Concrete Cubic Feet Calculator
Intuitive volume calculations, bag conversion equivalents, and load weight.
Concrete Cubic Feet Calculator: Get Volume, Bags & Yards for Any Project
Cubic feet is the most intuitive volume unit for concrete work. It connects directly to the dimensions on your tape measure, the coverage printed on every bag of pre-mix concrete, and the intermediate step in every ready-mix yard calculation. Whether you are calculating how many bags to buy for a post hole or converting your slab volume to cubic yards for a supplier order, cubic feet is the unit that ties everything together.
This concrete cubic feet calculator converts your project dimensions into cubic feet instantly for any shape slabs, cylinders, footings, walls, and stairs. From your cubic feet result, you can immediately determine how many 40 lb, 60 lb, or 80 lb pre-mix bags you need, what the equivalent volume is in cubic yards for a ready-mix order, and what the total weight of the pour will be for structural and logistics planning.
Use this calculator any time you need to work in cubic feet whether for bag calculations, mix design ingredient quantities, structural dead load assessments, or simply as the stepping stone to your cubic yard order. It is the most versatile single-unit concrete calculator available, and it covers every shape and conversion you will encounter in residential and commercial concrete work.
What Is a Cubic Foot of Concrete?
A cubic foot is a unit of volume equal to a cube measuring 1 foot on each side: 1 ft x 1 ft x 1 ft = 1 cubic foot. In concrete work, it is the most practical intermediate unit because:
- Bag yields are expressed in cubic feet—an 80 lb bag yields 0.60 ft³, a 60 lb bag yields 0.45 ft³, a 40 lb bag yields 0.30 ft³
- Mix design ingredient quantities are calculated per cubic foot of finished concrete
- Structural dead load calculations use lbs per square foot, derived from ft³
- The ready-mix conversion is simple: divide cubic feet by 27 to get cubic yards
- One cubic foot of standard concrete weighs approximately 150 lbs
Understanding the physical scale of a cubic foot helps you verify calculations:
- One cubic foot fills a space 12 inches x 12 inches x 12 inches
- One cubic foot covers 9 square feet at a 1.33-inch thickness
- One cubic foot covers 4.5 square feet at a 2.67-inch (roughly 2¾-inch) thickness
- One cubic foot covers 3 square feet at a 4-inch thickness
- 27 cubic feet = 1 cubic yard = one standard batching unit of ready-mix concrete
Key relationship: 1 cubic foot of concrete = 150 lbs = 0.037 cubic yards = 28.32 liters. These conversions are the backbone of all concrete quantity, weight, and cost calculations.
Concrete Cubic Feet Formulas for Every Shape
1. Rectangular Slab, Floor, Walkway, or Wall
Cubic Feet = Length (ft) x Width (ft) x Thickness (ft)
Thickness conversion: inches / 12 = feet | 2 in = 0.167 ft | 3 in = 0.250 ft | 4 in = 0.333 ft | 5 in = 0.417 ft | 6 in = 0.500 ft | 8 in = 0.667 ft
2. Cylinder (Column, Post Hole, Pier, Sonotube)
Cubic Feet = π x (Diameter / 2)² x Height (ft)
Convert diameter from inches to feet: diameter (in) / 12 = diameter (ft). Radius = Diameter (ft) / 2.
3. Rectangular Footing (Pad or Strip)
Cubic Feet = Length (ft) x Width (ft) x Depth (ft)
4. Concrete Wall
Cubic Feet = Length (ft) x Height (ft) x Thickness (ft)
For tapered walls: use average thickness = (Top + Bottom) / 2
5. Stair Steps
Cubic Feet per step = Step Number x Rise (ft) x Run (ft) x Width (ft)
Total Cubic Feet = Sum of all steps + Landing platform volume
6. Converting Cubic Feet to Other Units
Cubic Yards = Cubic Feet / 27
Cubic Meters = Cubic Feet x 0.02832
Weight (lbs) = Cubic Feet x 150
US Tons = (Cubic Feet x 150) / 2,000
7. Pre-Mix Bag Count from Cubic Feet
80 lb bags = Cubic Feet / 0.60 (round up)
60 lb bags = Cubic Feet / 0.45 (round up)
40 lb bags = Cubic Feet / 0.30 (round up)
8. Waste Factor Modifier
Adjusted Cubic Feet = Calculated Cubic Feet x Waste Factor
Standard: x 1.10 (10%) | Rough terrain: x 1.15 | Complex shapes: x 1.20
Step-by-Step Cubic Feet Calculation Guide
- Measure all project dimensions in feet and inches.
- Convert any inch measurements to feet: divide inches by 12.
- Apply the correct formula for your shape to get cubic feet.
- For multiple structures, calculate cubic feet for each and sum the results.
- Multiply total cubic feet by your waste factor (1.10 standard).
- Divide adjusted cubic feet by 27 to get cubic yards for ready-mix orders.
- Divide adjusted cubic feet by the bag yield for pre-mix bag count.
- Multiply adjusted cubic feet by 150 for total weight in pounds.
Real-World Cubic Feet Calculation Examples
Example 1: Small Patio Slab 10 x 12 ft, 4 inches thick
| Step | Calculation | Result |
|---|---|---|
| Convert thickness | 4 in / 12 | 0.333 ft |
| Cubic feet (exact) | 10 x 12 x 0.333 | 39.96 ft³ |
| Add 10% waste | 39.96 x 1.10 | 43.96 ft³ |
| Cubic yards | 43.96 / 27 | 1.63 yd³ → order 1.75 yd³ |
| 80 lb bags (alt.) | 43.96 / 0.60 | 74 bags |
| 60 lb bags (alt.) | 43.96 / 0.45 | 98 bags |
| Total weight | 43.96 x 150 | 6,594 lbs / 3.30 tons |
Example 2: Five Fence Post Holes 8-inch diameter, 2.5 ft deep
| Step | Calculation | Result |
|---|---|---|
| Convert diameter | 8 in / 12 | 0.667 ft |
| Radius | 0.667 / 2 | 0.333 ft |
| Cubic feet per hole | 3.14159 x (0.333)² x 2.5 | 0.873 ft³ |
| Cubic feet for 5 holes | 0.873 x 5 | 4.36 ft³ |
| Add 10% waste | 4.36 x 1.10 | 4.80 ft³ |
| 60 lb bags | 4.80 / 0.45 | 11 bags |
| 80 lb bags | 4.80 / 0.60 | 8 bags |
Example 3: Retaining Wall 20 ft long, 3.5 ft high, 8 inches thick
| Step | Calculation | Result |
|---|---|---|
| Convert thickness | 8 in / 12 | 0.667 ft |
| Cubic feet (exact) | 20 x 3.5 x 0.667 | 46.69 ft³ |
| Add 10% waste | 46.69 x 1.10 | 51.36 ft³ |
| Cubic yards | 51.36 / 27 | 1.90 yd³ → order 2.00 yd³ |
| Weight | 51.36 x 150 | 7,704 lbs / 3.85 tons |
| Supply method | — | Ready-mix preferred (over 1 yd³) |
Example 4: Mix Design Ingredients from Cubic Feet (1:2:3 ratio)
| Ingredient | Per Cubic Foot (1:2:3 mix) | For 43.96 ft³ (patio example) |
|---|---|---|
| Portland cement | 19.15 lbs (0.204 ft³) | 841 lbs / ~9 x 94 lb bags |
| Sand | 38.30 lbs (0.387 ft³) | 1,683 lbs / ~0.84 tons |
| Gravel (3/4 in) | 57.44 lbs (0.536 ft³) | 2,524 lbs / ~1.26 tons |
| Water (w/c 0.50) | 9.59 lbs (0.154 ft³) | 421 lbs / ~50 gallons |
Mix design note: Cubic feet is the working unit for site-mixed concrete ingredient calculations. Knowing your pour volume in cubic feet lets you calculate exact cement, sand, aggregate, and water quantities for any batch ratio without first converting to cubic yards.
Cubic Feet Quick Reference Tables
Pre-calculated cubic feet volumes for common project sizes. All values include a 10% waste factor.
Rectangular Slabs 4-Inch Thickness (with 10% waste)
| Slab Size | Exact ft³ | ft³ (+10%) | Cubic Yards | 80 lb Bags | Weight (lbs) |
|---|---|---|---|---|---|
| 6 x 6 ft | 12.0 ft³ | 13.2 ft³ | 0.49 yd³ | 22 bags | 1,980 lbs |
| 8 x 8 ft | 21.3 ft³ | 23.5 ft³ | 0.87 yd³ | 40 bags | 3,525 lbs |
| 8 x 10 ft | 26.7 ft³ | 29.3 ft³ | 1.09 yd³ | 49 bags | 4,395 lbs |
| 10 x 10 ft | 33.3 ft³ | 36.7 ft³ | 1.36 yd³ | 62 bags | 5,505 lbs |
| 10 x 12 ft | 40.0 ft³ | 44.0 ft³ | 1.63 yd³ | 74 bags | 6,600 lbs |
| 12 x 12 ft | 48.0 ft³ | 52.8 ft³ | 1.96 yd³ | 88 bags | 7,920 lbs |
| 12 x 16 ft | 64.0 ft³ | 70.4 ft³ | 2.61 yd³ | 118 bags | 10,560 lbs |
| 12 x 20 ft | 80.0 ft³ | 88.0 ft³ | 3.26 yd³ | 147 bags | 13,200 lbs |
| 16 x 20 ft | 106.7 ft³ | 117.3 ft³ | 4.34 yd³ | 196 bags | 17,595 lbs |
| 20 x 20 ft | 133.3 ft³ | 146.7 ft³ | 5.43 yd³ | 245 bags | 22,005 lbs |
| 20 x 24 ft | 160.0 ft³ | 176.0 ft³ | 6.52 yd³ | 294 bags | 26,400 lbs |
| 20 x 30 ft | 200.0 ft³ | 220.0 ft³ | 8.15 yd³ | 367 bags | 33,000 lbs |
| 24 x 40 ft | 320.0 ft³ | 352.0 ft³ | 13.04 yd³ | 587 bags | 52,800 lbs |
| 30 x 40 ft | 400.0 ft³ | 440.0 ft³ | 16.30 yd³ | 734 bags | 66,000 lbs |
Rectangular Slabs 6-Inch Thickness (with 10% waste)
| Slab Size | Exact ft³ | ft³ (+10%) | Cubic Yards | 80 lb Bags | Weight (lbs) |
|---|---|---|---|---|---|
| 10 x 10 ft | 50.0 ft³ | 55.0 ft³ | 2.04 yd³ | 92 bags | 8,250 lbs |
| 12 x 20 ft | 120.0 ft³ | 132.0 ft³ | 4.89 yd³ | 220 bags | 19,800 lbs |
| 16 x 20 ft | 160.0 ft³ | 176.0 ft³ | 6.52 yd³ | 294 bags | 26,400 lbs |
| 20 x 20 ft | 200.0 ft³ | 220.0 ft³ | 8.15 yd³ | 367 bags | 33,000 lbs |
| 20 x 40 ft | 400.0 ft³ | 440.0 ft³ | 16.30 yd³ | 734 bags | 66,000 lbs |
| 24 x 40 ft | 480.0 ft³ | 528.0 ft³ | 19.56 yd³ | 880 bags | 79,200 lbs |
| 30 x 40 ft | 600.0 ft³ | 660.0 ft³ | 24.44 yd³ | 1,100 bags | 99,000 lbs |
Cubic Feet per Inch of Thickness Coverage Reference
| Thickness | ft³ per sq ft of slab | sq ft per ft³ | Bags per 100 sq ft (80 lb) | Weight per sq ft (lbs) |
|---|---|---|---|---|
| 1 inch | 0.083 ft³/sq ft | 12.0 sq ft/ft³ | ~14 bags | 12.5 lbs/sq ft |
| 2 inches | 0.167 ft³/sq ft | 6.0 sq ft/ft³ | ~28 bags | 25.0 lbs/sq ft |
| 3 inches | 0.250 ft³/sq ft | 4.0 sq ft/ft³ | ~42 bags | 37.5 lbs/sq ft |
| 3.5 inches | 0.292 ft³/sq ft | 3.4 sq ft/ft³ | ~49 bags | 43.8 lbs/sq ft |
| 4 inches | 0.333 ft³/sq ft | 3.0 sq ft/ft³ | ~56 bags | 50.0 lbs/sq ft |
| 5 inches | 0.417 ft³/sq ft | 2.4 sq ft/ft³ | ~70 bags | 62.5 lbs/sq ft |
| 6 inches | 0.500 ft³/sq ft | 2.0 sq ft/ft³ | ~84 bags | 75.0 lbs/sq ft |
| 8 inches | 0.667 ft³/sq ft | 1.5 sq ft/ft³ | ~112 bags | 100.0 lbs/sq ft |
| 10 inches | 0.833 ft³/sq ft | 1.2 sq ft/ft³ | ~139 bags | 125.0 lbs/sq ft |
| 12 inches | 1.000 ft³/sq ft | 1.0 sq ft/ft³ | ~167 bags | 150.0 lbs/sq ft |
Cylinder Volumes in Cubic Feet Per Single Column or Post Hole
| Diameter | 1 ft Deep | 2 ft Deep | 3 ft Deep | 4 ft Deep | 6 ft Deep |
|---|---|---|---|---|---|
| 6 in (0.5 ft) | 0.196 ft³ | 0.393 ft³ | 0.589 ft³ | 0.785 ft³ | 1.178 ft³ |
| 8 in (0.667 ft) | 0.349 ft³ | 0.698 ft³ | 1.047 ft³ | 1.396 ft³ | 2.094 ft³ |
| 10 in (0.833 ft) | 0.545 ft³ | 1.091 ft³ | 1.636 ft³ | 2.182 ft³ | 3.272 ft³ |
| 12 in (1.0 ft) | 0.785 ft³ | 1.571 ft³ | 2.356 ft³ | 3.142 ft³ | 4.712 ft³ |
| 14 in (1.167 ft) | 1.069 ft³ | 2.138 ft³ | 3.207 ft³ | 4.276 ft³ | 6.414 ft³ |
| 16 in (1.333 ft) | 1.396 ft³ | 2.793 ft³ | 4.189 ft³ | 5.585 ft³ | 8.378 ft³ |
| 18 in (1.5 ft) | 1.767 ft³ | 3.534 ft³ | 5.301 ft³ | 7.069 ft³ | 10.603 ft³ |
| 24 in (2.0 ft) | 3.142 ft³ | 6.283 ft³ | 9.425 ft³ | 12.566 ft³ | 18.850 ft³ |
Cubic Feet to Other Units Complete Conversion Table
| Cubic Feet | Cubic Yards | Cubic Meters | Cubic Inches | Weight (lbs) | Weight (US tons) |
|---|---|---|---|---|---|
| 1 ft³ | 0.037 yd³ | 0.028 m³ | 1,728 in³ | 150 lbs | 0.075 tons |
| 3 ft³ | 0.111 yd³ | 0.085 m³ | 5,184 in³ | 450 lbs | 0.225 tons |
| 5 ft³ | 0.185 yd³ | 0.142 m³ | 8,640 in³ | 750 lbs | 0.375 tons |
| 10 ft³ | 0.370 yd³ | 0.283 m³ | 17,280 in³ | 1,500 lbs | 0.750 tons |
| 13.5 ft³ | 0.500 yd³ | 0.382 m³ | 23,328 in³ | 2,025 lbs | 1.013 tons |
| 20 ft³ | 0.741 yd³ | 0.566 m³ | 34,560 in³ | 3,000 lbs | 1.500 tons |
| 27 ft³ | 1.000 yd³ | 0.765 m³ | 46,656 in³ | 4,050 lbs | 2.025 tons |
| 40 ft³ | 1.481 yd³ | 1.133 m³ | 69,120 in³ | 6,000 lbs | 3.000 tons |
| 54 ft³ | 2.000 yd³ | 1.529 m³ | 93,312 in³ | 8,100 lbs | 4.050 tons |
| 81 ft³ | 3.000 yd³ | 2.294 m³ | 139,968 in³ | 12,150 lbs | 6.075 tons |
| 135 ft³ | 5.000 yd³ | 3.823 m³ | 233,280 in³ | 20,250 lbs | 10.125 tons |
| 270 ft³ | 10.000 yd³ | 7.646 m³ | 466,560 in³ | 40,500 lbs | 20.250 tons |
Pre-Mix Bag Coverage in Cubic Feet
| Bag Size | Cubic Feet per Bag | Bags per 10 ft³ | Bags per 27 ft³ (1 yd³) | Approx. Price |
|---|---|---|---|---|
| 40 lb | 0.30 ft³ | ~34 bags | ~90 bags | $5 – $7 |
| 50 lb | 0.37 ft³ | ~27 bags | ~73 bags | $6 – $8 |
| 60 lb | 0.45 ft³ | ~23 bags | ~60 bags | $7 – $9 |
| 80 lb | 0.60 ft³ | ~17 bags | ~45 bags | $9 – $11 |
Bag selection tip: For projects between 5 and 13 cubic feet (under 0.5 cubic yards), 60 lb bags offer the best balance of yield, manageable weight, and cost. For projects over 13.5 cubic feet (0.5 cubic yards), consider 80 lb bags or whether ready-mix is more practical.
Cubic Feet by Project Type: Common Applications
| Application | Typical Dimensions | Approx. Cubic Feet | Best Supply Method |
|---|---|---|---|
| Mailbox post hole | 6-in dia, 2 ft deep | 0.39 ft³ | 1–2 x 40 lb bags |
| Fence post hole (4x4) | 8-in dia, 2.5 ft deep | 1.09 ft³ | 2–3 x 60 lb bags |
| Fence post hole (6x6) | 10-in dia, 3 ft deep | 1.64 ft³ | 4 x 60 lb bags |
| Deck footing (12-in dia) | 12-in dia, 4 ft deep | 3.14 ft³ | 6 x 60 lb bags |
| Small step landing | 3 x 4 ft x 4 in | 4.00 ft³ | 7–8 x 60 lb bags |
| Small patio (8x8) | 8 x 8 ft x 4 in | 21.3 ft³ | Ready-mix or 40 bags |
| Standard patio (12x16) | 12 x 16 ft x 4 in | 64.0 ft³ | Ready-mix (2.37 yd³) |
| Garage floor (20x20) | 20 x 20 ft x 4 in | 133.3 ft³ | Ready-mix (4.94 yd³) |
| Driveway (12x40, 5 in) | 12 x 40 ft x 5 in | 200.0 ft³ | Ready-mix (7.41 yd³) |
| Retaining wall (20 ft) | 20 x 4 ft x 8 in | 53.3 ft³ | Ready-mix (1.97 yd³) |
Waste Factor Guide for Cubic Feet Calculations
| Scenario | Waste Factor | Cubic Feet Multiplier | Notes |
|---|---|---|---|
| Commercial, laser-leveled, steel forms | 5% | x 1.05 | Minimal waste, precise subgrade |
| Standard residential, wood forms | 10% | x 1.10 | Industry standard for most projects |
| Rough subgrade, uneven terrain | 15% | x 1.15 | Subgrade variation adds unpredictable volume |
| First-time DIY, complex shapes | 20% | x 1.20 | Extra buffer for measurement and mixing errors |
| Post holes and small pours | 10% | x 1.10 | Standard buffer covers soil edge collapse |
Cubic Feet vs. Cubic Yards: When to Use Each
| Use Cubic Feet When... | Use Cubic Yards When... |
|---|---|
| Calculating pre-mix bag quantities | Ordering ready-mix concrete from a supplier |
| Working out mix design ingredient weights | Estimating material cost per yard |
| Calculating structural dead load (lbs per sq ft) | Comparing with concrete truck capacity |
| Sizing small pours under 1 cubic yard | Planning large multi-structure combined pours |
| Working with mix design specifications | Communicating with batch plant dispatchers |
| Calculating ingredient dry volume (x 1.54) | Quoting concrete contractors and bids |
Practical workflow: Always calculate in cubic feet first. It is the natural intermediate unit that connects your tape measure readings to your bag count to your yard order. Cubic feet is where all the math happens. Cubic yards are simply the final step before you call the supplier.
Common Cubic Feet Calculation Mistakes
- Not converting inches to feet before multiplying: Using 4 inches as 4 instead of 0.333 in the formula inflates the cubic feet result by 12 times. A 10x10 slab at 4 feet gives 400 cubic feet. At 4 inches it gives 33.3 cubic feet. Always divide inch measurements by 12.
- Forgetting to include the waste factor before calculating bags: Dividing your exact cubic feet volume by the bag yield gives the minimum bags needed under perfect conditions. Real pours always require more. Apply the 10% waste factor first, then calculate bags.
- Dividing by 27 before applying waste factor: The correct sequence is: calculate cubic feet, add waste factor, then convert to cubic yards. Reversing the order produces a slightly different number because waste is added to the adjusted volume, not the raw one.
- Using bag yield volume for ready-mix ordering: Bag yields (0.60 ft³ per 80 lb bag) are for hand-mixed pre-mix concrete. Ready-mix is ordered in cubic yards, not cubic-foot bag equivalents. Convert your cubic feet to cubic yards and quote that number to the supplier.
- Confusing cubic feet with square feet: A slab area of 200 square feet is not 200 cubic feet of concrete. Volume requires three dimensions. Multiply the area by the thickness in feet to get cubic feet.
- Rounding down on bag counts: Always round up to the next whole bag. There is no half-bag. Stopping a pour one bag short is not an option. The cost of one extra bag is $7 to $11. The cost of running short mid-pour is a ruined slab.
Pro Tips for Working in Cubic Feet
- Memorize these four cubic foot values for the most common slab thicknesses: 4 in = 0.333 ft³/sq ft, 5 in = 0.417 ft³/sq ft, 6 in = 0.500 ft³/sq ft, 8 in = 0.667 ft³/sq ft. With these, you can calculate any rectangular slab volume in seconds.
- For post hole projects, use the cylinder cubic feet table above rather than recalculating each time: Find your diameter and depth intersection and multiply by the number of holes. Add 10% waste and divide by your bag yield.
- When specifying mix design ingredients for site-mixed concrete, always work in cubic feet not cubic yards: The dry volume multiplier (x 1.54) and ingredient ratios apply directly to cubic feet and are easier to verify at the batch level.
- For bag projects, bring 10% more bags than your calculation requires: Bags are easy to return unopened. Stopping a pour to make a hardware store run is not.
- On multi-structure projects, build a simple cubic feet takeoff table: One column per structure, one row for dimensions, one row for cubic feet. Sum the cubic feet column, add waste, then convert to yards once at the bottom. This avoids compounding rounding errors from converting each structure individually.
- Use the cubic feet coverage table (cubic feet per square foot per inch of thickness) for quick field verification: If your 20x20 slab at 4 inches should give about 133 cubic feet and your calculation shows 13.3, you have a decimal error. The table catches it.