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Concrete Calculator

Enter slab dimensions and depth to estimate concrete volume in cubic feet and yards, 80-lb premix bags needed, and cost by the yard or foot. Includes a waste allowance for spillage and uneven subgrades. An estimate — confirm barrow counts with your supplier.

Last updated: September 30, 2026

Your result · Volume

1.36cu yd

Volume: 36.7cu ft
80-lb bags (premix): 62
Estimated cost: $203.70

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What your result means

Volume in cubic yards is the headline: what ready-mix companies quote against. Order slightly over — trucks charge waiting time, not just concrete.

Bag count suits small repairs only; past a yard of concrete, delivery beats bags on price and your back.

How the calculation works

Convert dimensions to feet and multiply for cubic feet, dividing by 27 for cubic yards.
Add the waste percentage, then divide by 0.6 and round up for 80-lb bags.
Multiply volume by your price per yard (or foot) for cost.

Formula

volume = length × width × depth (+ waste); bags = volume ÷ 0.6; cost = volume × price

Volumelength × width × depth (converted to feet)
With wastevolume × (1 + waste %)
Bagsround up (cubic feet ÷ 0.6 per 80-lb bag)
Costvolume × price per yard (or foot)

Worked examples

Example: 10×10 ft slab at 4 in, 10% waste, $150/yd
dimUnit: ftlength: 10width: 10depthUnit: indepth: 4wastePct: 10priceUnit: cypricePerUnit: 150
Result: Volume 36.7 · Volume 1.36 · 80-lb bags (premix) 62 · Estimated cost $203.70
Example: small 4×4 ft pad at 4 in, priced per foot
dimUnit: ftlength: 4width: 4depthUnit: indepth: 4wastePct: 5priceUnit: cfpricePerUnit: 6
Result: Volume 5.6 · Volume 0.21 · 80-lb bags (premix) 10 · Estimated cost $33.60

Assumptions

What this calculation takes for granted — check these before relying on the result.

Assumptions:

Default values are an example 10×10 ft slab at 4 inches — replace with your measurements.
Depth is finished slab depth; waste covers forms, spillage, and subgrade variation.
Bag math assumes 80-lb premix bags yielding 0.6 cu ft each.
Outputs stay imperial (cu ft, cu yd) however you enter dimensions.

Limitations

Why your real figure may differ:

Footings, thickened edges, and slopes need separate sections — a flat slab figure understates them.
Premix bags past about 30 (≈1 yard) cost far more than ready-mix delivery with its short-load fees.
Reinforcement, admixtures, finishing, and labor are excluded.

Frequently asked questions

Bags or ready-mix delivery?

Bags suit under ~1 cubic yard (about 45 eighty-pound bags). Beyond that, ready-mix is cheaper per yard and far less labor — ask about short-load fees under 3–4 yards.

How thick should a concrete slab be?

4 inches for patios and walkways on firm ground, 5–6 inches for driveways, thicker at edges and under heavy loads — and always on compacted base, which this volume math assumes.

How much does an 80-lb bag cover?

About 0.6 cubic feet. A 60-lb bag covers about 0.45 — scale the bag count up by a third if that is what your store stocks.

Why add waste to concrete?

Forms flex, subgrades dip, and wheelbarrows spill. 5–10% extra beats a cold joint from running a barrow short — concrete waits for no one.

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