How thick should a concrete patio be?

There is no code answer, and that is worth knowing before somebody quotes you one. Neither patio nor driveway appears anywhere in IRC Chapters 4 or 5. Four inches is the usual convention and it is a sensible one for foot traffic. What the code does have is a strength requirement for exterior flatwork, which is a regional question rather than a thickness one.

How much concrete do I need for a 20x14 patio?

3.46 cubic yards at 4 inches, or 3.80 with a 10% allowance, which is 4.00 cu yd once the supplier rounds to the quarter yard. In bags at 0.6 cu ft that is 172, which is about three yards of hand mixing and a day of work for two people.

What strength concrete for a patio?

The nearest row in IRC Table R402.2 covers porches, carport slabs, exterior steps and garage floors: 2,500 psi in a negligible weathering region, 3,000 in moderate and 3,500 in severe, at 28 days. In moderate and severe the concrete must also be air-entrained at 5 to 7 percent by volume. Which region you are in comes from Table R301.2(1) and is local.

What is air entrainment and do I need it?

Deliberate microscopic bubbles in the mix, which give freezing water somewhere to expand into instead of spalling the surface. Table R402.2 makes it mandatory for exterior flatwork in moderate and severe weathering regions. It is a different mix rather than an upgrade, it has to be ordered, and it changes the finishing: air-entrained concrete should not be hard steel-trowelled or the surface can delaminate.

How much soil comes out of a patio excavation?

More than the slab, usually by half again. A 280 sq ft patio with a 4 in slab on a 4 in base standing an inch proud is a 7 in hole, which is 6.05 cubic yards of spoil against 3.46 of concrete. Excavated soil also bulks up, so the heap is bigger than the hole.

Is a thicker patio slab worth it?

An inch is 25 percent of the concrete, which is real money on a big patio and trivial on a small one. For foot traffic and furniture, thickness is rarely what fails: uniform subgrade support and joint placement are. If something is going to be parked on it, that is a different question and thickness starts to matter.

Do I need rebar or mesh in a patio?

Not by any code, because no code covers a patio. Reinforcement does not stop concrete cracking; it holds the crack tight once it happens, which keeps the two sides level and stops the joint opening. Whether that is worth it is a judgement about how visible the patio is, and this page does not size it.

Home › Concrete › Concrete Patio Calculator

Concrete Patio Calculator

The only code figure that touches a patio is Table R402.2

The word patio does not appear anywhere in IRC Chapters 4 or 5. Neither does driveway. There is no code minimum thickness for a patio slab, and the four inches everybody quotes has no source behind it.

Concrete patio takeoff

Patio length

Patio width

Slab thickness (no code sets one)

Compacted base (0 to skip)

Slab standing proud of grade

Weathering potential (Table R301.2(1), local)

Waste allowance (ours, subgrade dependent)

Bag yield (0 for ready-mix only)

cu ft

Priced as (the source's own row labels)

Volume, base and spoil from your dimensions. The strength row is IRC Table R402.2 as printed in the 2018 Seattle Residential Code. The money is Lawn Love's published range, attributed below.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Slab, base and the hole, in proportion

Worked example

A 20 ft by 14 ft patio, 4 in thick on a 4 in compacted base, standing an inch proud of grade, in a severe weathering region.

280 square feet is 3.46 cubic yards of concrete, and 3.80 once a 10% allowance goes on for what the subgrade takes. Ready-mix is sold in quarter yards, so that is 4.00 cu yd on the ticket.
Or 172 bags, which is the number that settles the argument. Sixty bags is roughly a cubic yard, so this is nearly three yards of mixing by hand and a full day for two people.
An inch of thickness is 25% of the concrete. Going to 5 in takes it to 4.75 cu yd. Going back down to 4 from 5 is only 20%, because the percentage is measured against a different base. Same inch, two different numbers, and quotes are argued over both.
The hole is 7 in deep, not 4. Slab plus base, less the inch standing proud. That is 6.05 cubic yards of spoil off a small patio, which is a skip, and it is the line most commonly missing from a quote.
And in a severe weathering region the mix is not ordinary. Table R402.2's nearest row wants 3,500 psi and air entrainment. Air is not an upgrade, it is a different mix, and a supplier will deliver whatever was ordered.

The patio itself is the cheapest part of the ticket. The excavation, the base and the mix specification are where the money and the failure modes both are.

The formula

Three volumes out of the same footprint, and the one people forget is the third:

concrete = A × t/12 base = A × b/12 spoil = A × (t + b − proud)/12

A: the patio area, which is the only figure most calculators use
t: slab thickness, and no code sets one for a patio
b: compacted base, a soil and drainage decision rather than a code one
proud: how far the finished slab stands above grade, which comes off the digging

All three are divided by 27 for cubic yards. The spoil is the biggest of the three and the only one nobody prices, and it does not compact back down: excavated soil bulks up, so the heap is larger than the hole it came out of.

There is no code minimum for a patio, and that is checkable

The word patio does not appear anywhere in IRC Chapters 4 or 5. Neither does driveway. Both chapters were read in full for this page, from Seattle's adopted code, and the count is zero. That is not an oversight: the IRC governs buildings, and a freestanding patio is not one.

So the four inches everybody quotes has no code behind it. It is a sensible convention, and it is what most people should build, but a calculator presenting it as a requirement is presenting a habit as a rule.

The nearest thing the code has to say is about strength, not thickness. Table R402.2 has a row for porches, carport slabs and steps exposed to the weather, and garage floor slabs. A patio is none of those four things, but it lives in the same weather, and that row is the honest benchmark: 2,500 psi in a negligible weathering region, 3,000 in moderate, 3,500 in severe, with air entrainment mandatory in the last two.

What that means practically is that the mix is a regional question and the thickness is not a code question at all. Which is the reverse of how most patio pages present it.

Reinforcement and control joints are also absent, from the code and from this page. Joint spacing is where flatwork cracks, and the guidance that exists is in ACI documents that are not free to read. A page that invented a spacing would be doing the thing this site exists not to do.

Prices for poured concrete, by finish

The published figures for poured concrete, by finish are on the concrete calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

The published price table, read carefully

The figures below the calculator are Lawn Love's, with their byline and date, and this site publishes no price of its own. Two things in that table are worth knowing before you use it.

Exposed aggregate is quoted below plain concrete. Aggregate comes in at $2 to $3 per square foot against $4 to $9 for a plain slab. An aggregate finish is a plain slab plus seeding, washing and sealing, so it cannot cost less than the slab under it. Something in that row is measuring a different thing, most likely the finish alone rather than the finished surface.

And the all-finishes row is wider than every finish in it. The whole-patio row runs $4 to $26 per square foot, while no individual finish in the same table tops $10. A range that contains none of its own members at the top end is a range built from a different survey.

Neither makes the table useless. It makes it a ranking with a plausible spread, which is what this site uses published figures as, and it is why the unit column matters more than the numbers.

What decides whether the patio cracks

None of it is in the volume. Concrete cracks, reliably, and the whole of the craft is deciding where.

The subgrade, compacted, and uniformly. A slab bridging a soft spot is a beam, and it was not designed as one. Uniform support matters more than strong support: a slab evenly bedded on poor soil outlasts one sitting half on rock and half on backfill.

The joints, and where they are. Control joints give the crack somewhere to happen. Too far apart and it happens between them, in a line of its own choosing. This page will not print a spacing, because the guidance lives in ACI documents that cannot be quoted here, but the principle is worth knowing: the joint is not preventing the crack, it is choosing it.

The water in the mix. More water is easier to place and weaker when set, and it shrinks more as it dries, which is what cracks it. A load that arrives stiff and gets a bucket of water added on site is a load that has quietly lost strength nobody will measure.

And the curing. Concrete does not dry, it reacts, and the reaction needs water for days. A slab poured on a hot windy day and left uncovered loses that water to the air before it has been used.

Which is why the mix specification above is worth taking seriously even though no code enforces it on a patio. Air entrainment in a freeze-thaw climate is the difference between a surface that lasts and one that scales off in the third winter.

Frequently asked questions

How thick should a concrete patio be?: There is no code answer, and that is worth knowing before somebody quotes you one. Neither patio nor driveway appears anywhere in IRC Chapters 4 or 5. Four inches is the usual convention and it is a sensible one for foot traffic. What the code does have is a strength requirement for exterior flatwork, which is a regional question rather than a thickness one.
How much concrete do I need for a 20x14 patio?: 3.46 cubic yards at 4 inches, or 3.80 with a 10% allowance, which is 4.00 cu yd once the supplier rounds to the quarter yard. In bags at 0.6 cu ft that is 172, which is about three yards of hand mixing and a day of work for two people.
What strength concrete for a patio?: The nearest row in IRC Table R402.2 covers porches, carport slabs, exterior steps and garage floors: 2,500 psi in a negligible weathering region, 3,000 in moderate and 3,500 in severe, at 28 days. In moderate and severe the concrete must also be air-entrained at 5 to 7 percent by volume. Which region you are in comes from Table R301.2(1) and is local.
What is air entrainment and do I need it?: Deliberate microscopic bubbles in the mix, which give freezing water somewhere to expand into instead of spalling the surface. Table R402.2 makes it mandatory for exterior flatwork in moderate and severe weathering regions. It is a different mix rather than an upgrade, it has to be ordered, and it changes the finishing: air-entrained concrete should not be hard steel-trowelled or the surface can delaminate.
How much soil comes out of a patio excavation?: More than the slab, usually by half again. A 280 sq ft patio with a 4 in slab on a 4 in base standing an inch proud is a 7 in hole, which is 6.05 cubic yards of spoil against 3.46 of concrete. Excavated soil also bulks up, so the heap is bigger than the hole.
Is a thicker patio slab worth it?: An inch is 25 percent of the concrete, which is real money on a big patio and trivial on a small one. For foot traffic and furniture, thickness is rarely what fails: uniform subgrade support and joint placement are. If something is going to be parked on it, that is a different question and thickness starts to matter.
Do I need rebar or mesh in a patio?: Not by any code, because no code covers a patio. Reinforcement does not stop concrete cracking; it holds the crack tight once it happens, which keeps the two sides level and stops the joint opening. Whether that is worth it is a judgement about how visible the patio is, and this page does not size it.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 4 FoundationsSeattle Department of Construction and Inspections. Read 2026-09-09. Read in full to confirm that neither patio nor driveway appears anywhere in it, and for Table R402.2, which is the only code figure on this page.
2015 Seattle Residential Code, Chapter 5 FloorsSeattle Department of Construction and Inspections. Read 2026-09-09. Also read in full for the same reason: it covers slabs on ground under buildings and says nothing about exterior flatwork.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

No code governs a patio slab, and this page does not pretend one does. The thickness is a convention, the base is a soil decision, and the strength row quoted is the nearest thing in Table R402.2 rather than a requirement that applies. Reinforcement and control joint spacing are not here because the guidance lives in ACI documents that are not free to read, and joint spacing is where flatwork actually cracks. No dollar figure on this page is this site's own.