Calculate how much concrete you need for any project. Get cubic yards, cubic metres and bag counts with waste included.
Calculating the right amount of concrete before a project starts is essential to avoid costly over-ordering or running short mid-pour. Both scenarios are expensive: too much concrete means wasted material and disposal costs; too little means a partial pour that must stop and restart, creating cold joints that weaken the finished structure. Our concrete calculator covers the four most common residential concrete applications — slabs, footings, columns and steps — and includes a configurable waste factor to account for spillage, irregular subgrade and form compression. Results are shown in cubic yards, cubic metres, cubic feet and bag counts for both 60lb and 80lb bags.
A concrete slab is the most common residential application — driveways, patios, garage floors, sidewalks and shed pads. The volume formula is straightforward: Length x Width x Depth (all in the same units). The depth is critical: standard slabs are 4 inches (0.333 feet) for pedestrian use and light vehicles, 5-6 inches for heavy vehicles, and 3.5 inches minimum for basement floors. A 10ft x 12ft patio slab at 4 inches depth: 10 x 12 x 0.333 = 40 cubic feet = 40/27 = 1.48 cubic yards before waste. Adding 10% waste: 1.48 x 1.10 = 1.63 cubic yards, or 74 bags of 80lb pre-mix. Always add at least 10% for waste — experienced contractors often add 15% for slabs with irregular perimeters. Use our area calculator to calculate the slab surface area first, then input the dimensions here for volume. Our unit converter handles any measurement conversions needed between imperial and metric.
| Slab Size | Depth | Cubic Yards | 80lb Bags | 60lb Bags |
|---|---|---|---|---|
| 10 x 10 ft | 4 in | 1.23 | 56 | 74 |
| 10 x 20 ft | 4 in | 2.47 | 112 | 148 |
| 12 x 20 ft | 4 in | 2.96 | 134 | 178 |
| 20 x 20 ft | 4 in | 4.94 | 224 | 296 |
| 20 x 30 ft | 4 in | 7.41 | 336 | 444 |
| 20 x 20 ft | 6 in | 7.41 | 336 | 444 |
Footings are below-grade concrete structures that transfer building loads to the soil. They are typically rectangular in cross-section and run continuously around the perimeter of a structure. The footing volume formula is: Length x Width x Depth. Width and depth are typically specified in inches in residential construction. A frost wall footing for a 20-foot run at 12 inches wide by 8 inches deep: 20 x (12/12) x (8/12) = 20 x 1 x 0.667 = 13.33 cubic feet = 0.49 cubic yards. Frost depth determines the minimum footing depth — footings must extend below the frost line in your region to prevent frost heave. Frost lines range from 0 inches in Florida to 60+ inches in Minnesota. Column footings (pads) are square or rectangular pads beneath individual posts or columns, calculated as a simple rectangular volume. Always add 15-20% waste for footing pours due to irregular excavation shapes and soil absorption.
The choice between pre-mixed bags and ready-mix concrete depends primarily on project volume and access. For projects under 0.5 cubic yards (approximately 27 bags of 80lb), mixing bags is usually more economical when factoring in the minimum order charge and short-load fees that ready-mix plants impose. For projects between 0.5 and 1.5 cubic yards, the economics are comparable — compare the total bag cost (including rental of a mixing drum if you have more than 20 bags) against the delivered ready-mix price including all fees. For projects over 1.5 cubic yards, ready-mix is almost always more economical per cubic yard and dramatically reduces labour. An 80lb bag yields approximately 0.60 cubic feet (0.022 cubic yards). A 60lb bag yields approximately 0.45 cubic feet. One cubic yard requires 45 bags of 80lb or 60 bags of 60lb. Ready-mix trucks carry 8-10 cubic yards. Use our discount calculator to compare supplier pricing and our percentage calculator to calculate cost per cubic yard across quotes.
| Project Type | Typical Size | Cubic Yards | Recommendation |
|---|---|---|---|
| Fence post hole | 1 post | 0.01-0.02 | 50lb fast-set bag |
| Mailbox base | 1 hole | 0.05 | 60/80lb bag |
| Small garden path | 3x10 ft, 3in | 0.28 | 13 x 80lb bags |
| Patio slab | 10x12 ft, 4in | 1.63 | Bags or ready-mix |
| Driveway | 12x20 ft, 4in | 3.26 | Ready-mix truck |
| Garage floor | 20x20 ft, 4in | 5.43 | Ready-mix truck |
Concrete compressive strength is rated in PSI (pounds per square inch) after 28 days of curing. The standard mix ratios and their applications: 2,500 PSI (1:3:5 ratio) is suitable for light-duty residential flatwork such as sidewalks and residential driveways with passenger vehicles only. 3,000 PSI (1:2:4 ratio) is the most common residential mix for slabs, footings and general construction — adequate for light vehicle traffic and residential foundations. 3,500-4,000 PSI (1:1.5:3 ratio) is used for commercial driveways, heavy vehicle traffic, industrial floors and structural elements. 5,000+ PSI mixes are used in high-strength structural applications including bridges, parking structures and high-rise building cores. Pre-mixed bags from hardware stores are typically formulated for 4,000 PSI, making them stronger than required for most residential applications — a quality advantage of bag mixes over imprecisely proportioned site-mixed concrete.
Proper curing is as important as the mix design in determining final concrete strength. Concrete does not "dry" in the conventional sense — it undergoes a chemical hydration reaction that requires water. Curing is the practice of maintaining adequate moisture and temperature for this reaction to proceed fully. Concrete that dries out too quickly (hot, sunny, windy conditions) develops surface cracking and achieves only 50-60% of its rated strength. Curing methods include: wet curing with plastic sheeting or burlap kept moist for 7 days, curing compounds sprayed on the surface immediately after finishing, and water ponding for slabs. Temperature affects curing rate significantly: curing slows below 50°F (10°C) and stops below 40°F (4°C). Fresh concrete must not freeze within the first 24 hours or permanent damage occurs. In cold weather, use heated enclosures or insulating blankets. In hot weather, pour in the early morning and keep the surface wet. The concrete reaches 28-day design strength only when cured properly throughout the first month.
Ready-mix concrete prices in the US range from approximately $120-180 per cubic yard for standard 3,000 PSI mix, depending on region, supplier and order volume. Additional charges include delivery ($100-200 per truck), short-load fees for orders under the minimum ($50-150 extra), Saturday delivery premiums, and fuel surcharges. A 20x20 ft garage floor at 4 inches thick requires approximately 5 cubic yards: at $150/yd plus $150 delivery = $900 in concrete materials. Pre-mixed 80lb bags typically retail for $6-9 each. The same 5 cubic yard project would require approximately 225 bags at $7 each = $1,575 in materials — significantly more expensive than ready-mix for this volume, not counting the considerable labour of mixing. Use our percentage calculator to calculate material cost per square foot for your project budget.
Plain concrete has high compressive strength but low tensile strength — it can bear heavy loads pushing down but cracks easily under bending or tension. Steel reinforcement (rebar or wire mesh) compensates for this weakness by providing tensile strength, resulting in reinforced concrete that resists both compression and tension. For residential slabs, 6-inch wire mesh (6x6 W1.4xW1.4 welded wire fabric) is standard for patios and sidewalks. Rebar (deformed steel bars) is used for driveways subject to vehicle traffic, footings and structural slabs. Common rebar sizes: #3 (3/8 inch diameter) for residential slabs and walkways, #4 (1/2 inch) for driveways and lightly loaded slabs, #5 (5/8 inch) for heavily loaded commercial applications. Rebar should be placed in the middle third of the slab depth, typically 2 inches from the bottom in a 4-inch slab. Lap splices where rebar pieces overlap should be at least 12 inches. For a 4-inch slab with #4 rebar at 12-inch spacing in both directions, you need approximately 165 linear feet of rebar per 100 square feet of slab. Control joints (saw cuts to 1/4 slab depth within 24 hours of pour) prevent random cracking by giving the concrete a predetermined place to crack. Space control joints at intervals equal to 2-3 times the slab thickness in feet -- every 8-10 feet for a 4-inch slab. Use our area calculator to determine total slab area and our percentage calculator to estimate the additional cost percentage for reinforcement versus plain concrete.