Concrete Volume Calculator for slabs, footings and post holes

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.

To use the tool, 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 rather than re-typing the same numbers. For post holes, Quantity multiplies the single-hole volume automatically.

Waste is your safety margin. A flat, well-formed slab might be fine at five percent, while rough augered holes or trenches often need ten to fifteen percent. The calculator applies the waste factor after computing the geometric volume so the “Total to order” reflects what you should actually bring to site.

Bag counts are shown for typical premix sizes. These are estimates meant for quick planning and small jobs. If you’re ordering ready-mix, focus on the total cubic yards or cubic meters and round up to your supplier’s minimum delivery increment. For example, if your total is 2.62 yd³ and the truck minimum is 0.5 yd³ increments, you’d order 2.5 or 3.0 yd³ based on your risk tolerance and site conditions.

Here is a simple example in imperial units: a 10 ft by 10 ft patio at 4 in thick is 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is 1.235 yd³. With 10% waste the total is 1.359 yd³. Selecting 60 lb bags (≈0.45 ft³ each) yields about 100 bags. The same job in metric would be 3.05 m × 3.05 m × 10 cm = 0.931 m³; with 10% waste that’s 1.024 m³, and 30 kg bags at ≈0.0135 m³ each would be about 76 bags.

Use the Notes field to capture context like finish, reinforcement, or access constraints. That note is echoed beneath the result so you can screenshot the summary and share it with a supplier or teammate without rewriting details.

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. These values vary by brand, aggregate and slump; treat results as estimates.

The calculator assumes uniform thickness and clean geometry. It does not account for beams, thickened edges, grade variation, voids, or embedded features. If your slab changes depth, divide it into rectangles at each thickness, run separate calculations, then add the totals before applying waste. For footings with bell bottoms or haunches, use the nearest simple shape that slightly overestimates the true volume to stay conservative.

Premix bag yields are based on typical finished volume under standard mixing practices. Extra water, high air content, or very stiff mixes can change the final yield. When mixing on site, keep batches consistent, measure water carefully, and consolidate the concrete in the forms to avoid honeycombing, which can prompt ordering more material mid pour.

Ordering tips: confirm truck access, chute reach, and any pump requirements; know the supplier’s minimum load, wait time, and return policy; and coordinate pour start so forms, reinforcement, and subgrade are ready. For hot or windy conditions, consider a retarder and have finishing tools staged to reduce cold joints. Proper curing improves strength and reduces cracking; plan for wet curing, curing compound, or both.