Concrete Calculator
Enter your pour dimensions and get the volume in cubic yards, the bag count for 80, 60 or 40 lb mixes, and the weight — with the 5% waste margin already applied.
Last updated: 2026-09-22
How the calculation works
- The engine computes the geometric volume of the selected shape in cubic feet.
- Cubic yards divide by 27 — the unit ready-mix is ordered in across North America.
- Bag counts use published yields: an 80 lb bag mixes to about 0.60 ft³, a 60 lb to 0.45 ft³, a 40 lb to 0.30 ft³.
- The recommended order adds 5% for uneven subgrade, spillage and forms that bulge.
Formula
Slab/Footing: V = L × W × t Wall: V = L × h × t Column/Hole: V = π/4 × d² × h yd³ = V_ft³ / 27 bags = V_ft³ / bag_yield
| Symbol | Meaning | Unit |
|---|---|---|
V | Concrete volume | ft³ → yd³ |
L, W, h, t | Length, width, height, thickness | ft or in |
bag_yield | Mixed yield per bag (0.6 / 0.45 / 0.3 ft³) | ft³/bag |
Worked example
Interpreting the result
Order ready-mix in 0.25 yd³ increments — round the bag-with-waste figure up to the nearest quarter yard. Under 1 yd³, bagged mix is reasonable; 1–2 yd³ is the painful middle where a small ready-mix truck or a trailer from a batch plant pays off. The weight figure matters for deck footings and raised slabs: concrete at 150 lb/ft³ is heavier than most people budget for.
Assumptions
- Nominal dimensions on the drawing, not as-dug excavation.
- Density 150 lb/ft³ for standard 3000–4000 psi structural mix.
- Published bag yields at correct water content — over-watering changes both yield and strength.
Limitations
- Openings, chamfers, haunches and keyways are not deducted.
- Post holes with belled bottoms or irregular excavation need manual adjustment.
- No account for form thickness or concrete lost in congested reinforcing zones.
Ready-mix vs bagged: where the crossover sits
One cubic yard is 45 bags of 80 lb mix — about 3,600 lb of dry material that must be mixed with roughly 5 gallons of water per bag and placed before it sets. Two people with a portable mixer sustain maybe 15–20 bags per hour; a full yard is a 2.5–3 hour mixing marathon, and the last batches are going down while the first are already curing. That labor math, not the material price, is why anything over a yard almost always goes to the ready-mix truck. The bag route wins for fence posts, small pads and repairs where a truck's minimum-load charge (often $100–150 for less than 3–4 yd³) dwarfs the concrete cost.
Cold joints: the mistake this calculator prevents
- A cold joint forms when fresh concrete lands on concrete that has already set — the two layers never bond chemically.
- For any pour over ~2 yd³, plan the delivery so the whole volume is placed within 90 minutes of batch time.
- If a second truck is needed, schedule the gap at a construction joint location, never mid-span of a slab.
Frequently asked questions
How much concrete do I need for a 20×20 4-inch slab?
20 × 20 × (4/12) = 133 ft³ = 4.94 yd³. Order 5.25 yd³ with the waste margin. That's about 233 bags of 80 lb mix — firmly ready-mix territory.
How many 80 lb bags make one cubic yard?
27 ft³ ÷ 0.60 ft³ per bag = 45 bags per cubic yard exactly. For 60 lb bags it's 60 bags; for 40 lb, 90 bags.
Should I include waste in the calculation?
Yes — the 5% margin is already in the recommended order figure. It covers subgrade irregularities and spillage. Running short mid-pour creates a cold joint, which is a permanent structural weakness; surplus concrete is merely a small cost.