Concrete Volume Calculator for slabs, footings and post holes
Compute concrete volume and bag counts
Concrete calculator — quick guide & assumptions
Quick guide
This concrete volume calculator for slabs, footings and post holes follows common field conventions so you can estimate materials quickly on a phone. In imperial mode, lengths are in feet and thicknesses in inches with results in cubic yards. In metric mode, lengths are in meters and thicknesses in centimeters with results in cubic meters. Add a waste factor to cover over-excavation, spillage and uneven subgrade.
- Rectangular slab: volume = length × width × thickness. Circular slab: π × (diameter ÷ 2)² × thickness.
- Continuous footing: length × width × depth along the run.
- Post holes (cylinders): quantity × π × (diameter ÷ 2)² × depth.
Set Units first so labels show the right symbols. Pick the Shape that matches your pour, then type dimensions. If you have several identical bays or pads, increase Quantity instead of re-typing. For post holes, Quantity multiplies the single-hole volume automatically.
Waste is your safety margin. Flat, well-formed slabs might be fine at 5%, while rough augered holes or trenches often need 10–15%. The calculator applies waste after the geometric volume so “Total to order” reflects what you should actually bring to site.
Bag counts are shown for typical premix sizes. For ready-mix, focus on total cubic yards/meters and round up to the supplier’s minimum increment.
Assumptions & conversions
Imperial yields (approx.): 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³. Metric yields (approx.): 20 kg ≈ 0.009 m³, 25 kg ≈ 0.011 m³, 30 kg ≈ 0.0135 m³, 40 kg ≈ 0.018 m³. One cubic yard = 27 ft³; 1 m³ = 1000 L. Values vary by brand, aggregate and slump; treat results as estimates.
The tool assumes uniform thickness and clean geometry. It doesn’t account for beams, thickened edges, grade variation, voids, or embedded features. For varying depths, split into simple shapes, compute each, add totals, then apply waste.
Concrete calculator FAQs
Should I include a waste factor?
Yes. 5–10% is common for flatwork; footings and rough holes may need more.
What if my slab has steps or beams?
Break complex shapes into rectangles/cylinders, calculate each, then add the totals. Apply one waste factor to the grand total.
Are bag yields the same everywhere?
No. Always check the bag’s stated yield and adjust the selection if it differs from the defaults.
Does this include reinforcement or fiber?
No. Volume and bag counts only. Reinforcement/admixtures are separate.
How precise should inputs be?
Nearest half-inch or centimeter is fine; consistency matters more than extreme precision.