Concrete Calculator
Enter a slab, footing or column size and get the concrete in cubic yards and the number of 40, 50, 60, 80 or 90 lb bags.
The allowance covers spillage, uneven ground and form bulge; the percentage is your own choice.
Concrete needed
1.3 cubic yards (35 cu ft)
0.99 m³ · about 4,900 lb placed
| Bag size | Yield per bag | Bags needed |
|---|---|---|
| 40 lb | 0.3 cu ft | 117 |
| 50 lb | 0.375 cu ft | 94 |
| 60 lb | 0.45 cu ft | 78 |
| 80 lb | 0.6 cu ft | 59 |
| 90 lb | 0.675 cu ft | 52 |
What this page cannot tell you
Ready-mix plants sell in fixed increments and often have a short-load fee; ask how they round. Bag yields are for QUIKRETE Concrete Mix; other mixes yield differently, so check the bag. Reinforcement, base gravel and forms are not included.
How to calculate concrete
- Choose the pour. Slab, footing or round column.
- Enter the size. Length, width and thickness, or diameter and height.
- Add an allowance. Your own percentage for spillage and uneven ground.
- Read yards and bags. Cubic yards for ready-mix and bag counts for each size.
10 concrete jobs this sizes
Patio slab
A homeowner pouring a 10 × 12 patio at 4 in finds the cubic yards to order.
Shed base
Someone pricing bags for a small shed pad.
Driveway
A contractor estimates ready-mix for a driveway.
Fence and deck posts
A DIYer counts bags for round post holes.
Footings
A builder sizes continuous footings.
Hot tub pad
A homeowner checks bags for a pad.
Steps and landings
Someone pours a landing at the back door.
Bag versus truck
A DIYer sees whether bags or a truck make more sense.
Mailbox post
A homeowner sets a post in concrete.
Checking a quote
A homeowner compares a contractor's yardage with their own.
How the concrete is worked out
Volume = length × width × thickness (thickness in inches ÷ 12), or π × radius² × height for a round column, × quantity × (1 + allowance). Cubic yards = cubic feet ÷ 27.
Bags use the yields on the QUIKRETE Concrete Mix No. 1101 data sheet: 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675. The placed weight uses the sheet's 'approximately 140 lb/ft³'.
Limits: other mixes yield differently, so check the bag. Ready-mix suppliers sell in set increments and may charge short-load fees. Rebar, base gravel and forms are not included. The page does not design slab thickness or reinforcement.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab 4 in thick?
33.3 cu ft, or 1.23 cubic yards, before any allowance.
How many 80 lb bags are in a yard of concrete?
At 0.60 cu ft per bag, 45 bags make 27 cu ft.
How do I convert cubic feet to cubic yards?
Divide by 27.
Should I add extra?
Many people add a small allowance for spillage and uneven subgrade; the percentage is your choice.
How heavy is concrete?
About 140 lb per cubic foot once placed, per the QUIKRETE data sheet.
Is it cheaper to use bags or ready-mix?
It depends on local prices and delivery fees; enter a price per yard to compare.
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