Ordering concrete is one of the easiest places to overspend on a home project. Too little and the truck leaves you mid-pour, with a cold joint you can never fully hide. Too much and you pay for wet concrete you dump in a hole. This concrete calculator takes the length, width and thickness of your project and returns the exact number of cubic yards, the bags you would need if you mixed by hand, and an estimated material cost — with a built-in waste allowance so you are not short.
A standard slab pours out in cubic yards (one cubic yard equals 27 cubic feet). Punch in your dimensions and every figure updates instantly. Switch between 80-lb, 60-lb and 40-lb bags to compare a ready-mix truck against bagged mix, and adjust the waste percentage to see how a small overage protects the pour.
How to Use
- Enter the Slab Length in feet — the long side of the rectangle you are pouring.
- Enter the Slab Width in feet — the short side.
- Set the Slab Thickness. A patio or walkway is usually 4 inches; a driveway that sees cars is 4–6 inches; a footing under a wall is 8–12 inches.
- Pick a Bag Size if you plan to mix by hand. Leave it at 80 lb for the most common sack; switch to 60 or 40 lb to compare.
- Set the Ready-Mix Price per cubic yard. Use the default $150 if you do not yet have a quote; local delivered prices commonly run $120–$200.
- Adjust Waste / Overage. 10% is the safe default for a first-time pour; fine, straight-edged work can use 5%.
The result card shows the raw volume in cubic feet and cubic yards, the yards you should actually order once waste is added, the number of bags required, and the estimated material cost rounded up to whole yards and whole bags.
The Concrete Formula
Concrete volume is plain rectangular-box geometry, converted to yards:
Cubic Yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
Divide inches by 12 to get feet, then divide cubic feet by 27 because one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
Worked on a 10 ft × 10 ft × 4 in slab: 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, and 33.33 ÷ 27 = 1.23 cubic yards. That is the figure ready-mix suppliers price against.
Bags vs. Ready-Mix
An 80-lb bag of premixed concrete yields roughly 0.6 cubic feet once watered and placed. Because a cubic yard is 27 cubic feet, you need about 45 eighty-pound bags per yard. The yield changes with bag size:
- 80-lb bag ≈ 0.60 cu ft → about 45 bags per cubic yard.
- 60-lb bag ≈ 0.45 cu ft → about 60 bags per cubic yard.
- 40-lb bag ≈ 0.30 cu ft → about 90 bags per cubic yard.
For the same 10×10×4 slab (1.23 yd³) at 45 bags per yard, that is about 56 eighty-pound bags. Ready-mix almost always wins on price and labor once a project reaches roughly one full yard; bags make sense for small patches, fence post holes and stepping stones where a truck would be overkill.
Why You Always Add Waste
Concrete cannot be returned. A form that sits slightly off-level, a few shovel-fulls spilled on the ground, or a patch that settles deeper than planned can eat 5–10% of your estimate. The calculator applies the waste percentage on top of the raw volume: order 1.23 × 1.10 = 1.36 yards rather than the bare 1.23. Most suppliers will let you order to the nearest quarter yard and charge by what is actually loaded, so rounding up a little costs very little.
Examples
Example 1 — A backyard patio. A homeowner pours a 10 ft × 10 ft, 4-inch patio. The volume is 1.23 cubic yards. With 10% waste they order 1.36 yards, and at $150 per yard the ready-mix material cost is about $204. If they mix by hand with 80-lb bags, they need about 62 bags after waste.
Example 2 — A car driveway. A 20 ft × 24 ft driveway at 5 inches thick needs 20 × 24 × 5 ÷ 12 ÷ 27 = 7.41 cubic yards raw. Adding 10% waste means ordering about 8.2 yards, roughly $1,230 in material at $150 per yard — before rebar, forms and labor.
Example 3 — Fence posts. Twelve holes, each 1 ft × 1 ft × 2 ft deep, total 24 cubic feet = 0.89 cubic yards. That is under a full yard, so mixing by hand — about 40 eighty-pound bags — is cheaper and simpler than a truck minimum-load fee.
FAQ
How many bags of concrete do I need?
Divide your total cubic yards by the yield of the bag. An 80-lb bag covers about 0.6 cubic feet (45 bags per cubic yard); a 60-lb bag covers 0.45 cubic feet (60 per yard); a 40-lb bag covers 0.30 cubic feet (90 per yard). Multiply your cubic yards by the right bag count, round up, and add a small waste margin.
How many cubic yards is a 10×10 slab at 4 inches?
A 10 ft × 10 ft × 4 inch slab is 33.33 cubic feet, which equals 1.23 cubic yards. Plan on ordering about 1.36 yards with 10% waste, or roughly 62 eighty-pound bags.
What thickness should my concrete slab be?
A pedestrian patio or walkway is usually 4 inches thick. A driveway that holds cars should be 4–6 inches over a compacted gravel base, reinforced with rebar or mesh. Footings under load-bearing walls are typically 8–12 inches thick and wider than the wall above them.
Should I order ready-mix or buy bags?
Ready-mix is cheaper and faster once a project is near or above one cubic yard — it is batched to the right slump and delivered by the chute. Bags are better for very small jobs: post holes, small patches and stepping stones, where a truck's minimum load fee would exceed the material itself.
How much does concrete cost per yard?
Delivered ready-mix typically runs $120–$200 per cubic yard in most US markets, with the price rising for short loads, weekend pours, fiber reinforcement, and special mixes like high-early or fiber-reinforced concrete. Use the price slider on this page with a local quote for an exact estimate.
Why do you recommend adding 10% waste?
Concrete is poured wet and cannot be returned or saved. A slightly uneven form, spillage, or a hole that settles deeper than measured commonly consumes 5–10%. Ordering 10% extra guarantees you finish the pour in one continuous load, avoiding cold joints that weaken the slab.
Disclaimer
This concrete calculator is provided for informational purposes only and does not replace engineering, structural, or construction advice. Estimates are based on the rectangular-slab volume formula and standard bag yields; actual yardage, bag counts and prices vary by site, subgrade, weather, and supplier. Always confirm slab thickness, reinforcement, and finished quantities with a qualified concrete contractor or supplier before ordering.