How many cubic yards of ready-mix do I need for a 20 by 20 foot slab?

A 20 by 20 foot slab poured 4 inches thick has a volume of 133.3 cubic feet, which is 4.94 cubic yards. Adding a 10% allowance for uneven subgrade, spillage and form movement gives 5.43, which this guide rounds up to 5.5 cubic yards. Each extra inch of thickness adds about 1.36 cubic yards with the same allowance. The 10% figure is an assumption, so ask your supplier how they round and whether short-load fees apply.

Does the producer price index tell me what a cubic yard costs?

No. The BLS index for concrete products (series WPU133) stood at 410.255 in September 2026 against 393.678 a year earlier, up 4.2%. That is index points on a 1982 = 100 base, not dollars. It shows which way producer prices are moving, not what your local plant charges for a delivered cubic yard. Get the dollar price from a written supplier quote and use the index only to judge direction.

How much of a slab bill is labor and how much is concrete?

In the worked example here, the concrete itself is about 30% to 32% of the total, while labor after the burden multiplier is about 57% to 59%, with forms, base and other materials making up the rest. Those shares depend on the assumed cubic yard price, crew hours and a 2 to 3 times burden multiplier. If your quote shows a very different split, ask which input differs.

What single question exposes a padded slab quote?

Ask: how many cubic yards and how many crew-hours did you price? A 400 square foot, 4 inch slab needs roughly 5.5 cubic yards and, in this guide's assumption, about 24 crew-hours. If a bid assumes 8 cubic yards or 60 hours without a stated reason such as thick edges, a pump or difficult access, the padding is in that line, and the contractor should be able to explain it in a sentence.

Are national wages good enough to judge a local quote?

They set a floor for sanity checks, not a verdict. The national median for cement masons and concrete finishers is $27.41 per hour, with a middle half of workers between $22.60 and $32.15. Local wages, ready-mix prices, haul distance and season can move a bid outside the range built here. Use the numbers to find which line item to question, then compare at least two written local quotes.

Ready-Mix Concrete Prices and What They Add to a Slab

A 20 by 20 foot, 4 inch slab needs about 5.5 cubic yards of ready-mix once waste is added. Under the assumptions shown below, installed cost lands near $2,600 to $3,800, or about $6.50 to $9.40 per square foot, with the concrete itself roughly 30% of that. The cubic yard price is an assumption, not a measured figure.

Key points

A 400 square foot slab at 4 inches thick holds 4.94 cubic yards, and about 5.5 cubic yards should be ordered with a 10% allowance.
The national median wage for cement masons and concrete finishers is $27.41 per hour, and the middle half of workers earn between $22.60 and $32.15.
Federal producer prices for concrete products rose 4.2% in the twelve months to September 2026, while hydraulic cement fell 1.2%.
In the worked example, concrete is about 30% to 32% of the bill; labor after a 2 to 3 times burden multiplier is the larger share.
This page cannot see your local plant price, permits, access or season, so the result is a test for a quote, not a replacement for one.

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What does this calculation cover, and what does it leave out?

It covers a plain, flat, residential slab on grade: forms, a gravel base, a vapor barrier, reinforcing mesh, ready-mix concrete, placing, finishing and curing. It does not cover demolition, hauling away old concrete, permits, a pump truck, decorative finishes, drainage work or soil problems. Those are discussed later in the section on what the numbers cannot see.

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The method is simple arithmetic you can argue with. Volume comes from geometry. Labor comes from a federal wage multiplied by an assumed number of crew-hours and an assumed burden factor. Materials are an allowance. Nothing on this page comes from contractor surveys, collected quotes or site visits. This site has done none of those, and the figures below are derived calculations, not observed bills.

Three kinds of statement are kept apart:

Official observations: the BLS wages and producer price index levels, each linked where it appears.
Assumptions: crew size, hours, burden multiplier, waste, cubic yard price and the other materials allowance.
Calculated results: every dollar total, which is only as good as the assumptions beneath it.

How much concrete does a 400 square foot slab need?

It needs 4.94 cubic yards for the exact volume, and about 5.5 cubic yards to order. The math: 20 feet × 20 feet × (4 inches ÷ 12) = 133.3 cubic feet. Divide by 27 cubic feet per cubic yard and you get 4.94 cubic yards.

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Real pours never use the exact figure. Subgrade is not perfectly flat, forms bow slightly, and some concrete stays in the truck chute or gets spilled. This guide assumes a 10% allowance, which gives 5.43 cubic yards, rounded up to 5.5. The 10% is an assumption. A flat, well-prepared base might need less, and a rough base more.

Thickness matters more than most homeowners expect. Each extra inch over a 400 square foot area adds 400 ÷ 12 ÷ 27 = 1.23 cubic yards, or about 1.36 cubic yards with the 10% allowance. A 5 inch slab therefore needs roughly 6.8 cubic yards, not 5.5. Thicker slabs for vehicles or heavy loads are a design decision that belongs with the contractor or an engineer, not with this page.

What do federal wage data say a mason is paid?

The wage table below comes straight from the BLS Occupational Employment and Wage Statistics series for cement masons and concrete finishers, occupation code SOC 47-2051, survey year 2025, United States.

Measure	Hourly wage paid to the worker
25th percentile	$22.60
Median	$27.41
Mean	$28.87
75th percentile	$32.15
First-line construction supervisors, median	$38.42

BLS counts 206,170 people employed in this occupation nationally. These are wages paid to workers. They are not prices billed to a customer, and they exclude the employer’s payroll taxes, insurance, truck, tools, office time and profit.

The spread inside the occupation is worth noticing. The 75th percentile wage is $9.55 per hour above the 25th, which makes it about 42% higher. A crew built around workers at the top of the pay range costs noticeably more per hour in wages alone, before any burden is applied. The supervisor figure of $38.42 per hour is for first-line supervisors of construction trades in general, so it is an approximate stand-in for a crew lead, not a concrete-specific number. The BLS Occupational Outlook Handbook profile describes the duties of this occupation, its entry requirements and its pay in more detail.

Can I compare metro areas for this slab?

Not honestly. The source data supplied for this page is national only: one United States row for wages and one for each price index. There is no metro wage row, so any city-to-city spread here would be invented. What can be said is that the national wage varies by 42% between the 25th and 75th percentiles, and local labor markets sit somewhere inside and beyond that spread. If you want a local sanity check, substitute your own area’s wage for the $27.41 median in the tables below and keep everything else the same.

What is the burden multiplier, and why is it not the price?

A homeowner pays more than the wage because the contractor must also cover payroll taxes, workers’ compensation, liability insurance, vehicles, fuel, tools, office time, warranty reserve and profit. Multiplying the wage by a single factor is a rough way to account for all of that.

This guide applies a burden multiplier of 2.0 to 3.0 times the wage and uses 2.5 as the middle case. That range is a working assumption for small residential contractors, not a measured figure. A larger, better-insured firm might sit higher, and a one-person operation with few overheads might sit lower. The multiplier is applied to labor only. Materials are carried at cost in this example, and many contractors add a margin on materials as well, which is covered under the quote section.

What does the full cost build-up look like?

Here are the inputs, all of them assumptions except the wages:

Crew: three workers at the national median mason wage of $27.41 per hour, for 8 hours, which is 24 crew-hours. This is an assumption for forming, placing and finishing a small flat slab in one day.
Supervision: 2 hours of a lead at $38.42 per hour. Many small jobs have a working owner instead, so this line may be absent in a real quote.
Concrete: 5.5 cubic yards at an assumed $140, $180 or $220 per cubic yard delivered. These are placeholders for you to replace with a supplier quote. They are not BLS data.
Other materials: an assumed flat $350 for gravel base, vapor barrier, mesh, form lumber and curing supplies.

The labor wage line first: 24 hours × $27.41 = $657.84, plus 2 hours × $38.42 = $76.84, for a wage total of $734.68.

Step	Low case	Middle case	High case
Wage total (24 crew-hours plus 2 lead hours)	$734.68	$734.68	$734.68
Burden multiplier (assumption)	2.0	2.5	3.0
Billed labor	$1,469.36	$1,836.70	$2,204.04
Ready-mix, 5.5 cu yd at assumed price	$770.00 ($140)	$990.00 ($180)	$1,210.00 ($220)
Other materials allowance (assumption)	$350.00	$350.00	$350.00
Total	$2,589.36	$3,176.70	$3,764.04
Per square foot (400 sq ft)	$6.47	$7.94	$9.41

The result is roughly $2,600 to $3,800, or $6.50 to $9.40 per square foot, with a middle case near $3,200. Concrete is $770 of $2,589 in the low case (about 30%), $990 of $3,177 in the middle case (about 31%) and $1,210 of $3,764 in the high case (about 32%). Billed labor is about 57% to 59% of the total across the three cases.

This is a derived range, not a market average. It tells you how the pieces fit together. It does not tell you what any particular contractor will charge.

What do ready-mix prices add to the slab, and what do the producer price indexes show?

Ready-mix adds about $770 to $1,210 in this example, and the supplied federal data cannot narrow that further, because the BLS publishes index levels, not delivered dollar prices. The BLS Producer Price Index for concrete products (series WPU133) was 410.255 in September 2026, against 393.678 a year earlier. That is a rise of 4.2% over twelve months. The index for hydraulic cement (series WPU1322), the binder inside concrete, was 356.209 against 360.462, a fall of 1.2%.

These are index points on a 1982 = 100 base. They are not dollars and must not be turned into a price per cubic yard. What they do show is direction. Producer prices for finished concrete products rose over the year while the cement ingredient eased, so the movement is not simply a cement story. The broader concrete products series also covers items other than ready-mix, so it is only a rough guide to what a local plant is doing.

Used with care, the index helps one illustrative question: if a delivered price for the middle case, $990, had moved in line with the 4.2% index change, the concrete line would be about $42 higher than a year before. That is about 1.3% of the $3,177 middle-case total. The conclusion is that a 4.2% index move is small next to the uncertainty in labor hours and the burden multiplier. The cubic yard price you are quoted still matters, but it is not the biggest lever on a slab this size.

How sensitive is the answer to hours, thickness and cubic yard price?

The answer is most sensitive to the burden multiplier, then to crew-hours, then to thickness, and least to the producer price movement. The two tables below hold everything else at the middle case.

Crew-hours (3 masons at $27.41, no lead)	Wage total	Billed labor at 2.5 times
16 hours	$438.56	$1,096.40
24 hours	$657.84	$1,644.60
32 hours	$877.12	$2,192.80

Eight crew-hours either way shifts billed labor by about $550 at 2.5 times. That is larger than the whole difference between a $140 and a $180 cubic yard on 5.5 cubic yards, which is $220.

Thickness moves both concrete and labor. Going from 4 to 5 inches adds about 1.36 cubic yards. At an assumed $180 per cubic yard that is about $245 of extra concrete, plus whatever additional finishing time the contractor sees. Going the other way, thinning a slab to save money can weaken it, so treat thickness as a structural decision.

Wage level matters too. If the crew is paid at the 75th percentile of $32.15 rather than the median, 24 crew-hours cost $771.60 in wages instead of $657.84. At 22.60 the 25th percentile gives $542.40. Those are differences of $113.76 and $115.44 before any burden multiplier is applied.

What can this calculation not see?

It cannot see anything that is specific to your property or to the day of the pour. The following can each move a real bid well outside the range above:

Permits and inspections. Some jurisdictions require a permit for a new slab or for work near property lines. Fees and inspection timing are local and are not in this range.
Demolition and disposal. Replacing an old slab means breaking it out and hauling it away. Neither is included here, and both can exceed the cost of the new pour on a thick or reinforced slab.
Code upgrades. Frost depth, drainage, reinforcement and footings may be required by local code, which would change the design and the quantities.
Access. If a truck cannot reach the site, the concrete may need a pump or wheelbarrows. A pump adds equipment and labor hours that this example leaves out.
Site difficulty. Slopes, tight forms, trees and poor soil change the base work and the finishing time.
Season. Cold weather may require blankets or heated water, and hot weather may need retarders and faster finishing. Demand is also seasonal for contractors.
Short loads and delivery fees. Suppliers often charge extra for small loads, long hauls or waiting time. Ask for the delivered price, not the plant price.
Emergency call-out premiums. Rush jobs and repairs scheduled outside normal hours cost more than planned work, and this range assumes planned work.

If two or more of these apply, treat the range above as a lower bound and expect a quote to sit above it.

How should I read a real slab quote against these numbers?

Start by asking for line items. A fair quote should separate the following, so you can compare each against the build-up above:

Concrete quantity and unit price. Cubic yards, price per cubic yard, and any delivery or short-load fee.
Base and forms. Gravel depth, vapor barrier and form materials.
Reinforcement. Mesh or rebar type and spacing.
Labor. Crew size and hours, or a labor lump sum with its stated assumptions.
Finishing and curing. Surface type, joints and sealing if any.
Extras. Permit, pump, demolition, haul-away and anything contingent.

Mind the markup on materials. Contractors commonly carry materials at cost plus a margin. As an illustration, a 15% margin on the $1,340 of materials in the middle case would add $201. That is a normal pricing practice and the quote may be fair even with it, but it should be visible rather than buried.

Which single question exposes a padded bid?

Ask: “How many cubic yards and how many crew-hours did you price?” A 400 square foot, 4 inch slab needs about 5.5 cubic yards and, under the assumption here, about 24 crew-hours. If the bid assumes 8 cubic yards or 60 hours with no reason such as thickened edges, a pump or hard access, the padding is in that line. A reasonable contractor can answer in one sentence. A vague answer is the signal to ask for a rewritten quote.

To compare a quote against this page, divide the bid by the area to get a per-square-foot figure and place it against $6.47 to $9.41. Landing inside the range does not prove the bid is fair, because the range rests on assumptions. Landing far outside means one of the inputs differs from yours, and the line-item question will show which.

What should I check before signing?

Quantity: the stated cubic yards match your dimensions, thickness and a reasonable waste allowance.
Price basis: the concrete price is delivered, and short-load or wait-time fees are stated.
Labor: crew size and hours are given, or the lump sum comes with a written scope.
Exclusions: demolition, disposal, permits, pump and drainage are marked as included or excluded.
Specification: thickness, reinforcement, base depth and finish are written down.
Insurance and licensing: ask for proof of liability coverage and workers’ compensation, and check licensing with your state or local authority.
Payment terms: a deposit that is small compared with the materials, and a final payment tied to completion.
Second opinion: at least two written local quotes, compared line by line rather than by total.

Frequently asked questions

The answers to the most common questions about this calculation are in the FAQ section of this page’s metadata and repeat the figures above: 4.94 cubic yards before waste, 5.5 to order, $27.41 as the national median wage, 4.2% for the concrete products index and about 30% to 32% as the concrete share of the total.

How current is this and where does the data come from?

The wage figures are BLS Occupational Employment and Wage Statistics for survey year 2025. The price index levels are BLS Producer Price Index series for September 2026. They were pulled from the BLS public API on 2026-11-05, which is the date this page was last checked against its sources. The dollar price of concrete, the crew-hours, the burden multiplier and the other materials allowance are the author’s assumptions, shown in the tables so you can replace them with your own.