Unit system
Length
Width
Depth/Thickness
Full breakdown (click to copy)
Before You Order
Bags vs. ready-mix
Under about 0.5 cubic yards, bags are usually cheaper and more practical. Over 1 cubic yard, ready-mix delivered by truck is typically faster and more cost-effective despite the delivery minimum — check both before committing.
The bag count already includes waste
The 60lb/80lb bag figures shown already include a 10% overage for spillage and uneven subgrade — you don't need to add further margin on top of these numbers.
Confirm your shape is rectangular
This calculator computes rectangular volume (length × width × depth). For round columns or piers, calculate volume separately and enter the equivalent rectangular dimensions — see the note above the calculator.
How to Use the Concrete Calculator
- Select unit system — feet (with depth in inches) for US/imperial projects, metres for metric projects.
- Enter length and width — the horizontal dimensions of the area to be filled.
- Enter depth/thickness — for slabs, the thickness in inches (imperial) or metres (metric). A standard driveway is 4 inches; a patio is 3–4 inches; a structural slab is 6+ inches.
- Click Calculate — see volume in cubic yards, cubic feet, cubic metres, and the number of 60lb and 80lb bags needed.
- Add 10% for waste — the calculator automatically adds a 10% overage. Always buy slightly more than calculated — running short means a second trip and potential colour/batch mismatch.
🏗️ Ready-mix vs bags: For projects under 0.5 cubic yards (small repairs, fence posts), bags are practical. For 0.5–1 cubic yard, bags are cheaper but labour-intensive. For over 1 cubic yard, ready-mix concrete delivered by truck is faster, more consistent, and often more economical despite minimum delivery charges (typically 1 cubic yard minimum).
Understanding Concrete
🧱 What Is Concrete?
A mixture of Portland cement (binding agent), sand (fine aggregate), gravel/crushed stone (coarse aggregate), and water. The cement reacts chemically with water (hydration) to form a hard crystalline structure that bonds the aggregates together. Concrete is strong in compression but weak in tension — steel reinforcement (rebar) compensates for tensile weakness.
📊 Concrete Mix Ratios
Standard mix (1:2:3): 1 part cement, 2 parts sand, 3 parts aggregate by volume. Strong mix (1:1.5:3): more cement for higher strength. For 2,500 PSI (typical residential): 1:2:3. For 3,500 PSI (driveways, structural): 1:1.5:3. For 5,000 PSI (heavy structures): specialist mixes. Bagged concrete (Quikrete, Sakrete) has the correct proportions pre-mixed.
⏰ Setting vs Curing
Setting: concrete stiffens within 2–8 hours (depending on temperature and mix). Curing: the strength-gaining process continues for 28 days. At 7 days, concrete reaches approximately 70% design strength. At 28 days: 100% design strength. At 1 year: 110–115%. Never load concrete before 7 days; avoid vehicle traffic for 28 days on driveways.
🌡️ Temperature Effects
Cold weather: water freezes before hydration completes, causing permanent weakness. Do not pour concrete below 40°F (4°C) without cold-weather precautions (heated enclosures, warm water, insulating blankets). Hot weather: water evaporates too quickly, weakening the surface. Pour in early morning, keep moist during curing with wet burlap or curing compound.
💧 Water-Cement Ratio
The single most important factor in concrete strength: lower water-cement ratio = stronger concrete. A ratio of 0.45 produces strong concrete; 0.65 produces weaker concrete but is more workable. Never add extra water to improve workability — use admixtures (plasticisers) instead. Each additional gallon of water per bag reduces compressive strength by 200–300 PSI.
🔧 Reinforcement
Rebar: steel bars wired together before pouring. Provides tensile strength concrete lacks. Minimum 2 inches of concrete cover over rebar. Wire mesh: welded wire fabric for slabs — controls crack width but provides less structural reinforcement than rebar. Fibre reinforcement: synthetic or steel fibres mixed in for crack control in non-structural applications.
Concrete Project Planning
Slab thickness guide
Thickness directly affects concrete volume and structural capacity. Sidewalk (pedestrian only): 3–4 inches minimum. Patio (furniture, light use): 3–4 inches. Driveway (passenger cars): 4 inches minimum, 5 inches for SUVs and trucks. Garage floor: 4–6 inches. Foundation/structural slab: 6+ inches with rebar. Each additional inch of thickness increases material cost by 25% for the same area — correct thickness selection balances cost with structural requirements.
Estimating and ordering
Always order 5–10% more concrete than calculated volume. Concrete cannot be easily returned once ordered and wastage from forms, uneven subgrade, and spillage is inevitable. For ready-mix: concrete is sold by the cubic yard and has minimum order quantities (typically 1 yard). Partial loads incur short-load fees. For bags: one 80lb bag makes approximately 0.60 cubic feet (0.022 cubic yards), so a 1-yard pour requires about 45 bags; one 60lb bag makes approximately 0.45 cubic feet, so the same pour requires about 60 bags — time-consuming to mix manually.
Site preparation is critical
The concrete itself is only part of a successful slab. Subgrade preparation determines long-term performance. Remove organic matter (soil settles and compresses). Compact subgrade to prevent differential settling. Add 4–6 inches of compacted gravel base for drainage and stable support. Install vapour barrier (6-mil plastic) for indoor slabs to prevent moisture migration. Set forms at correct height and level. These preparation steps prevent the cracking and settling that account for most concrete failures.
🏗️ Concrete project checklist: Subgrade compacted and level ✓. Gravel base installed (4–6 inches) ✓. Forms set at correct height ✓. Rebar or wire mesh positioned ✓. Concrete ordered with 10% overage ✓. Tools ready (screed, float, edger, broom) ✓. Curing compound or wet burlap available ✓. Weather checked (no rain or frost within 24 hours) ✓. These eight steps prevent the most common concrete project failures.
Concrete Volume Formula
Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12. Bags needed = Volume × 1.1 (waste) ÷ Bag Yield (0.45 ft³ for 60lb, 0.60 ft³ for 80lb).
Worked example
A 10ft × 12ft slab at 4 inches deep: Volume = 10 × 12 × 4 ÷ 12 = 40 ft³ = 1.48 yd³. With 10% waste: 44 ft³. Bags needed: 44 ÷ 0.45 = 98 bags of 60lb mix, or 44 ÷ 0.60 = 74 bags of 80lb mix. Ready-mix order: 1.48 × 1.1 = 1.63 yd³.
Avoid These Mistakes
Mixing up feet and inches for depth
In the feet/inches mode, length and width are in feet but depth is in inches — entering depth as feet (e.g. "4" meaning 4 feet instead of 4 inches) overestimates volume 12×.
Ordering exactly the calculated amount
Uneven subgrade, form leakage, and spillage are normal. The 10% waste allowance already built into these figures is a minimum, not a guaranteed buffer — round up further on irregular or hand-dug sites.
Using a rectangular formula for round shapes
Columns, piers, and post holes need π × radius² × height, not length × width × height. Using this calculator directly on a round shape without converting will give the wrong volume.
Skipping subgrade preparation
Correct volume doesn't fix a poorly prepared site — uncompacted subgrade is the leading cause of cracking and settling regardless of how accurately the concrete was ordered.
Frequently Asked Questions
How much concrete do I need?
Multiply length × width × depth to get volume. For a 10ft × 12ft × 4in slab: 10 × 12 × (4/12) = 40 cubic feet = 1.48 cubic yards. Always add 10% for waste and spillage.
How many bags of concrete per cubic yard?
A 60lb bag makes about 0.45 cubic feet (0.017 cubic yards). An 80lb bag makes about 0.60 cubic feet (0.022 cubic yards). You need about 60 × 60lb bags or 45 × 80lb bags per cubic yard (27 ÷ 0.45 = 60, 27 ÷ 0.60 = 45).
When should I use ready-mix vs bags?
For small projects under 1 cubic yard, bags are cheaper and more practical. For larger projects (driveways, foundations), ready-mix concrete delivered by truck is more cost-effective and ensures consistent mixing quality.
References
📄 Bag manufacturer data sheets
Quikrete, Sakrete, and other major bagged-concrete brands publish yield-per-bag figures on their technical data sheets — the 0.45 ft³ (60lb) and 0.60 ft³ (80lb) figures used here match published values, but always confirm against your specific product's packaging.
🏗️ Your local building code
Minimum slab thickness, rebar requirements, and frost-depth rules for footings vary by jurisdiction. Check your local building code before finalizing dimensions for anything structural.
📞 Your ready-mix supplier
Minimum order quantities, short-load fees, and delivery windows vary by supplier — confirm directly before planning a pour date.