Concrete Calculator
A 10 ft × 10 ft slab 4 inches thick needs about 1.36 cu yd of concrete with 10% extra, or 62 bags of 80 lb.
Updated · US units: feet, inches, cubic yards · Built and checked by AI, which can make mistakes
Work out how much concrete you need
Result
This is an estimate. Real quantities depend on uneven ground, settling, compaction and the exact product, and bag sizes and weights vary by brand. Check the bag or ask your supplier before you buy.
Built and checked by AI. AI wrote this calculator and its explanations, researched the rates and ran the tests. AI can make mistakes, and the checks may not catch them all, so confirm anything important with an official source or a qualified professional. How we use AI.
Quick answer
A 10 ft × 10 ft slab 4 inches thick needs about 1.36 cu yd of concrete with 10% extra, or 62 bags of 80 lb.
In Australia? Use our Australian concrete calculator, in cubic metres and litres.
How to use the concrete calculator
- Choose the shape of the area: a rectangle, a circle, or enter the area if you already know it.
- Enter the measurements in feet and the thickness in inches.
- Pick the bag size you plan to buy, and the extra to allow for spillage.
- Optionally open “Compare costs” to see whether bags or bulk is cheaper.
- Press Calculate to see how much you need, with the steps.
How it’s calculated
Area = length × width (sq ft), or π × (diameter ÷ 2)² for a circle
Volume (cu ft) = area (sq ft) × depth (in) ÷ 12
Cubic yards = cubic feet ÷ 27
Concrete to order = volume × (1 + extra % ÷ 100)
Bags = cubic feet ÷ yield per bag, rounded up
Worked example
Concrete for a 10 ft × 10 ft slab at 4 inches
- Area: 10 ft × 10 ft = 100 sq ft.
- Volume: 100 sq ft × 4 in ÷ 12 = 33.33 cu ft.
- Add 10% for spillage and uneven ground: 36.7 cu ft.
- In cubic yards: 36.67 cu ft ÷ 27 = 1.36 cu yd.
- Bags: 36.67 cu ft ÷ 0.6 cu ft per bag = 61.11, rounded up to 62 bags.
Bags or ready-mix?
Bag yields come from Quikrete’s data sheet: an 80 lb bag makes about 0.60 cu ft of concrete, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. That’s 45 bags of 80 lb for each cubic yard.
For larger slabs, ready-mixed concrete delivered by truck is usually easier than mixing dozens of bags. Suppliers sell it by the cubic yard and many have minimum orders, so ask before you book. Concrete work can be dangerous and may need permits. Follow your plans, the product instructions and local building rules.
When you’d actually use a concrete calculator
- Pouring a small slab or path. Work out how many bags to buy, with a little extra.
- Setting fence or deck posts. Use the circle option for each post hole and multiply by the number of holes.
- Choosing bags or ready-mix. Compare the cost once you know the volume.
What this calculator doesn’t include
- Irregular shapes. Split the area into rectangles and circles and add the results.
- Reinforcement, footings that change depth, and steps. Calculate each part separately.
- Delivery fees, minimum orders and local prices, which vary by supplier.
How we calculate
Area comes from your measurements. Volume is area × thickness ÷ 12, converted to cubic yards by dividing by 27. We add 10% extra unless you change it. Bag yields for concrete come from the manufacturer’s figures shown in the bag list. We calculate at full precision and round only for display. Bag counts are rounded up to whole bags.
Sources:
Frequently asked questions
How many bags of concrete do I need?
Divide the volume in cubic feet by the bag’s yield. An 80 lb bag makes about 0.60 cu ft, so a cubic yard (27 cu ft) takes 45 bags.
How much extra concrete should I allow?
Many people add about 5 to 10% for spillage, uneven ground and forms that bow. The calculator adds 10% by default, and you can change it.
Should I use bags or ready-mix?
Bags suit small jobs such as posts and small pads. For bigger slabs, ready-mixed concrete delivered by the cubic yard is usually easier, but suppliers often have minimum orders.