Concrete Calculator
Calculate ready-mix concrete volume in cubic yards, cubic feet, and cubic meters, plus premixed bag counts (40lb, 50lb, 60lb, 80lb) for slabs, footings, and round piers. Add multiple project sections to compute a complete combined jobsite takeoff.
Construction & Framing Takeoff Worksheet
Date: September 3, 2026
Source: ProTrade Calculators
Interactive Slab Geometry
Live Scale: 10′ Length × 10′ Width × 4″ Thickness
Job Dimensions & Geometry
Rounded up to nearest 0.25 yd³ for ready-mix dispatch.
1. What the Concrete Calculator Does
This utility solves one of the most common and expensive trade problems: accurately estimating ready-mix concrete delivery volume and bagged concrete counts without running short on the jobsite or over-ordering costly surplus.
Unlike simple single-rectangle calculators, our engine allows you to model complex multi-section projects in a single worksheet. For example, you can calculate a patio slab (Section 1), a continuous perimeter footing (Section 2), and four round post piers (Section 3) simultaneously to produce a single combined ready-mix order recommendation.
2. How to Calculate Concrete Volume: Core Formulas
Rectangular Slabs & Continuous Footings
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)
Volume (cu yd) = Volume (cu ft) ÷ 27
Always convert thickness from inches to feet first by dividing by 12 (e.g. 4" ÷ 12 = 0.333 ft).
Round Columns, Post Holes & Sonotubes
Volume (cu ft) = π × Radius² (ft) × Depth (ft)
Radius = Diameter (ft) ÷ 2
For a 12" hole (1 ft diameter, 0.5 ft radius), radius squared is 0.25 sq ft.
3. Cubic Feet vs. Cubic Yards: The 27 Rule
In North American construction, commercial ready-mix concrete is sold exclusively by the cubic yard (yd³), while measurements are taken in feet and inches.
Why do you divide cubic feet by 27?
1 linear yard = 3 feet. Therefore, a 1-yard cube measures 3 ft × 3 ft × 3 ft = 27 cubic feet. When you calculate volume in cubic feet, you must always divide by 27 to determine how many cubic yards to order from the concrete supplier.
4. How Much Waste Percentage Should You Add?
Ordering the exact mathematical volume almost always leads to a shortage. Physical jobsite conditions require ordering surplus based on your project type:
| Project Type & Condition | Recommended Waste | Reasoning |
|---|---|---|
| Standard Slab on Laser-Graded Base | 5% | Smooth, well-compacted crushed stone base with stiff form boards. |
| Driveways, Patios, Garage Floors (Standard) | 10% | Standard trade rule. Compensates for uneven subgrade and form deflection. |
| Earthen Trenches & Rough Footings | 12% – 15% | Irregular trench sidewalls, soil over-excavation, and root voids. |
| Long Pump Line or Boom Placements | +0.5 to 1.0 yd³ | Concrete retained inside pump hopper and delivery pipelines. |
5. How to Measure an Irregular Project Layout
If your patio or driveway is L-shaped, T-shaped, or curved:
- Decompose into Simple Rectangles: Divide complex shapes into individual rectangular blocks. Add each block as a separate section in the calculator above.
- Curved Borders: Treat curved edges as partial circles or approximate with average width across equal intervals (Simpson's rule).
- Thickened Edges / Monolithic Slabs: Calculate the main slab area as Section 1 (e.g. 4" thick), and calculate the thickened perimeter footing trench as Section 2.
6. Common Concrete Takeoff Mistakes
1. Mixed Unit Multiplying
Multiplying 10 ft × 10 ft × 4 inches = 400 (wrong!). Thickness must always be converted to feet (4 ÷ 12 = 0.333 ft) first.
2. Formwork Bowing
Wet concrete exerts heavy lateral hydraulic pressure. Inadequately staked 2×4 forms bulge outward, increasing required volume.
3. Ignoring Slope & Swales
If subgrade isn't graded at the same slope as finished top concrete, slab thickness will vary, requiring extra material.
7. Ready-Mix Truck Delivery vs. Bagged Concrete Mix
Ready-Mix Concrete (Truck Delivery)
Best for: Pours 1.0 cubic yard or larger (patios, driveways, foundations).
Pros: Consistent ASTM C94 mix quality, poured in minutes, minimal manual mixing labor.
Watch out for: Short-load delivery fees on orders under 4–5 cubic yards, and rigid 90-minute discharge time windows.
Premixed Bagged Concrete (Quikrete / Sakrete)
Best for: Pours under 1.0 cubic yard (fence posts, deck footings, small steps).
Pros: Available on demand from home centers, mix only what you need, no truck access required.
Watch out for: Physical labor (mixing 45 eighty-pound bags by hand equals 3,600 lbs of lifting).
Need Crushed Stone or Gravel for Your Slab Base?
Every long-lasting slab requires a 4-inch compacted gravel or #57 crushed stone cushion. Use our Gravel & Aggregate Calculator to estimate tons, cubic yards, and dump truckloads with compaction factors.
9. Frequently Asked Questions
How do I calculate how many cubic yards of concrete I need?
Multiply the length (in feet) by the width (in feet) by the thickness (in feet) to find total cubic feet, then divide by 27. For example, a 10 ft × 10 ft slab at 4 inches thick (0.333 ft) is 10 × 10 × 0.333 = 33.33 cubic feet. Dividing by 27 gives 1.23 cubic yards. Adding 10% waste equals approximately 1.35 cubic yards.
How many 80 lb bags of concrete make 1 cubic yard?
It takes 45 bags of 80 lb concrete mix to yield 1 cubic yard (27 cu ft ÷ 0.60 cu ft per bag = 45 bags). For 60 lb bags (0.45 cu ft yield), it requires 60 bags per cubic yard. For 50 lb bags (0.375 cu ft yield), it requires 72 bags per cubic yard.
How thick should a concrete patio, sidewalk, or driveway slab be?
Standard sidewalks and residential patios are typically poured 4 inches thick (using 2×4 form boards). Residential driveways and garage slabs supporting passenger vehicles should be 4 to 5 inches thick with rebar or wire mesh reinforcement. Heavy-duty truck driveways, RV pads, and equipment slabs should be 6 inches thick.
Why should I add 5% to 10% waste to my concrete order?
Excavations are rarely laser-flat. Sub-base variations of just 1/4 inch across a large slab, formwork bowing outward under the hydrostatic pressure of wet concrete, spillage, and concrete left in pump truck lines will quickly consume 5% to 10% more material than nominal plan dimensions. Running short during a pour results in costly cold joints.
When should I order ready-mix concrete instead of mixing bags?
As a rule of thumb, projects requiring 1 cubic yard (approx. 45 eighty-pound bags) or more are significantly faster, more consistent, and structurally superior when ordered from a ready-mix batch plant. Projects under 1 cubic yard (such as mailbox posts, small steps, or minor footing patches) are usually more economical with premixed bags due to truck short-load delivery fees.
How do I calculate concrete for round post holes or sonotubes?
Use the cylinder volume formula: π × radius² × depth. Divide the diameter in inches by 2 to get the radius, convert all dimensions to feet, multiply π × r² × depth in feet, and divide by 27 for cubic yards. For example, a 12-inch diameter hole (radius = 0.5 ft) that is 3 feet deep requires 3.1416 × 0.25 × 3 = 2.36 cubic feet (about 0.087 cubic yards, or 4 eighty-pound bags).