Estimate volume, bag count, weight, mixer truckloads and cost for slabs, footings, columns, curbs and stairs.
No matter the shape of your project, every concrete calculation follows the same core idea: find the volume of space you need to fill, convert it to cubic yards and add a waste buffer. The calculator above handles the math automatically but understanding the steps makes you a smarter buyer at the ready-mix plant and helps you catch errors before they cost money.
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft) then divide by 27 to get cubic yards. Convert inches to feet first by dividing by 12. Add 10% for waste.
Select Slab for flat surfaces like driveways, patios and floors. Choose Column for round footings and fence posts. Pick Tube for hollow cylinders, Curb and Gutter for road edging or Stairs for stepped pours. Each tab uses the correct volume formula for that geometry.
Type length, width and thickness (or diameter and depth for round shapes). Every input has a unit selector. Choose feet, inches, yards, meters or centimeters. You can mix units freely such as length in feet and thickness in inches. The calculator converts everything automatically.
The results panel includes a waste slider from 0 to 20%. For most residential pours, 10% is the industry standard. Uneven subgrade, irregular shapes and spills all eat into your order so leaving the slider at 10% is the safest starting point.
The panel shows cubic yards, cubic feet, cubic meters, weight in lbs and kg, bag counts for 40, 60 and 80 lb bags, mixer truckloads and estimated cost. Enter a price per cubic yard to get a project cost estimate on the spot.
Convert thickness: 4 in ÷ 12 = 0.333 ft. Volume = 10 × 12 × 0.333 = 40 ft³. Divide by 27 = 1.48 yd³. Add 10% waste = 1.63 yd³ to order. At roughly $150/yd³ that is about $245 in ready-mix or approximately 68 bags of 60 lb concrete.
The same logic applies to every shape. The only thing that changes is the area formula used before multiplying by depth. Columns use π × r², tubes subtract the inner cylinder, curbs combine a rectangle and a step and stairs sum individual tread volumes. The tabs above handle each one automatically.
Each shape tab in the calculator uses a different geometric formula. Here is a full reference so you can verify any result or run a quick hand-calculation when you are on-site without a device.
| Shape | Formula (cubic feet) | Variables | Common use |
|---|---|---|---|
| Slab / Wall | L × W × H |
Length, Width, Height/Thickness (all in ft) | Driveways, patios, floors, footings |
| Column / Round Footing | π × (d÷2)² × h |
Diameter (ft), Height (ft) | Deck footings, fence posts, sonotubes, piers |
| Tube (hollow cylinder) | π × [(d1÷2)² − (d2÷2)²] × h |
Outer dia., Inner dia., Height (all ft) | Culverts, hollow columns, circular forms |
| Curb & Gutter | L × [(gw + cd) × ft + ch × cd] |
Length, Gutter width, Curb depth, Curb height, Flag thickness | Road edging, driveway aprons |
| Stairs | Σ (run × rise×n × width) + platform |
Run, Rise, Width, Platform depth, Number of risers | Exterior steps, entry stairs |
After calculating cubic feet, converting to cubic yards is always the same: divide by 27. To convert to cubic meters divide cubic feet by 35.3147. The calculator shows all three in the results panel at the same time.
| Convert from | To cubic feet | To cubic yards | To cubic meters |
|---|---|---|---|
| Inches | Divide by 12 first then use ft formula | Divide ft³ by 27 | Divide ft³ by 35.315 |
| Feet | Already in base unit | ÷ 27 | ÷ 35.315 |
| Yards | × 3 | Already in yd³ after calc | × 0.7646 |
| Meters | × 3.28084 | ÷ 27 after converting to ft | Already in m³ |
| Centimeters | × 0.0328084 | ÷ 27 after converting to ft | ÷ 1,000,000 (cm³ to m³) |
Pre-mixed concrete bags are sold by the bag but their yield is listed in cubic feet not cubic yards. The Quikrete concrete calculator and Sakrete concrete calculator both convert bag counts into cubic feet for this reason. Our tool matches this convention. Bag counts are calculated against your total cubic feet then grouped by bag weight.
Bagged concrete from brands like Quikrete and Sakrete is the go-to choice for small pours under about 1 cubic yard. Each bag has a fixed yield depending on its weight. The bag count in the results panel is calculated automatically but here is the formula so you can double-check any figure:
Number of bags = Total cubic feet ÷ Yield per bag. Always round up to the nearest whole bag. Never round down. Running short mid-pour forces you to leave a cold joint which is a structural weak point.
| Project | Size | Cubic Yards | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| Small patio | 8×8 ft, 4 in | 0.79 yd³ | 72 | 48 | 36 |
| Standard patio | 10×10 ft, 4 in | 1.23 yd³ | 112 | 74 | 56 |
| Deck footing ×4 | 12 in dia, 48 in deep | 0.98 yd³ | 88 | 59 | 44 |
| Driveway | 12×20 ft, 5 in | 3.70 yd³ | 334 | 223 | 167 |
| Garage slab | 20×20 ft, 5 in | 6.17 yd³ | 556 | 371 | 278 |
All figures above include a standard 10% waste factor. For sonotube or post-hole projects use the Column tab for individual hole volumes. Multiply by the number of holes and the bag count updates instantly.
Mixing bags by hand is practical up to about 1 cubic yard which is roughly 45 bags of 80 lb concrete. Beyond that labour and mixing time make ready-mix trucks the smarter choice. Most suppliers have a 1 yd³ minimum order. For a driveway or garage slab you almost always want a truck.
The cubic yard is the standard unit for ordering ready-mix concrete in the United States. One cubic yard is a cube measuring 3 feet on every side: 3 ft × 3 ft × 3 ft = 27 cubic feet. Everything at the ready-mix plant including pricing, minimum orders and truck capacity is measured in cubic yards so this is the number you need before you make a call.
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27. Convert thickness from inches to feet first. Divide thickness in inches by 12 to get thickness in feet.
| Slab Size | Thickness | Cubic Feet | Cubic Yards (no waste) | +10% Waste — Order This |
|---|---|---|---|---|
| 10×10 ft | 4 in | 33.3 | 1.23 yd³ | 1.35 yd³ |
| 12×12 ft | 4 in | 48.0 | 1.78 yd³ | 1.96 yd³ |
| 20×20 ft | 4 in | 133.3 | 4.94 yd³ | 5.43 yd³ |
| 24×24 ft | 5 in | 240.0 | 8.89 yd³ | 9.78 yd³ |
| 30×30 ft | 5 in | 375.0 | 13.89 yd³ | 15.28 yd³ |
| 12×20 ft | 5 in | 100.0 | 3.70 yd³ | 4.07 yd³ |
Under 1 yd³ (about 45 bags of 80 lb) bags from a hardware store are usually cheaper and more flexible. Between 1 and 3 yd³ weigh the labour cost of mixing bags against a short-load delivery fee which is typically $15 to $25 per yd³ under the minimum. Over 3 yd³ ready-mix is almost always the right call. Use the cost estimator in the results panel to compare both options for your specific volume.
One important note: ready-mix trucks typically carry 8 to 10 cubic yards per load. If your project needs more than one truck schedule the pours as close together as possible to avoid cold joints between loads.
Concrete pricing varies by region, mix design and supplier. Understanding the typical price range helps you spot a bad quote and budget your project with confidence. The cost estimator built into every results panel lets you enter your local price per cubic yard and instantly see total project cost.
Total cost = Cubic yards (with waste) × Price per cubic yard. Enter your price in the results panel. The calculator applies your waste factor before multiplying so the estimate reflects what you will actually order and not just the base volume.
Ready-mix suppliers price concrete by the cubic yard. Most have a minimum order of 1 cubic yard with a short-load surcharge for anything below their standard minimum which is often 5 to 7 yards. Always call at least two suppliers. Prices for the same mix can vary by $20 to $40 per yard depending on your location and the season.
| Project Type | Typical Size | Cubic Yards | Est. Cost @ $150/yd³ | Est. Cost @ $175/yd³ |
|---|---|---|---|---|
| Small patio | 10×10 ft, 4 in | 1.35 yd³ | ~$202 | ~$236 |
| Driveway | 12×20 ft, 5 in | 4.07 yd³ | ~$611 | ~$712 |
| Garage slab | 20×20 ft, 5 in | 6.79 yd³ | ~$1,019 | ~$1,188 |
| Large driveway | 24×24 ft, 5 in | 9.78 yd³ | ~$1,467 | ~$1,712 |
| Deck footings ×6 | 12 in dia, 48 in deep | 1.47 yd³ | ~$221 | ~$257 |
All estimates above include the 10% waste factor. These are material-only figures. Labour, formwork, finishing and reinforcement like rebar, mesh and fiber are separate costs. For a full concrete slab cost including labour use the price field in the results panel and add your contractor's per-yard labour rate to the material price before entering it.
Many ready-mix suppliers add a fuel surcharge of $5 to $15 per yard on top of the base price. Saturday or after-hours deliveries can cost $50 to $200 extra per truck. Always ask for a fully loaded quote that includes delivery, fuel and any weekend fees before comparing suppliers.
As a quick sanity check, residential concrete slabs typically cost $4 to $8 per square foot for materials and basic labour at standard 4-inch thickness. A 10×10 patio = 100 sq ft × $6 average = about $600 total installed. Use this to gut-check any contractor quote. A price that is significantly higher or lower warrants a follow-up question.
Thickness is the single biggest driver of how much concrete you need. Increase a 10×10 slab from 4 inches to 6 inches and your cubic yards jump by 50%. Getting the thickness right for your project type is essential. Too thin and the slab cracks under load. Too thick and you are pouring money into the ground.
| Project Type | Min. Thickness | Recommended | Rebar / Mesh? | Notes |
|---|---|---|---|---|
| Sidewalk / Path | 3.5 in | 4 in | Optional mesh | Pedestrian only. Grade well for drainage. |
| Patio | 3.5 in | 4 in | Optional mesh | Control joints every 8 to 10 ft to prevent cracking. |
| Residential driveway | 4 in | 4–5 in | Mesh or #3 rebar | Use 5 in for heavier vehicles or clay soils. |
| Garage floor | 4 in | 5–6 in | #3 or #4 rebar | Vapor barrier under slab is strongly recommended. |
| Commercial driveway | 5 in | 6–8 in | #4 rebar required | Trucks and delivery vehicles. Engineer sign-off often required. |
| Deck / post footing | Varies | Below frost line | Optional | Depth matters more than diameter. Check local frost depth. |
| Foundation wall | 6 in | 8–12 in | #4–#5 rebar required | Structural. Always follow engineer specifications. |
A 20×20 ft garage slab at 4 inches needs 4.94 yd³. The same slab at 6 inches needs 7.41 yd³ which is 50% more concrete. If you are on the fence about thickness use the Slab tab above to compare volumes side by side before making a decision. It takes 10 seconds.
Rebar and wire mesh do not change the volume of concrete you need because they sit inside the pour. However adding reinforcement is almost always worth it for driveways, garage floors and any slab subject to freeze-thaw cycles. The small extra cost in steel is far cheaper than a cracked slab repair later.
A 5-inch slab on poorly compacted fill will crack faster than a 4-inch slab on properly compacted gravel. Before pouring remove organic material, compact the subgrade to at least 95% Proctor density and add 4 to 6 inches of compacted gravel base. This is especially important in areas with expansive clay soils or significant frost heave.
Even the best concrete calculator is only as accurate as the measurements you give it. Small errors in dimension entry — particularly thickness — compound quickly and can leave you short on pour day. These practical tips help you go from calculator to confident order.
For slabs measure length and width at multiple points because forms are rarely perfectly parallel. Use the smallest measurement as your input if you want to be conservative or use the average for a more typical estimate. A 2-inch error on a 20-foot dimension adds almost a cubic yard on a large slab.
The 10% default waste factor accounts for uneven subgrade, form bulging, spills and the inevitable overpour at edges. For irregular shapes or rocky subgrades bump it to 15%. For very flat and well-prepared surfaces 5 to 7% may be sufficient. Running out mid-pour creates a cold joint which is a permanent structural weakness.
This is the most common manual calculation mistake. Four inches is 0.333 feet not 4. If you are spot-checking the calculator's result always divide your inch measurement by 12 before multiplying. The calculator's unit selectors handle this automatically but it is worth understanding when verifying on paper.
An L-shaped driveway or a patio with a cut-out cannot be calculated as one rectangle. Break the shape into simple rectangles, run each one through the Slab tab and add the results. The calculator shows cubic yards for a single shape per tab so multi-shape projects require multiple calculations.
Ready-mix suppliers deliver in 0.25 yd³ increments at minimum. If your calculation gives 3.12 yd³ with waste order 3.25 yd³ and not 3.0. The cost difference of a quarter yard is far less than the cost of a second delivery or a cold joint repair. When in doubt the extra concrete can go into a small footing or post hole.
Your excavation depth includes both the concrete thickness and the gravel base below it. If you are digging 10 inches total but only pouring 4 inches of concrete enter 4 inches in the calculator and not 10. The gravel volume is calculated separately and does not count toward your concrete order.
Ready-mix concrete has a working window of about 90 minutes from when water is added at the plant. For large pours requiring multiple trucks confirm the delivery interval so each load arrives before the previous one starts to set. Most suppliers can schedule trucks 30 to 45 minutes apart for continuous pours.
Post holes and deck footings are cylindrical not rectangular. Calculating them as a slab overestimates volume significantly. Use the Column tab, enter the diameter and depth of a single hole, set quantity to the number of holes and get an exact bag count. For a 12-inch diameter hole at 48 inches deep you need about 2 bags of 80 lb concrete per hole and not the 5 or more that a slab calculation would suggest.
This calculator estimates volume of concrete. It does not specify mix design, PSI strength or water-cement ratio. For structural foundations, retaining walls and commercial projects always consult a structural engineer or follow your local building code for the correct mix specification. The standard residential mix is 3,000 to 4,000 PSI but footings and foundations often require 4,000 to 5,000 PSI.
✓ Dimensions measured in multiple spots and averaged
✓ Thickness entered in the correct unit (inches vs feet)
✓ Waste factor set to at least 10%
✓ Irregular shapes broken into separate calculations
✓ Order quantity rounded up to the nearest 0.25 yd³
✓ Gravel base depth not included in concrete thickness
✓ Supplier called for a firm delivered price including fuel surcharge
With these steps in place the estimate from the calculator above will match your actual order within a few percent. The biggest source of surprise cost on concrete projects is not calculation error — it is forgetting to add the waste factor and the delivery surcharges. The results panel handles both automatically once you set your waste percentage and enter the supplier's full delivered price per yard.
The most common questions about calculating concrete, ordering ready-mix and using this tool answered clearly.
Multiply length × width × thickness all in feet to get cubic feet. Divide by 27 to get cubic yards. Add 10% for waste. That is the complete calculation for any rectangular slab, wall or footing.
For round shapes like columns and post holes the formula changes to π × (diameter÷2)² × depth. For stairs each tread is calculated separately. The calculator above handles all five shapes automatically. Select the right tab, enter your dimensions and the result appears instantly.
A 10×10 ft slab at 4 inches thick requires 1.23 cubic yards of concrete before any waste allowance. With the standard 10% overage added you should order 1.35 cubic yards.
The calculation: convert 4 inches to feet (4 ÷ 12 = 0.333 ft) then multiply 10 × 10 × 0.333 = 33.3 cubic feet ÷ 27 = 1.23 yd³. Add 10%: 1.23 × 1.10 = 1.35 yd³.
Each 80 lb bag of concrete yields 0.60 cubic feet. Since there are 27 cubic feet in a cubic yard you need approximately 45 bags of 80 lb concrete per cubic yard (27 ÷ 0.60 = 45).
For other bag sizes: a 60 lb bag yields 0.45 ft³ so you need about 60 bags per cubic yard. A 40 lb bag yields 0.30 ft³ requiring about 90 bags per cubic yard.
The formula for a rectangular slab is: Volume (yd³) = [Length (ft) × Width (ft) × Thickness (ft)] ÷ 27
The key step most people miss is converting thickness from inches to feet first. Divide your inch measurement by 12 before using it in the formula. For example 4 inches = 4 ÷ 12 = 0.333 feet.
Full example: 12 ft × 20 ft driveway at 5 inches thick. Convert: 5 ÷ 12 = 0.417 ft. Volume = 12 × 20 × 0.417 = 100 ft³ ÷ 27 = 3.70 yd³. Add 10% waste = 4.07 yd³ to order.
The calculator uses standard geometric volume formulas identical to those used by structural engineers and ready-mix suppliers. Given accurate input measurements the mathematical result is exact.
Real-world accuracy depends on your measurements. Common sources of error include forms that are not perfectly parallel, subgrade that varies in depth and rounding dimensions to the nearest foot. For most projects the calculator result will be within 2 to 5% of the actual concrete needed before the waste factor is applied.
The decision comes down to volume and labour. Here is the simple rule of thumb used by most contractors:
Under 1 cubic yard: use bags. You can mix them one at a time, work at your own pace and avoid delivery minimums. Roughly 45 bags of 80 lb concrete per cubic yard.
1 to 3 cubic yards: it depends. If you have a mixer and help bags may still be practical. If not a ready-mix short load is often worth the delivery fee.
Over 3 cubic yards: order ready-mix. The labour and time cost of mixing that many bags by hand almost always exceeds the price difference.
Cement is an ingredient in concrete. It is the fine grey powder (Portland cement) that acts as the binding agent. Concrete is the finished material — the hardened mixture of cement, water, sand (fine aggregate) and gravel or crushed stone (coarse aggregate).
A common analogy: cement is to concrete what flour is to bread. You cannot build with cement alone any more than you can make a sandwich with just flour.
This calculator estimates the volume of finished concrete needed for your project and not the quantity of cement powder. If you are mixing concrete from raw materials a standard mix ratio is approximately 1 part cement : 2 parts sand : 3 parts gravel by volume.
Multiply your square footage by thickness in feet then divide by 27. The formula is: yd³ = (sq ft × thickness in ft) ÷ 27
Example: you have 200 square feet of patio area at 4 inches thick. Convert 4 inches to feet: 4 ÷ 12 = 0.333 ft. Then: 200 × 0.333 = 66.6 ft³ ÷ 27 = 2.47 yd³. Add 10% = 2.72 yd³ to order.
If you know the square footage but not the separate length and width you can still use the Slab tab above. Just enter the square root of your area for both length and width or enter 1 for length and your full square footage as the width. The volume result will be identical.
Concrete does not simply dry. It cures through a chemical reaction called hydration. Here are the key milestones for standard 3,000 PSI residential concrete:
24 to 48 hours: Initial set. Foot traffic is possible but avoid concentrated loads. Do not drive on it.
7 days: Approximately 70% of design strength. Light vehicle traffic may be acceptable depending on thickness and use.
28 days: Approximately 99% of design strength. Full load bearing. This is the standard engineering benchmark for concrete strength testing.
Keep freshly poured concrete moist for the first 7 days to maximize strength. Cover it with burlap or plastic sheeting and mist it daily. Concrete that dries too quickly loses strength. In hot weather or direct sun curing is especially critical.
A cubic yard is a volume equal to 3 feet × 3 feet × 3 feet = 27 cubic feet. Picture a cube about the size of a standard washing machine. That is roughly one cubic yard.
In practical terms for concrete: one cubic yard weighs approximately 4,050 lbs (about 2 tons) for standard mix. It can fill a 10×10 ft slab to just under 4 inches thick or a 10×12 ft patio to about 3.3 inches.
Ready-mix concrete is always priced and ordered in cubic yards in the United States. Typical ready-mix truck capacity is 8 to 10 cubic yards. Most suppliers have a minimum order of 1 cubic yard with short-load fees for smaller quantities.