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Concrete · Method Guide

How to calculate concrete

The same five steps work for a fence-post footing and a commercial foundation. Learn them once and every calculator on this site becomes a double-check instead of a crutch.

Contents

  1. 1.The five-step method
  2. 2.Unit conversions you actually need
  3. 3.Worked example: L-shaped patio
  4. 4.Worked example: from volume to bags
  5. 5.Odd shapes: circles, slopes and steps

The five-step method

  1. 1

    Measure the pour

    Measure length, width and depth at several points and record the averages. Depth is measured from the compacted base to the finished surface, not from string lines.

  2. 2

    Convert to one unit

    Convert every dimension to feet (or meters) before multiplying. 4 inches = 0.333 ft, not 0.4.

  3. 3

    Compute the volume

    Multiply length × width × depth. For irregular shapes, split the pour into rectangles and sum them.

  4. 4

    Add a waste allowance

    Multiply by 1.05–1.15: 5% for formed pours on a laser-leveled base, 10% typical, 15% for earth-formed trenches or hand mixing.

  5. 5

    Convert to order units

    Divide cubic feet by 27 for cubic yards, or use cubic meters directly. Round up to the supplier's increment — 0.25 yd³ for ready-mix, whole bags for site mixing.

Formula

Order (yd³) = (L × W × D) ÷ 27 × (1 + waste)

L, W, D in feet. Working metric? Skip the ÷27: cubic meters are the order unit.

Unit conversions you actually need

The five conversions that cover 95% of concrete work. Densities for other materials are in the density chart.
FromToMultiply / divide by
InchesFeet÷ 124 in = 0.333 ft
Cubic feetCubic yards÷ 27
Cubic yardsCubic meters× 0.765
Cubic feet80 lb bags÷ 0.60Bag yield, not weight
Cubic metersMetric tons× 2.4Normal-weight concrete

Worked example: L-shaped patio

Worked example

An L-shaped patio: main area 18 × 12 ft plus a 8 × 6 ft leg, poured 4 in thick.

  1. 1

    Split at the inside corner

    Rectangle A: 18 × 12 · Rectangle B: 8 × 6

  2. 2

    Areas

    216 ft² + 48 ft² = 264 ft²

  3. 3

    Volume at 4 in (0.333 ft)

    264 × 0.333 = 87.9 ft³

  4. 4

    Waste and conversion

    87.9 × 1.10 ÷ 27 = 3.58 yd³

Result: Order 3.75 yd³ (next quarter-yard increment).

Worked example: from volume to bags

Worked example

Setting 12 fence posts in 10 in holes, 30 in deep, 4×4 posts (3.5 in actual).

  1. 1

    Hole area (10 in ⌀ = 0.417 ft radius)

    π × 0.417² = 0.545 ft²

  2. 2

    Hole volume at 30 in (2.5 ft) deep

    0.545 × 2.5 = 1.36 ft³

  3. 3

    Subtract the post (0.29 × 0.29 × 2.5)

    1.36 − 0.21 = 1.15 ft³ per hole

  4. 4

    All twelve holes

    1.15 × 12 = 13.8 ft³

  5. 5

    As 80 lb bags with 10% waste

    13.8 × 1.10 ÷ 0.60 = 25.3

Result: 26 bags of 80 lb mix — about two bags per post, the rule of thumb confirmed.

Odd shapes: circles, slopes and steps

Circles use π × r² × depth — that's the whole trick, and the column calculator does it for you. Sloped slabs use the average of the high-side and low-side thickness. Steps decompose into stacked rectangles: each tread is a small slab sitting on the one below, so a 3-step stoop is three volumes summed. Voids (pipe sleeves, blockouts) only matter above about 2% of the pour — smaller ones live inside the waste factor.

Frequently asked questions

Now run your numbers

Every calculator applies this method with the unit conversions handled for you.

Open the Concrete Calculator

Shape-specific calculators

Concrete Slab CalculatorConcrete Footing CalculatorConcrete Wall CalculatorConcrete Column Calculator

Go deeper

Same method, different material

How to Calculate AsphaltAsphalt Volume Calculator

Sources & references

  1. [1]Design and Control of Concrete Mixtures, 17th ed. — Portland Cement Association, 2021
  2. [2]ASTM C94/C94M: Standard Specification for Ready-Mixed Concrete — ASTM International, 2024