Work out how much concrete a slab, footing or round column needs in cubic feet, yards and 40/60/80 lb bags, then price the materials and labor and copy a quote line.
For concrete contractors and builders · updated October 2026
An estimate. Bag counts use QUIKRETE Concrete Mix yields and include waste; forms, base and rebar are extra.
How to use this concrete calculator
- Pick the shape: a slab (patio, shed pad, driveway), a footing (a long trench pour) or round columns (post holes, piers, sonotubes).
- Enter the dimensions. Lengths are in feet; slab thickness, footing width and depth, and column diameter are in inches.
- Add a waste allowance. The default is 10%.
- Choose ready-mix by the cubic yard or bagged mix, and enter your local price.
- Add a labor rate per square foot or per cubic yard if you're quoting the job, then copy the quote line into your estimate or invoice.
How to calculate concrete
Concrete is sold by volume, so every job starts with cubic feet, converted to cubic yards. There are 27 cubic feet in a cubic yard.
Worked example: a 20 × 20 ft slab, 4 inches thick
- Volume: 20 × 20 × 4 ÷ 12 = 133.33 cu ft, or 133.33 ÷ 27 = 4.94 cu yd.
- With 10% waste: 4.94 × 1.10 = 5.43 cu yd. Ready-mix is ordered in quarter-yard steps, so you order 5.5 cu yd.
- Concrete at $140 per yard: 5.5 × $140 = $770.
- Labor at $4 per sq ft: 400 × $4 = $1,600.
- Quote total: $2,370, just under $6 per sq ft.
A 40 ft footing, 16 inches wide and 8 inches deep, works out the same way: 40 × (16 ÷ 12) × (8 ÷ 12) = 35.56 cu ft, or 1.32 cu yd. Four 12-inch columns 4 ft tall come to 12.57 cu ft, or 0.47 cu yd.
How many bags of concrete do I need?
Divide the volume by what one bag makes. These yields come from the QUIKRETE Concrete Mix data sheet: a 40 lb bag makes about 0.30 cu ft, a 60 lb bag about 0.45 cu ft and an 80 lb bag about 0.60 cu ft.
| Bag size | Yield per bag | Bags per cubic yard | 10 × 10 ft slab, 4 in, +10% waste |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 62 bags |
| 60 lb | 0.45 cu ft | 60 | 82 bags |
| 40 lb | 0.30 cu ft | 90 | 123 bags |
The 20 × 20 ft slab above would take 245 bags of 80 lb mix, which is why bigger pours almost always go to a ready-mix truck. Bags make sense for post holes, small pads and repairs. High-yield and lightweight mixes make more volume per bag, so check the yield printed on the bag you actually buy.
Concrete cost per yard and per square foot
These ranges come from HomeGuide's 2026 concrete cost guides and its contractor pricing guide, read in October 2026. Use them to sanity-check your own local prices.
| Item | Typical range |
|---|---|
| Ready-mix, delivered | $119 – $147 per cu yd ($120 – $160 in the contractor guide) |
| Short load (under 10 cu yd) | +$53 per cu yd |
| Slab materials | $3 – $7 per sq ft |
| Slab labor, forms included | $3 – $5 per sq ft |
| Basic broom-finished slab, installed | $4 – $8 per sq ft |
| Driveway, installed | $6 – $12 per sq ft |
| Stamped concrete, installed | $8 – $18 per sq ft |
| Installed price per cubic yard | $300 – $600 |
| Gravel base | $20 – $50 per ton |
A short load matters on small jobs. If you're ordering less than a full truck, add the surcharge to the price per yard you enter, or the materials line will come out low.
How to price a concrete job
The calculator covers concrete and labor. A full bid also needs the rest of your costs and a profit margin on top.
- List every material. Base gravel, forms, rebar or mesh, expansion joints and sealer belong on the bid. HomeGuide's contractor guide suggests a miscellaneous materials line of 3% to 5% of material cost for stakes, form oil and tie wire.
- Use loaded labor rates. The same guide puts loaded crew rates at $35 to $75 an hour once payroll taxes, insurance and benefits are in.
- Spread your overhead. Divide a year of insurance, vehicles, tools and office costs by the jobs you do in a year, or add it as a percentage. HomeGuide cites 10% to 20% of revenue.
- Price to a margin. HomeGuide puts a healthy concrete margin at 10% to 25%, with many established contractors aiming for 15% to 20% net. To get a 20% margin, divide total cost by 0.80.
That last step trips people up. A 20% markup on $5,000 of cost is $6,000, which is only a 16.7% margin. $5,000 ÷ 0.80 = $6,250 gives you the full 20%. The profit margin calculator does the conversion for any numbers.
What homeowners pay for a concrete slab
When a client asks whether your number is fair, these are the national averages they'll find. HomeGuide puts an installed slab at $6 to $12 per sq ft, with labor making up 40% to 50% of the total.
| Slab | Average cost installed |
|---|---|
| 10 × 10 ft (100 sq ft) | $600 – $1,200 |
| 20 × 20 ft (400 sq ft) | $2,400 – $4,800 |
| 30 × 30 ft (900 sq ft) | $5,400 – $10,800 |
| 4 in thick, per sq ft | $6 – $8 |
| 6 in thick, per sq ft | $8 – $10 |
Source: HomeGuide's concrete slab cost guide, February 2026. Prices run higher for thick or reinforced slabs, hard-to-reach sites and decorative finishes.
Common concrete estimate mistakes
- Mixing inches and feet. A 4 inch slab is 0.333 ft thick, not 4. It's the easiest slip to make with any concrete slab calculator.
- Ordering exactly the volume. Subgrade is never perfectly flat. Running short mid-pour can leave a cold joint and a second delivery charge.
- Forgetting the short-load fee. Small orders cost more per yard. Ask your supplier and build it into the price.
- Quoting bags for a big pour. Hundreds of bags means hours of mixing labor you may not have priced.
- Leaving out prep. Excavation, base, forms and reinforcement are often more work than the pour itself.
- Pricing without a site visit. Slope, access for the truck and soil conditions change the job.
How to put a concrete estimate on an invoice
Send the quote first, as a regular document or message (see estimates vs invoices). After the pour, invoice the same lines, each as quantity × rate, so the client sees exactly what they're paying for:
- Ready-mix concrete: 5.5 cu yd × $140 = $770
- Slab labor, forming, pouring and finishing: 400 sq ft × $4 = $1,600
- Gravel base, rebar or mesh, and any removal: one line each.
In Onigiri, add these line items to your invoice, apply a tax rate or discount, and send it by email, as a link or as a PDF. The free plan includes 3 invoices a month, unlimited clients and payment status. PRO is $12 a month and adds unlimited invoices, item templates and card payments through your own Stripe account. Need a single invoice now? The free invoice generator makes a PDF without sign-up, and the contractor invoice template is laid out for trade jobs.
Concrete calculator FAQ
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 5% to 10% for waste before you order.
How many 80 lb bags of concrete make a yard?
About 45. QUIKRETE rates an 80 lb bag of Concrete Mix at roughly 0.60 cu ft, and a cubic yard is 27 cu ft.
How many bags of concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cu ft. With 10% waste that's 62 bags of 80 lb mix, 82 bags of 60 lb or 123 bags of 40 lb.
How much concrete does a cubic yard cover?
One cubic yard covers 81 sq ft at 4 inches thick, about 65 sq ft at 5 inches and 54 sq ft at 6 inches.
How much does concrete cost per yard?
HomeGuide puts delivered ready-mix at about $119 to $147 per cubic yard, with about $53 per yard added for loads under 10 yards. Prices vary by region and mix strength.
How much should I charge per square foot for concrete?
HomeGuide's contractor guide puts basic broom-finished flatwork at about $5 to $9 per sq ft installed, and stamped or decorative work at $8 to $20 or more. Build your own rate from your costs, overhead and margin.
How much waste should I add for concrete?
Add 5% to 10% to the calculated volume for spills, uneven subgrade and over-pours. Running short mid-pour costs more than a little leftover concrete.
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