Monolithic
Monolithic Slab Calculator
A monolithic slab is a floor and its edge beam poured together. The edge hangs below the slab, so it is not just length times width times 4 inches.
Your region
Feet, inches, cubic yards, and 40–80 lb bags
Concrete Calculator
Example measurements — replace them with yours.
Common sizes
Most orders add 5–10% for spillage, uneven ground, and the last bit left in the chute.
Cost, truck size, and concrete weight
Leave a price blank to skip that estimate. Use the number your supplier quoted.
This job
Add each pour — slab, piers, steps — and Concrete Calculator totals them.
How to use a monolithic slab calculator
The floor is a normal slab. The edge beam hangs below it. Only the extra depth below the slab thickness is added for the beam. Corners would be double-counted if you used full perimeter × width — Concrete Calculator subtracts the four corner squares.
Edge size belongs on the foundation plan. A 12×12 in edge under a 4 in slab is a common sketch, not a universal detail. Frost-protected and post-tension slabs need the engineer’s section, not a blog default.
Worked example with the monolithic slab calculator
Slab = 30 × 40 × 0.333 = 400 ft³. Extra edge depth = 8 in = 0.667 ft. Perimeter strip adjusted for corners ≈ (2×(30+40)×1 − 4×1) × 0.667 ≈ 90.7 ft³. Total ≈ 490.7 ft³ = 18.2 yd³ neat.
If you wrongly added a full 12 in edge depth without subtracting the 4 in already in the floor, you would over-order the beam. If you skip the corner correction, you count each corner twice. Re-run the Monolithic Slab Calculator after thickness changes — depth moves yardage faster than length.
When to use separate footings instead
Some plans still want a trench footing poured separately from the floor slab. In that case use the footing calculator plus the slab calculator, not this combined model. Monolithic means the edge and floor are one pour with the geometry above.
Common mistakes
Adding the full edge depth without subtracting the slab thickness already in the floor.
- Double-counting corners.
- Using a sketch edge when the plan differs.
Bags, yards, or a truck?
House-sized monolithics are multi-truck pours. Confirm the plant can stage them.
The formula Concrete Calculator uses
Slab = L×W×T. Edge = (perimeter×beam width − 4×beam width²) × (beam depth − T).
Who this is for
Builders pouring slab-on-grade with integral edge beams.
Monolithic slab foundation — edge math checks
Verify beam depth is greater than slab thickness; otherwise the edge term is zero and you have a plain slab. Verify beam width on the plan — a 12 in edge and an 8 in edge are different yards on a house-sized perimeter.
Corner correction matters more as beam width grows. Skipping it on a 40×60 slab with a wide edge invents concrete at four corners. Concrete Calculator subtracts those overlaps automatically when you use this shape.
House-sized example intuition
A 40×60 slab at 4 inches is 29.6 yd³ neat for the floor alone. A 12×12 edge with 8 inches of extra depth around that perimeter adds on the order of several more yards after corner correction — enough to change truck counts. Always run the numbers; do not add a flat “10% for the edge” guess.
Related calculators and guides
Open the pages that match the next question on the job — volume is only the first number.
Frequently asked questions
What is a monolithic slab?
A slab-on-grade poured with a thicker edge that acts as the footing.
Why subtract corners?
Perimeter × width counts each corner twice.
Edge no deeper than slab?
Extra volume is zero — you have a plain slab.
Separate trench footing?
Use footing + slab calculators instead of this combined model.