Concrete Slab Calculator
By Vinod Kondeti · Updated
A slab is a flat rectangle: length × width × thickness. Most errors come from units, because length and width are in feet and thickness is in inches. This calculator converts the thickness to feet for you (4 in = 0.333 ft), then turns the volume into yards and bags.
How do I measure for a slab pour?
- Measure the inside of the forms, not the outside.
- If the subgrade is uneven, measure the thickness in a few places and use the average.
- For an L-shaped slab, split it into two rectangles and set the quantity for each, or run it twice and add the results.
Slab concrete questions
How much concrete do I need for a 10×10 slab at 4 inches?
10 × 10 × (4 ÷ 12) = 33.3 ft³, which is 1.23 yd³. That is about 56 bags of 80 lb mix before waste, or 62 bags with 10% extra.
Does an extra inch of thickness matter much?
Yes. Going from 4 in to 5 in adds 25% more concrete, so a 1.23 yd³ slab becomes 1.54 yd³.
Bag yields: 0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb standard concrete mix (manufacturer data sheets). The concrete guide has the formulas and worked examples.
Sources
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