Concrete Calculator: Slabs, Footings, Columns & Bag Estimator
Use this concrete calculator to calculate cubic yards and exact bag counts for slabs, footings, round piers, and foundation walls. Compares premix 40, 60, and 80 lb bags against ready-mix truck deliveries with editable yields.
Buy 62 bags of 80 lb ready-mix concrete (or 82 bags of 60 lb). Rent a portable cement mixer for easy blending.
| Net Concrete Volume | 33.33 cu ft (1.23 yd³) |
|---|---|
| Excavation & Spillage Waste Margin | 10% |
| Total Volume Required (with Waste) | 36.67 cu ft (1.36 yd³) |
| 80 lb Premix Bags Required (yield 0.6 cu ft) | 62 bags |
| 60 lb Premix Bags Required (yield 0.45 cu ft) | 82 bags |
| 40 lb Premix Bags Required (yield 0.3 cu ft) | 123 bags |
| Ready-Mix Truck Order | 1.50 yd³ (rounded to 1/4 yd) |
📐 Show Formula and Default Assumptions
We believe all home improvement estimates should be transparent and verifiable. Below is the exact math running in this browser tool:
-
Slab Volume Conversion:
Cubic Yards = (Length ft × Width ft × (Thickness in ÷ 12)) ÷ 27Converts inch-based slab depths into fractional feet before dividing by 27 cu ft/cu yd. -
Cylinder/Pier Volume:
Volume cu ft = π × (Radius ft)² × Depth ftCalculates round post sonotube pier volumes. -
Bag Yield Ratios:
80 lb = ~0.60 cu ft | 60 lb = ~0.45 cu ft | 40 lb = ~0.30 cu ftStandard pre-mixed dry concrete yields when mixed with recommended water.
Note: All default assumptions (waste %, coverage rates, container yields) can be adjusted directly in the input fields above to match your project specifications.
How to Measure Forms for Concrete Pours
- Measure the internal form dimensions: Always measure the interior face of your wooden forms where the wet concrete actually makes contact.
- Verify uniform slab depth: Screed boards often sag or ground bases undulate. Measure down from your top string line at multiple points to confirm true slab thickness (e.g. 4" vs 3.5").
- Check round pier diameters: For deck footings using cardboard sonotubes, measure the inside diameter (usually 8", 10", or 12") and the below-frost depth.
- Add 10% for subgrade variance: Excavated subbase soil settles under the heavy weight of wet concrete. Form bowing also consumes extra mix.
- Decide between bags and ready-mix: If your pour exceeds 1.5 to 2.0 cubic yards (equivalent to 90 to 120 eighty-pound bags), ordering a ready-mix truck is significantly faster and physically easier.
- Support rebar on chairs: If installing #4 (1/2-inch) steel rebar or welded wire mesh, never lay it flat on the gravel. Support the rebar on plastic or concrete chairs at mid-depth of the slab (2 inches high in a 4-inch pour) so the steel sits in the tension zone where it provides true structural reinforcement.
- Plan control joints every 8 to 10 feet: Uncontrolled concrete always cracks. Cut control joints to a depth of 1/4 the slab thickness (1 inch deep for a 4-inch slab) spaced no farther apart than 24 to 30 times the slab thickness using a grooving tool or early-entry diamond saw within 12 to 24 hours of pouring.
How the Concrete Calculation Works (Worked Example)
Let us calculate a 10 ft by 10 ft patio slab poured at a standard 4-inch thickness (using nominal 2x4 forms):
- 1. Convert Thickness to Feet: 4 inches ÷ 12 = 0.333 ft.
- 2. Net Volume in Cubic Feet: 10 ft × 10 ft × 0.333 ft = 33.33 cu ft.
- 3. Convert to Cubic Yards: 33.33 ÷ 27 = 1.23 cu yds.
- 4. 10% Spillage & Base Allowance: 1.23 × 1.10 = 1.35 cubic yards (or 36.67 cu ft).
- 5. Bags of 80 lb Mix: 36.67 cu ft ÷ 0.60 cu ft/bag = 61.1 bags -> Round UP to 62 bags.
- 6. Bags of 60 lb Mix: 36.67 cu ft ÷ 0.45 cu ft/bag = 81.5 bags -> Round UP to 82 bags.
- 7. What to Buy: Buy 62 eighty-pound bags or order 1.5 cubic yards from a local ready-mix concrete batch plant.
Concrete Bag Yields, Weights & Ready-Mix Thresholds
| Bag Weight | Dry Cubic Foot Yield | Bags Per 1 Cubic Yard | Water Required Per Bag |
|---|---|---|---|
| 40 lb (18.1 kg) | 0.30 cu ft (0.0085 m³) | 90 Bags | 1.5 Quarts (1.4 L) |
| 60 lb (27.2 kg) | 0.45 cu ft (0.0127 m³) | 60 Bags | 2.5 Quarts (2.4 L) |
| 80 lb (36.3 kg) | 0.60 cu ft (0.0170 m³) | 45 Bags | 3.0 Quarts (2.8 L) |
| Ready-Mix Truck | 27.0 cu ft (0.7645 m³) | 1 Yard (Order in 0.25 yd increments) | Batched at plant |
Frequently Asked Questions About Concrete Calculator
When is it cheaper to order a concrete truck instead of mixing bags?
Ordering a ready-mix truck becomes cost-competitive and physically sensible around 1.5 to 2 cubic yards (equivalent to 68 to 90 eighty-pound bags). While ready-mix companies often charge a "short load" fee for under 4 or 5 yards, the savings in manual labor and joint uniformity are immense.
Why do 2x4 forms not produce a true 4-inch slab?
A standard nominal 2x4 board actually measures 1.5 inches by 3.5 inches. If you pour flush with the top of a 2x4 form set directly on a flat grade, your slab will be 3.5 inches thick rather than 4 inches, consuming about 12% less concrete.
How many 80 lb bags of concrete make one cubic yard?
It takes exactly 45 eighty-pound bags to yield one cubic yard of concrete (45 × 0.60 cu ft = 27 cu ft). With a standard 10% spillage margin, you should order 50 bags per yard.
How deep should deck footings be poured?
Deck footings must extend below your geographic frost depth line (typically 36" to 48" in northern climates, and 12" to 18" in southern states) to prevent winter frost heaving. Always check local building codes before digging.
How long does concrete take to cure before walking or driving on it?
You can walk on fresh concrete after 24 to 48 hours. Keep vehicle traffic off residential driveways for at least 7 days. Concrete reaches approximately 70% of its full design strength at 7 days and completes its standard cure cycle at 28 days.
What is concrete slump and why does excess water ruin concrete?
Slump measures the consistency and workability of wet concrete (typically 4 to 5 inches for residential slabs). Adding extra water at the jobsite makes raking easier but dramatically dilutes the water-cement ratio, drastically reducing the cured PSI compressive strength and causing surface scaling and spiderweb cracking.