Concrete Calculator
Use this free concrete material calculator to estimate how much concrete you need for almost any pour — a slab, a wall, a footing, or a post hole. Enter your dimensions, pick imperial or metric, add a waste factor, and instantly see cubic yards, cubic feet, the number of bags by size, and an optional material cost.
Total volume required
1.36Cubic Yards
Cubic feet
36.67
Cubic meters
1.04
Bags (80 lb)
62
Est. material cost
Add a price
Bags by size
40 lb
123
50 lb
98
60 lb
82
80 lb
62
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Choose the shape that matches your pour (slab, wall, footing, or post hole).
- 2Switch between Imperial and Metric, then enter the dimensions — length and width in feet, thickness in inches.
- 3Set a waste factor (10% is a common default) to cover spillage and uneven subgrade.
- 4Read your cubic yards, bags, and (optionally) cost. Copy or print the estimate.
Formula & rounding
Rectangular pours use volume = length × width × thickness, converted to cubic yards by dividing cubic feet by 27. Post holes use π × radius² × depth. Bag counts always round up.
10 ft × 10 ft slab, 4 inches thick
- Volume = 10 ft × 10 ft × (4 in ÷ 12) = 33.33 cubic feet
- With a 10% waste factor: 33.33 × 1.10 = 36.67 cubic feet
- Cubic yards = 36.67 ÷ 27 = 1.36 cubic yards
- 80 lb bags = 36.67 ÷ 0.60, rounded up = 62 bags
= ≈ 1.36 cubic yards, or 62 × 80 lb bags
Which concrete calculator should you use?
Start with this general calculator when you only know the shape. Use a dedicated calculator when your buying decision depends on bags, blocks, or post holes.
| Project | Best calculator | Why it fits |
|---|---|---|
| Patio, walkway, pad, or shed base | Slab calculator | Focuses on length × width × thickness and ready-mix yards |
| Bagged mix from a home center | Bag calculator | Compares 40, 50, 60, and 80 lb bag counts side by side |
| Fence, mailbox, deck, or pergola posts | Post-hole calculator | Uses cylinder math, depth, diameter, and number of holes |
| CMU wall or stem wall | Block calculator | Estimates blocks, mortar bags, and optional core fill |
All tools use planning estimates; structural design, reinforcement, and code compliance remain local/project-specific.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. Add a waste factor of about 10% so you do not run short. This calculator does all of that for you and also shows the number of bags.
How do I know how many bags of concrete to buy?
It depends on the bag size. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45, a 50 lb bag about 0.375, and a 40 lb bag about 0.30. Divide your waste-adjusted volume by the yield and round up. The bag comparison shows all four sizes at once.
How much is a cubic yard of concrete?
One cubic yard equals 27 cubic feet. Ready-mix prices vary a lot by region and supplier, so enter your local price per cubic yard to get an estimate — this tool does not assume a national price.
Is this concrete calculator free?
Yes. Every calculator on this site is free, requires no sign-up, and saves none of your project data.
Assumptions & sources
- Cubic yard
- 1 yd³ = 27 cubic feet (exact conversion).
- Bag yields
- ≈0.30 / 0.375 / 0.45 / 0.60 cu ft for 40 / 50 / 60 / 80 lb bags (common industry figures; confirm on the bag).
- Waste factor
- Default 10%; raise for uneven subgrade or over-excavation. Bag counts always round up.
- Cost
- No national price is assumed — cost uses the price per cubic yard or per bag that you enter.
- Round holes
- Post-hole volume uses π × radius² × depth; footing depth may be code-governed locally.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- How to calculate concrete — breaks down the volume formulas, waste factor, cubic yards, and bag conversion used across the site.
- Concrete bag sizes & yields — compares common bag weights if your estimate is small enough to hand-mix.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.