Concrete Slab Calculator
Turn slab length, width, and thickness into a concrete order volume plus a transparent whole-bag comparison.
- Net slab volume
- 20 ft³ (0.74 yd³)
- Volume with allowance
- 21 ft³ (0.78 yd³)
- Whole concrete bags
- 35 × 80 lb
- Bag rounding excess
- 0 ft³ (0 yd³)
This is a rectangular volume estimate. It does not choose slab thickness, base, reinforcement, joints, or concrete specification.
Show calculation
10 ft × 6 ft × 4 in ÷ 12 × (1 + 5%) = 21 ft³Transparent method and editable product assumptions. Verify current labels and project requirements.
How this calculator works
The concrete slab calculator converts thickness from inches to feet, then multiplies length × width × thickness. The result appears in cubic feet and cubic yards for a US ready-mix order, or cubic meters after switching to metric inputs.
One editable concrete allowance is applied to the unrounded net volume. The page then divides that same order volume by the bag yield and rounds up, so the ready-mix and bag views do not receive separate hidden waste factors.
The 0.60 ft³ default is the approximate yield published for an 80 lb QUIKRETE Concrete Mix bag. Product yield controls the bag count; bag weight by itself does not. Slab thickness, subbase, reinforcement, joints, slope, strength, and placement method must come from the project requirements.
Formula
Slab volume = length × width × thickness; order volume = slab volume × (1 + allowance); bags = ceil(order volume ÷ bag yield)The live substitution appears with your result.Control example: 10 × 6 ft slab at 4 in
Net volume is 10 × 6 × 4 ÷ 12 = 20 ft³, or about 0.74 yd³.
With 5% allowance, order volume is 21 ft³ or 0.78 yd³. At 0.60 ft³ per 80 lb bag, buy 35 bags.
Assumptions and limits
- The slab is a rectangular prism with uniform thickness.
- Dimensions describe concrete volume, not excavation or aggregate base.
- The allowance covers ordering uncertainty once.
- Bag yield is replaced when the selected product label differs.
Common measuring mistakes
- Multiplying by 4 without converting a 4-inch thickness to feet.
- Dividing cubic feet by 27 twice.
- Using bag weight as if it were hardened concrete volume.
- Treating this quantity result as slab design or reinforcement advice.
Sources
Product-specific values remain editable. These sources support the method and stated examples; they are not a universal substitute for your supplier’s current label.
- QUIKRETE concrete calculation training Shows length × width × thickness for a 10 × 6 × 4 in slab and 0.60 ft³ yield for an 80 lb bag.
- QUIKRETE Concrete Mix data sheet Publishes approximate yields for 40, 50, 60, 80, and 90 lb bag sizes.
Questions people ask
How many cubic yards of concrete are in a slab?
Multiply length and width in feet by thickness in feet, then divide cubic feet by 27. Add the chosen allowance once.
How many 80 lb bags of concrete do I need?
Divide order volume by the actual bag yield. The reference 80 lb mix yields about 0.60 ft³, but the selected label controls.
Does this calculator choose slab thickness?
No. Thickness affects safety and performance and must come from the project specification or qualified guidance.
Should I add waste to concrete?
Use one explicit allowance if appropriate for uneven subgrade, forms, spillage, or ordering increments. Do not add it to dimensions and again as a percentage.
Can I use the result for ready-mix?
The cubic-yard or cubic-meter order volume is a quantity starting point. Confirm minimum orders, increments, access, and delivery rules with the supplier.