browse all construction calculators on EntireCalc — paint, flooring, drywall, roofing and more.
Bags vs Ready-Mix Concrete
Choosing between bagged concrete and ready-mix depends mainly on volume and convenience.
- Under ~1 cubic yard — Bagged concrete (40, 60 or 80 lb) is usually practical. You can mix on site with a wheelbarrow or small mixer and avoid short-load fees.
- 1–2+ cubic yards — Ready-mix delivered by truck is almost always more consistent, faster, and cheaper per yard. Most suppliers have a minimum order (often 1 cubic yard) and may charge extra for small loads.
This calculator shows both options side-by-side so you can compare bag counts with the cubic-yard volume needed for a ready-mix order. Always confirm current pricing and minimums with your local supplier.
Typical Concrete Thickness by Project
Thickness drives volume as much as length and width. Confirm minimums with local code before you pour.
| Project | Common thickness |
|---|---|
| Patio / sidewalk / shed pad | 4 inches |
| Walkway with light use | 4 inches |
| Driveway / garage floor | 5–6 inches |
| Heavy equipment pad | 6+ inches (design-dependent) |
| Deck / post footings | Depth per frost line (often 12–48 in) |
Same footprint at 6 inches needs 50% more concrete than at 4 inches. Enter your actual thickness in the calculator — do not assume every “slab” is 4 inches.
PSI Strength & Curing (Quick Reference)
| Typical use | Common PSI range |
|---|---|
| Residential patio / sidewalk | 2,500–3,000 PSI |
| Driveway / garage floor | 3,000–4,000 PSI |
| Structural / code-specified | Follow engineer or listing |
- Volume ≠ strength: This calculator estimates how much concrete to order, not which mix design to buy.
- Curing: Keep new concrete moist for at least 3–7 days (longer in hot, dry, or windy weather). Poor curing is a common cause of weak surface and early cracks.
- Reinforcement: Mesh or rebar is separate from volume. Use the Rebar Calculator for bar count and weight, then confirm spacing with local code or an engineer.
Tips for Ordering Concrete
- Always add waste: 5% for simple rectangular slabs with good forms, 10% for most projects, and up to 15% for uneven ground or many small sections.
- Prepare the base: A level gravel or crushed-stone base improves drainage and slab performance. Use the gravel calculator to estimate cubic yards for the sub-base.
- Small jobs vs large jobs: Under about 1 cubic yard → bagged concrete is usually practical. Over 1–2 yards → ready-mix delivery is almost always cheaper and more consistent.
- Thickness matters: A 4-inch slab needs half the concrete of an 8-inch slab for the same area. Doubling thickness doubles the volume.
- Common strengths: Most residential slabs use 2500–3000 PSI concrete. Driveways and garage floors often use 3000–4000 PSI. Confirm the required strength with your local code or supplier.
- Curing matters: Keep new concrete moist for at least 3–7 days (longer in hot, dry, or windy weather). Proper curing significantly improves final strength and reduces surface cracking.
- Reinforcement: Many slabs benefit from wire mesh or rebar. Use the Rebar Calculator for bar count and weight — this tool estimates concrete volume only. Confirm design with local codes or an engineer.
- Check local codes: Frost depth, soil type, and load requirements affect footing size and minimum slab thickness. If you need a concrete landing or pad at the bottom of stairs, cross-check dimensions with the stair calculator first.
- Order a little extra: It is better to have leftover concrete than to stop a pour halfway through. Cold joints weaken the slab.
Important: This calculator estimates volume and bag counts from the dimensions and waste factor you enter. It does not design mix strength (PSI), reinforcement, frost depth, or structural capacity. Confirm thickness, base, and strength with local code and your supplier before ordering or pouring.