How Much Concrete Do I Need?

A complete concrete quantity guide

CB·01 concrete·ContractorBuilds Editorial·Published 2026-09-12 · Updated 2026-09-12·14 min read

✓Reviewed 2026-09-12 — independent technical, source and arithmetic review; every worked result reproduced in the named calculator

Step-by-step method to measure, calculate and order the right amount of concrete for slabs, footings, post holes and more.

01Quick answer

To work out how much concrete you need: measure the length, width and thickness of the space you are filling, all in the same unit. Multiply them — volume = L × W × T. If you measured in feet, that gives cubic feet; divide by 27 to get cubic yards, which is how ready-mix is commonly ordered. Then convert the volume into bags using your product's stated yield, or into a ready-mix order, and decide separately how much of an allowance to add.

Work out your slab now

The rest of this guide shows the working behind that number: what to measure for each project shape, the arithmetic step by step, and how a calculated volume becomes an order.

02What you need to measure

For any rectangular pour — a slab, a patio, a shed base — you need three numbers: length, width and thickness. For a footing trench, measure its width, depth and total length. For post holes, measure the hole diameter and hole depth, and note the post size, because the post displaces concrete. Measure the hole you dug, not the post going into it.

The most common measuring trap is mixing units. Thickness is usually measured in inches while length and width are in feet, and multiplying them as-is gives a meaningless number. Convert inches to decimal feet by dividing by 12: 4 in ÷ 12 = 4/12 = exactly 1/3 ft (0.3333… ft); 6 in = 0.5 ft. A measurement like 12 ft 6 in becomes 12.5 ft. If you'd rather not do the conversion by hand, the Feet & Inches Calculator converts feet-and-inches measurements to decimal feet, and the Square Footage Calculator finds the area of the surface you're covering.

03The concrete volume formula

Concrete quantity is a volume problem. For any rectangular shape:

All three measurements must be in the same unit. In feet, the result is cubic feet (ft³); in metres, cubic metres (m³). The formula doesn't change with the project — footings, steps and driveways below all reduce to the same multiplication, applied to their shapes.

Cubic feet to cubic yards

By definition, one cubic yard is a cube three feet on each side:

So to convert cubic feet to cubic yards, divide by 27. This single division is the step most estimates get wrong or skip, and it matters because ready-mix concrete is commonly quoted and sold by the cubic yard. The Cubic Yard Calculator converts measurements straight to cubic yards if you want the conversion done for you.

Metric: metres and cubic metres

In metric, measure length, width and thickness in metres and multiply the same way; the result is cubic metres directly, with no equivalent of the 27 division. The exact conversions between the systems are unit facts:

This guide gives US customary figures first, with the metric working alongside in the examples.

04Worked example: a slab

Take a 10 ft × 12 ft slab poured 4 in thick. The 4 in thickness here is an example input, not a recommendation — your slab's thickness is a design decision this guide doesn't make.

Whether you then order 1.5 yd³, 2 yd³, or something else is a separate decision covered in Calculated quantity vs what to order — the 1.48 yd³ is the net calculated volume, not the order.

Reproduce this in the Concrete Calculator with L = 10 ft, W = 12 ft, T = 4 in and the waste allowance set to 0 % — that shows this net calculated figure. Left at the calculator's labelled Suggested default, the result includes an ordering allowance on top of it, which is the calculated-vs-order distinction covered below.

05Other project shapes

Every shape below is the same idea — measure the space, compute its volume — with geometry adjusted to fit. Each formula's result converts to cubic yards the same way: divide cubic feet by 27.

Footings

A footing trench is a long, narrow rectangular volume:

Example: a trench 16 in wide (16/12 = 4/3 ft), 8 in deep (8/12 = 2/3 ft) and 24 ft long is 4/3 × 2/3 × 24 = 64/3 = 21.33 ft³ ≈ 0.79 yd³. The Footing Calculator takes the trench dimensions directly and handles the unit conversions.

Post holes

A post hole is a cylinder, and the post takes up part of it, so the concrete per hole is the cylinder minus the post below grade:

Example: a 12 in diameter hole (radius 0.5 ft), 30 in deep (2.5 ft), around a 4×4 post — which in practice commonly measures about 3½ in on each side (3.5/12 = 0.2917 ft): hole π × 0.5² × 2.5 = 1.96 ft³, post 0.2917 × 0.2917 × 2.5 = 0.21 ft³, so about 1.75 ft³ of concrete per hole; by hand, multiply by the number of holes. The Post Hole Calculator takes the hole dimensions, post size and number of holes, and returns the total for the whole job — pooling bags across the job total rather than rounding up per hole.

Round slabs and piers

A circular pad or pier is a cylinder:

Example: a 6 ft diameter pad (radius 3 ft) at 4 in (1/3 ft) is π × 9 × 1/3 = 3π = 9.42 ft³ ≈ 0.35 yd³. Remember the formula uses the radius — half the diameter — squared; using the diameter quadruples the answer.

Steps

Treat a set of steps as rectangles stacked front to back: each step is a box as wide as the stair, one riser high, and as deep as all the treads beneath it combined.

Example: three steps, 4 ft wide, 12 in treads, 6 in risers. Bottom box 4 × 3 × 0.5 = 6 ft³; middle 4 × 2 × 0.5 = 4 ft³; top 4 × 1 × 0.5 = 2 ft³; total 12 ft³ ≈ 0.44 yd³.

Driveways and patios

A driveway or patio is a large slab, and the slab formula applies unchanged — the numbers are just bigger, which is why the cubic-yard conversion matters most here. Example: a 20 ft × 24 ft driveway at 5 in (5/12 ft) is 20 × 24 × 5/12 = 200 ft³ ≈ 7.41 yd³. The 5 in is again an example input only. How thick a driveway should be depends on loads, subgrade and local practice — that is a structural design question, and a quantity calculator is not a structural design tool: it tells you how much concrete fills the shape you describe, not what shape the structure should be.

06Cubic feet, cubic yards and cubic metres

Concrete quantities move between three volume units, and the relationships between them are fixed unit facts: 27 ft³ = 1 yd³ (by definition), 1 yd³ = 0.764555 m³, and 1 m³ ≈ 35.31 ft³. In practice, ready-mix suppliers in the US commonly quote and sell by the cubic yard ("a yard of concrete" means a cubic yard), and by the cubic metre in metric markets; bagged concrete is bought by the bag and converted through the product's yield, covered in the next section.

Turned around, the same arithmetic answers "how far does a yard go?": coverage = 27 ÷ thickness in feet, so at 4 in — exactly 1/3 ft — one cubic yard covers 27 ÷ (1/3) = 27 × 3 = 81 ft².

Concrete per square foot by thickness

Derived arithmetic: ft³ per 100 ft² = 100 × (thickness in inches ÷ 12); yd³ = ft³ ÷ 27. Scale linearly for your area.

07Bags: how many, and why bag weight is not the answer

The number on a bag of concrete mix is its weight — the mass of dry material inside. What you need is its yield — the volume of placed concrete one mixed bag produces — and that is a figure the manufacturer states for each specific product on its data sheet. Two 80 lb bags from different products show why: Quikrete Concrete Mix No. 1101 — 80 lb has a stated yield of approximately 0.60 ft³ per bag [1], while Sakrete MAXIMIZER Concrete Mix — 80 lb has a stated yield of approximately 0.92 ft³ per bag [2] — the same weight, and more than half again the yield. Bag weight alone is not a reliable way to compare yield across different products. So there is no valid rule of the form "an 80 lb bag makes X cubic feet"; use the stated yield for the exact product and bag size you are buying.

The arithmetic, once you have the stated yield:

For the worked slab above (40 ft³), using Quikrete Concrete Mix No. 1101 — 80 lb, whose manufacturer states an approximate yield of 0.60 ft³ per mixed bag [1]:

40 ÷ 0.60 = 66.7 → 67 bags of Quikrete Concrete Mix No. 1101 — 80 lb (stated yield ≈ 0.60 ft³/bag) — always round bags up, since you can't buy two-thirds of a bag. By the same figure, one cubic yard is 27 ÷ 0.60 = 45 bags of that product.

Bagged mix is commonly sold in weights such as 40, 50, 60, 80 and 90 lb, and each size of each product carries its own stated yield. The Concrete Bag Calculator computes bag counts from the stated yields of specific products, so the figure you use matches the bag you'll actually buy.

08Calculated quantity vs what to order

The volume you calculated is the net quantity — the concrete that exactly fills the shape you measured, assuming the subgrade is level and compacted at an even depth and the forms are straight and true. The order quantity is a planning decision layered on top, and the two are different numbers for real-world reasons: subgrade is rarely perfectly even, excavations go slightly over-depth, forms bow outward under wet concrete, and some concrete is lost to spillage and to what stays in the mixer and barrow.

How much allowance to add is a judgement you make based on how much your site departs from the clean geometry — not a fixed percentage, because a hand-dug trench in soft ground and a laser-screeded slab on compacted stone are different situations. Our calculators show a labelled Suggested default allowance precisely so you can see it and change it; it is a starting point, not a rule.

For ready-mix, one more step sits between your number and the order: suppliers round orders to increments, and both the increment and any minimum vary by supplier and region — confirm them with your supplier when you order, and let their increments, not your rounding, set the final figure.

09Bags or ready-mix?

Bags and ready-mix deliver the same material by different routes, and the choice turns on volume, labour, time, access and consistency. Bags mean you control timing and can pour in stages, but every bag must be carried, mixed and placed, and batch-to-batch consistency is on you. Ready-mix arrives mixed and uniform and places fast, but the whole load comes at once, the truck needs access to the forms (or you need a wheelbarrow chain), and you're working at the truck's pace.

There is no fixed crossover volume where one route becomes right. Weigh your project's volume against the mixing and placement labour and time you can sustain, whether a truck can reach the forms, and how much batch-to-batch consistency matters for the finish — and since the economics depend on your local bag and delivery prices, put your own prices into the Concrete Cost Calculator and compare the two routes for your actual volume.

10Common estimating mistakes

• Mixing units — multiplying feet by inches without converting. Convert everything to feet (or metres) first.
• Reading inches as feet — a "4 ft thick" slab in a calculator when you meant 4 in produces a result twelve times too large.
• Forgetting thickness entirely — area alone (ft²) is not a volume; every quantity needs the third dimension.
• Using bag weight as yield — an 80 lb bag is a weight; the volume it makes is the product's stated yield, and it varies by product.
• Ignoring over-dig and uneven subgrade — the hole you actually dug is bigger than the one you drew.
• Ordering exactly the net quantity — leaving no allowance means any real-world loss shows up as a short pour.
• Measuring the post instead of the hole — the concrete fills the hole; the post only subtracts from it.
• Rounding down — round bag counts and order quantities up; concrete you don't have can't be stretched.

11Project reference table

Every example quantity above is shown with the inputs that produce it; results for your project require your measurements. Calculator names are linked from the sections above and the related-calculators block below.

12FAQ

13Sources, methodology and how we label facts

Every number in this guide is one of two things: derived in front of you — the arithmetic is on the page, and you can reproduce it in the named calculator with the inputs shown — or labelled with where it comes from. Unit relationships such as 27 ft³ = 1 yd³ and 1 yd³ = 0.764555 m³ are mathematical facts. Manufacturer figures, such as a product's stated bag yield, are that manufacturer's data, marked as such and cited to the product's data sheet. Statements about how the trade commonly works — that ready-mix is sold by the yard, that order increments vary by supplier — are conventions, worded as conventions, never as rules. Where our calculators pre-fill a value, it is shown as a labelled Suggested default you can change.

The "How we label facts" box on this page explains the provenance chips used across ContractorBuilds. For how the calculators themselves derive and label every figure, see our methodology.