Concrete Calculator
En español: Calculadora de concreto →
- Materials
- Labor
- Profit
Profit is 20.0% of the price. Ready-mix is ordered in quarter or half yards, so round the truck order up.
Turn this into a quote
A one-page PDF for your customer, filled in from the numbers above. Your markup isn't shown.
How the concrete calculation works
Volume is length × width × thickness
Convert inches to feet first. A 20 × 20 ft slab at 4 in thick is 20 × 20 × 0.333 = 133.3 cu ft.
Divide cubic feet by 27 for cubic yards
133.3 ÷ 27 = 4.94 yd³. Add 5–10% for spillage and uneven ground: with 10% it is 5.43 yd³.
Bags come from the yield on the bag
An 80 lb bag makes about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. Divide the cubic feet by the yield and round up.
Price the whole job
Add crew hours × your hourly cost, then your markup on the total cost. The markup is your profit.
Worked example. A 20 × 20 ft patio slab 4 in thick with 10% waste needs 5.43 yd³ of concrete (245 bags of 80 lb). At $165 per yard the concrete costs $896.30. With 12 labor hours at $60 ($720) and a 25% markup ($404.07), the job price is $2,020.37.
Concrete FAQ
How do I work out what a concrete slab or pad costs?
Enter the slab or pad size and thickness to get cubic yards, then your ready-mix price per yard. Switch on labor and markup to turn it into a job price. Prices vary by area, so use the quote from your local supplier.
How many yards of concrete do I need for a 20 × 20 slab?
At 4 in thick, about 4.94 yd³, or 5.43 yd³ with 10% extra. At 6 in thick it is 7.41 yd³ before waste.
How many bags of concrete do I need?
Divide the cubic feet by the bag yield: about 0.6 cu ft for an 80 lb bag and 0.45 for a 60 lb bag. One cubic yard takes about 45 bags of 80 lb or 60 bags of 60 lb.
When should I order ready-mix instead of bags?
Above about 1 yd³, a truck is usually faster and costs less than mixing 45+ bags per yard by hand. Check whether your plant charges a short-load fee for small orders.
How thick should a concrete slab be?
4 in is standard for patios, walks and shed floors. Driveways for cars are usually 5–6 in, and heavier loads need an engineer's spec.
How much should I charge to pour concrete?
Add materials, crew hours at your full hourly cost and any equipment, then apply your markup. Many contractors also quote per square foot; divide the job price by the slab area to compare.
How much concrete goes in a sonotube or post hole?
Volume = π × (diameter ÷ 2)² × depth. A 12 in tube 3 ft deep holds about 2.36 cu ft (0.087 yd³), so about 4 bags of 80 lb. Pick "Round columns or post holes" above and enter how many.
Is a cement calculator the same as a concrete calculator?
Close enough for most jobs. Cement is the powder that binds concrete; concrete is cement mixed with sand, gravel and water. Bagged concrete mix already contains all of them, so the bag count above is bags of concrete mix, not bags of plain cement.
How much extra concrete should I order?
5–10% is common for slabs on a well-prepared base. Running short mid-pour costs more than a little leftover, so round the order up.