Monolithic Slab Concrete Calculator (Slab + Thickened-Edge Footing)

Planning the Pour for a Monolithic Slab Foundation

A monolithic slab combines the flat slab body and the integral perimeter footing into a single concrete pour. Because the thickened-edge beam and the slab are cast simultaneously, the volume calculation has two components: the rectangular slab body (length × width × slab thickness) and the perimeter footing extension that drops below the slab around the entire perimeter. Accurate estimation matters because the footing extension can add 15–30% to the volume of the slab body alone — underordering means a short load on pour day, while overordering wastes money. The slab-body portion uses the same method as our pillar guide on how to calculate concrete for a slab.

This calculator accepts length and width in ft, in, yd, m, or cm; thickness and footing dimensions in in, ft, cm, or mm; and outputs volume in cubic yards, cubic feet, and cubic meters along with pre-mix bag counts for 40 / 50 / 60 / 80 lb bags. Because the thickened edge is really an integral footing, our reference on concrete footing calculations explains how that beam is sized; review how to reinforce a concrete slab for the rebar and mesh; and for hand-mixed pours, see how many concrete bags a slab needs.

Entering Your Monolithic Slab Dimensions

1. 1
   Select Quick or Advanced mode. Quick gives you volume and bag counts in seconds. Advanced mode adds a waste percentage and a ready-mix cost estimate field.
2. 2
   Click your project type preset — Garage, House Foundation, or Shed / ADU. The preset fills typical length, width, slab thickness, footing depth, and footing width values. All fields are editable.
3. 3
   In Step 2, adjust the Length and Width to match your actual slab plan area. Each field has its own unit selector (ft, in, yd, m, or cm).
4. 4
   Set the Slab Thickness — the flat part of the slab body above the thickened edge. The ACI / IRC minimum for residential slabs is 4 inches. Enter the value in your preferred unit.
5. 5
   In Step 3, enter the Total Footing Depth — the distance from the top of the slab to the bottom of the perimeter footing. This must be greater than the slab thickness. For most residential monolithic slabs this is 12 to 16 inches.
6. 6
   Enter the Footing Width — the width of the thickened perimeter edge. Typical residential values are 12 to 16 inches.
7. 7
   In Advanced mode, adjust the Waste Percentage. Use 5% for standard pours and up to 10% for large pours or sites with uneven subgrade.
8. 8
   Optionally enter your ready-mix price per cubic yard in Advanced mode to add a cost estimate to the results.
9. 9
   Press Calculate. The results panel shows the slab body and perimeter footing extension as separate volumes, then the combined total in yd³, ft³, and m³.
10. 10
    Review the Volume Breakdown tile to verify the split between slab body and footing extension. The footing extension = 2(L + W) × footing width × (footing depth − slab thickness).
11. 11
    Check the Bag Counts tile if you plan to use pre-mix bags. For pours above about 1.5 yd³, ready-mix delivery is more economical than bagged concrete.
12. 12
    Click Print / Save to export a clean PDF summary of all inputs, volumes, bag counts, and cost. Select Save as PDF in your browser's Print dialog.

The Monolithic Slab Volume Formula

• 1) Slab Body VolumeV_slab (ft³) = L × W × T_slab
  L and W are length and width in feet; T_slab is slab thickness in feet. This covers the entire plan area at the uniform slab depth.
• 2) Perimeter Footing Extension VolumeV_foot (ft³) = 2(L + W) × W_foot × (D_foot − T_slab)
  W_foot is footing width in feet; D_foot is total footing depth from top of slab to bottom of footing in feet; (D_foot − T_slab) is the depth the footing extends below the slab body. The factor 2(L + W) is the full perimeter length.
• 3) Total VolumeV_total (ft³) = V_slab + V_foot
  Convert to yd³ by dividing by 27; convert to m³ by multiplying by 0.028317.
• 4) Waste-Adjusted VolumeV_order (ft³) = V_total × (1 + waste% / 100)
  Bag counts are calculated on V_order. The standard waste allowance is 5–10% for monolithic slab pours.
• 5) Pre-Mix Bag CountsBags = ⌈V_order (ft³) / yield per bag⌉ (ceiling, rounded up)
  Bag yields: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³ (Quikrete / Sakrete standard yields).

Typical Monolithic Slab Dimensions

All values assume normal-weight concrete (150 pcf), no waste added. Use the calculator above for your exact dimensions.

Add 5–10% waste for a realistic order quantity. For pours above 1.5 yd³, ready-mix delivery is typically more economical than pre-mix bags.

Why Monolithic Slab Pours Come Up Short

• Skipping the waste allowance. The default 5–10% waste factor covers spills, uneven subgrade, and end-of-pour remainder. On a monolithic pour — where the footing trench adds its own variability — leaving this at zero is the fastest way to come up short a truck load.
• Guessing at footing depth instead of checking the frost line. IRC R403.1.4 requires footings to extend below the local frost line. The calculator's 12-inch default warning threshold suits mild climates only — cold-climate frost depths of 24–48 inches mean a much deeper, higher-volume footing.
• Ignoring the footing-vs-slab-thickness geometry rule. Total footing depth must be greater than slab thickness, or the footing extension formula produces an invalid (negative) volume. The calculator flags this before showing results — don't override it without rechecking your inputs.
• Ordering bags for a pour ready-mix should handle. Above roughly 1.5 yd³ combined slab-and-footing volume, ready-mix delivery is faster, cheaper per yard, and gives a more consistent mix than hand-mixing dozens of bags.
• Treating 4 inches as the slab thickness for every load. 4 in is the ACI 318 / IRC R506 practical minimum for residential slabs, but areas carrying vehicle traffic or heavy equipment may need 5–6 inches — check the load before locking in the preset thickness.

Specs & References

Monolithic slab-on-grade designs integrate the footing and floor slab in a single pour; turndown depth must reach the local frost line and soil bearing capacity must be confirmed by a licensed geotechnical engineer before forming.