How to Use This Calculator
- Choose the shape that matches your pour: rectangular slab/footing, round column, curb & gutter, or stairs.
- Enter your measurements: length, width, and thickness (or diameter and height for round pours). Switch to UK units if you're working in metres and centimetres.
- Add a waste/overage percentage. We default to 10%, which covers spillage, uneven subgrade, and form loss.
- Read your results instantly: total volume in cubic yards or cubic metres, the number of pre-mixed bags you'll need at common bag sizes, the estimated weight of the pour, and a rough cost range.
- Repeat for additional sections and add the totals together, or use the "add another section" option if building multiple pours into one order.
- Use your total to place a ready-mix order or buy the right number of bags. Always round up to the next full bag or truck load.
How We Calculate Concrete Volume
Rectangular pours (slabs, footings, walls)
Concrete volume is simply length × width × thickness. The only trick is unit consistency: thickness is almost always given in inches, so it has to be converted to feet before multiplying.
Formula (US): Volume (cubic feet) = Length (ft) × Width (ft) × (Thickness (in) ÷ 12)
Cubic Yards = Cubic Feet ÷ 27
Worked example: A 10 ft × 12 ft patio slab poured 4 inches thick:
10 × 12 × (4 ÷ 12) = 40 cubic feet → 40 ÷ 27 = 1.48 cubic yards (round up to 1.5 to 2 yards to cover waste).
Formula (UK): Volume (cubic metres) = Length (m) × Width (m) × Thickness (m)
Worked example: A 3 m × 4 m slab poured 100 mm (0.1 m) thick:
3 × 4 × 0.1 = 1.2 cubic metres.
Round pours (columns, sonotubes, post holes, piers)
Round forms use the cylinder volume formula, since a sonotube or post hole is essentially a cylinder.
Formula: Volume = π × (Diameter ÷ 2)² × Height
Worked example: A 12-inch diameter sonotube, 4 feet tall: Radius = 0.5 ft → π × 0.5² × 4 = 3.14 cubic feet ≈ 0.12 cubic yards per tube. Multiply by the number of tubes needed (e.g., 6 fence posts = 0.7 cubic yards total).
Curb & gutter
Curb and gutter cross-sections are irregular, so the calculator treats the cross-sectional area (face height × depth, plus the gutter lip) as a fixed shape multiplied by total linear run.
Formula: Volume = Cross-sectional Area × Total Length
Stairs
Each concrete step is treated as a rectangular block, then stacked and summed, since each step is progressively shorter as it rises.
Formula: Volume per step = Rise × Run × Width; Total Volume = Sum of all steps + supporting stringer/footing volume
Irregular shapes and sloped or uneven slabs
Not every pour is a clean rectangle or circle. Two adjustments handle most real-world sites:
- Irregular footprints: Break the area into simpler rectangles, triangles, or circles, calculate each section's volume separately, then add them together.
- Sloped or uneven ground: Use an average thickness. Measure the depth at the shallowest and deepest points, then average the two: Average Thickness = (Min Thickness + Max Thickness) ÷ 2. Use that average in place of a single thickness figure.
Watch: How to Measure Your Slab for Concrete
A quick visual walkthrough of measuring a slab area before you plug numbers into the calculator above. This is useful if this is your first pour.
Concrete Strength (PSI)
Concrete is rated by compressive strength in PSI (pounds per square inch), and using the right grade matters as much as getting the volume right:
| Project Type | Recommended PSI |
|---|---|
| Sidewalks, patios | 3,000 PSI |
| Foundations & footings | 3,000 to 4,000 PSI |
| Residential driveways | 3,500 to 4,000 PSI |
| Driveways for trucks/RVs, heavy-duty use | 4,000 to 4,500 PSI |
Higher PSI mixes use more cement and hold up better against freeze-thaw cycles in colder climates. When ordering ready-mix, ask your supplier for the PSI grade that matches your project type. Bagged mixes typically list their PSI rating on the packaging.
How Long Does Concrete Take to Cure?
Curing is the gradual hardening process after a pour, and it happens on a fairly predictable timeline:
- 24 to 48 hours: Safe for light foot traffic
- 7 days: Reaches roughly 70% of final strength, and is generally safe for vehicle weight
- 28 days: Reaches about 90% of final strength, and is considered fully cured for structural loads
- Ongoing: Concrete continues gaining strength gradually for months to years afterward
Cold weather slows curing significantly, while hot, dry conditions can cause the surface to dry too fast and crack. Keeping a new pour damp (misting or covering) for the first several days improves strength and reduces cracking risk.
How Many Bags of Concrete Do You Need
Pre-mixed bags are sold by weight, and each bag size yields a specific volume once mixed with water. Here's the standard yield used in our bag calculation:
| Bag Size | Yield cu ft | Bags per Cubic Yard |
|---|---|---|
| 40 lb bag | 0.30 cu ft | ~90 bags |
| 60 lb bag | 0.45 cu ft | ~60 bags |
| 80 lb bag | 0.60 cu ft | ~45 bags |
| 25 kg bag (UK) | 0.0125 cu m | ~80 bags per cubic metre |
For 1.5 cubic yards of concrete, you'd need roughly 68 bags at 60 lb. Always round up and add a few extra bags for waste. This yield applies to standard pre-mixed bags from any major brand (e.g., Quikrete, Sakrete, or equivalent UK ready-mix bags). Check your specific bag's label, since yield can vary slightly by formulation (fast-setting vs standard).
How Much Does Concrete Weigh
Standard concrete weighs approximately 150 lbs per cubic foot, or 4,050 lbs (about 2 tons) per cubic yard. In metric terms, that's roughly 2,400 kg per cubic metre. This matters for structural loading calculations, delivery truck planning, and disposal cost estimates if you're removing old concrete.
- 1 cubic yard ≈ 4,050 lbs (2.03 tons)
- 1 cubic metre ≈ 2,400 kg (2.4 tonnes)
- A standard 10×10 ft slab at 4" thick ≈ 5,000 lbs
Estimating Concrete Cost
Ready-mix concrete typically costs $125 to $180 per cubic yard delivered in the US, or £100 to £150 per cubic metre in the UK, before labor. Bagged concrete works out more expensive per yard (roughly $170 to $250 per cubic-yard-equivalent in bags) but makes sense for small pours where a full truck isn't practical. For a full cost breakdown by project type, including driveways, patios, footings, and foundations, use our Foundation Cost Calculator.
Quick Reference: Concrete Conversions
| Convert | To | Multiply/Divide |
|---|---|---|
| Cubic feet | Cubic yards | ÷ 27 |
| Cubic feet | Cubic metres | × 0.0283 |
| Cubic yards | Cubic metres | × 0.7646 |
| Square feet | Cubic yards (at 4" thick) | ÷ 81 |
| Square feet | Cubic yards (at 6" thick) | ÷ 54 |
| Cubic yards | Weight (lbs) | × 4,050 |
| Cubic metres | Weight (kg) | × 2,400 |
| Inches | Feet | ÷ 12 |
| Centimetres | Metres | ÷ 100 |
Tips for Accurate Concrete Ordering
- Always round up. Ready-mix trucks charge by the yard regardless of leftover, and running short mid-pour risks a visible cold joint.
- Add 5% to 10% waste for uneven subgrade, form seepage, and spillage. Add more on sloped or irregular sites.
- Measure thickness at multiple points if your subgrade isn't level; use the deepest point for a conservative estimate.
- Confirm bag yield on the label. Fast-setting and high-strength mixes sometimes yield slightly less volume per bag than standard mixes.
- For pours over 1 to 2 cubic yards, ready-mix delivery is usually cheaper and faster than buying and mixing dozens of bags by hand.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
At the standard 4-inch residential thickness, a 10×10 ft slab needs about 1.23 cubic yards of concrete before waste. Round up to 1.5 yards to be safe.
How many bags of concrete make a yard?
It takes about 90 bags at 40 lbs, 60 bags at 60 lbs, or 45 bags at 80 lbs to equal one cubic yard.
How much does a cubic yard of concrete weigh?
A cubic yard of standard concrete weighs approximately 4,050 lbs, or just over 2 tons.
How do I calculate concrete for a round column or sonotube?
Use the cylinder formula: π × radius² × height. Our round-pour module handles this automatically. Just enter the tube's diameter and height.
What's the difference between the concrete calculator and the concrete slab calculator?
This calculator covers every pour shape, including slabs, footings, columns, curbs, and stairs, in one tool. Our dedicated Concrete Slab Calculator goes deeper specifically on slab thickness standards, cost breakdowns, and slab-specific FAQs.
How much concrete do I need for fence post holes?
It depends on the hole diameter and depth. A typical 10-inch diameter, 2-foot-deep hole needs roughly 0.06 cubic yards per post. For a full fence project, use our Fence Calculator alongside this tool.
Should I add rebar to my concrete pour?
Most slabs over 4 inches thick and any structural footing benefit from rebar reinforcement. Use our Rebar Weight Calculator to size it correctly.
How much does concrete cost per cubic yard?
Ready-mix delivered concrete typically runs $125 to $180 per cubic yard in the US, varying by region, mix strength, and delivery distance.
Can I use this calculator for UK measurements?
Yes. Toggle to UK units to switch all fields to metres, centimetres, and cubic metres, with pricing shown in £.
How thick should a concrete slab be?
Standard residential slabs (patios, walkways, sheds) are typically 4 inches thick; driveways and areas with vehicle loads should be 5 to 6 inches, often with rebar or mesh reinforcement.
How do I calculate concrete for sonotubes or post holes?
Sonotubes and post holes are cylinders, so the formula is π × radius² × height. For example, a 10-inch diameter tube (5-inch radius = 0.42 ft) that's 4 feet tall needs about 0.08 cubic yards. Multiply by how many tubes you're filling to get your total order.
What's the quickest way to estimate concrete for a 4-inch slab?
Divide your total square footage by 81. A 200 sq ft slab, for example, needs roughly 2.47 cubic yards at 4 inches thick. See the "Quick Trick" callout above for the full explanation and how to adjust it for other thicknesses.
How much concrete do I need for a curb and gutter run?
Multiply the cross-sectional area of your curb-and-gutter profile (its height, depth, and gutter lip combined) by the total linear length of the run. Our curb & gutter module handles this automatically once you enter your profile dimensions.
How much concrete do I need for steps or a staircase?
Calculate each step as rise × run × width, then add the volumes of all the steps together, plus any supporting footing beneath them. Our stairs module totals this for you automatically based on the number of steps you enter.
What PSI concrete should I use for a driveway?
Residential driveways typically need 3,500 to 4,000 PSI concrete; driveways that see trucks or RVs regularly should use 4,000 to 4,500 PSI for better durability.
How long before I can walk or drive on new concrete?
Light foot traffic is generally safe after 24 to 48 hours. Vehicles should wait about 7 days, once the slab has reached roughly 70% of its final strength. Full structural load is reached around 28 days.
How do I calculate concrete for an irregular or L-shaped area?
Break the footprint into simpler rectangles, triangles, or circles, calculate the volume of each section separately using the formulas above, then add the totals together.
Does this calculator work in square metres and cubic metres?
Yes. Switch to UK units and every field, including the output, converts to metres, centimetres, and cubic metres automatically.