What Is Concrete Compressive Strength (PSI)?

Quick answer

Concrete compressive strength is the squeezing load a concrete mix can carry before it fails, divided by the area carrying it, and it is reported in pounds per square inch (psi). It is measured by crushing test cylinders, usually at 28 days.

Engineers write the target as f′c, the specified strength. It is the most common performance number in concrete plans, and it says nothing by itself about cracks, water or soil.

Quick facts

The key facts about Concrete Compressive Strength, with sources:

Unit: Pounds per square inch (psi) in US units; megapascals (MPa) in SI ¹
How it is measured: Breaking cylinders in a compression-testing machine, tested under ASTM C 39 ¹
Usual test age: 28 days in most specifications ¹²
Symbol on plans: f′c, the specified strength, used by designers ¹
Range quoted by NRMCA: About 2,500 psi for residential concrete up to 4,000 psi and higher in commercial work ¹
Example code table: California’s 2025 residential code table lists 2,500 to 3,500 psi by location and weathering ⁷
Not the same as: Tensile strength, crack resistance or watertightness ⁵

Key takeaways

Compressive strength is load divided by area, reported in psi, found by crushing cylinders ¹.
The usual test age is 28 days, and the specified value is written f′c in the plans ¹.
One low cylinder does not prove bad concrete. Most low results trace to how cylinders were handled, cured or tested ².
A residential code table in one state sets different minimum psi values for walls, slabs and exposed concrete, so “how many psi” depends on where the concrete sits ⁷.
High psi does not stop cracks. Concrete is weak in tension and shrinks as it dries ⁵.
A psi number on a repair quote or spec describes the material. It does not say whether the repair suits your house.

What concrete compressive strength is

Compressive strength is a squeeze test. Take a concrete cylinder, press it in a machine until it breaks, and divide the load at failure by the cross-section area.

The answer is in pounds per square inch, or psi. The National Ready Mixed Concrete Association (NRMCA) says it is the most common performance measure engineers use when designing buildings and other structures ¹.

The number you see on plans is the specified strength, written f′c. The concrete supplier then designs the mix to run a little stronger on average, so the job still meets the spec given normal variation ¹.

This page explains the measure. It does not say what strength your foundation needs. That comes from the structural plans, the code your jurisdiction adopted and the engineer. Our reinforced concrete foundation page shows where the steel fits in.

General information, not a diagnosis of your house

General information, not a diagnosis of your house. A strength figure in this page is a quoted example from a named source, not a design value and not advice for your foundation.

How strength is tested

On a job, a technician takes a sample of the delivered concrete and fills cylinder molds.

Cylinders for acceptance testing are made and cured under the standard-cure procedures in ASTM C 31, then broken under ASTM C 39 ¹.

NRMCA lists 6 by 12 inch or 4 by 8 inch cylinders for acceptance testing ¹.

One test result is the average of at least two cylinders from the same sample, tested at the same age. In most cases the strength requirement is for an age of 28 days ¹. Concrete keeps gaining strength after that, but the 28-day break is the usual yardstick.

Lab quality matters. NRMCA says ASTM C 1077 requires certified laboratory technicians, and that most deviations from standard cylinder practice give a lower measured strength ¹.

Process From truck to test result

SampleA technician samples the delivered concrete
Cast cylindersMolds are filled and cured by the standard method
CureCylinders are kept in standard conditions until test age
BreakA machine loads each cylinder to failure
CalculateLoad divided by area, averaged, reported in psi

Source: [1]

Where a psi number shows up

Each row is a place to find the number, not a value for your house.
Place	What the number is	What to ask
Structural plans	The specified strength f′c the designer chose ¹	Which sheet or note gives it, and for which parts of the foundation
Concrete ticket or mix design	The mix the supplier ordered to meet the spec ⁶	Does the ticket match the plan?
Cylinder break report	Measured strength at a test age, usually 28 days ¹	Who ran the test, and was it to ASTM standards?
Building code table	A minimum specified strength by location and weathering ⁷	Which edition and amendments does your jurisdiction use?
Repair specification	The strength asked of a new pier, patch or grout	Who wrote it, and what does it apply to?

Example: how a code table is organized

The 2025 California Residential Code, Section R402.2, gives minimum specified strength in a table by type of concrete and weathering potential. It is one state’s adopted code, shown here as reproduced by a third-party site. The values below are quoted, not recommended. Check the official text and your own jurisdiction.

Concrete location	Negligible	Moderate	Severe
Basement walls, foundations and other concrete not exposed to weather	2,500 psi	2,500 psi	2,500 psi
Basement and interior slabs on grade, except garage floors	2,500 psi	2,500 psi	2,500 psi
Foundation and exterior walls exposed to weather	2,500 psi	3,000 psi	3,000 psi
Porches, carport slabs, steps exposed to weather, garage floor slabs	2,500 psi	3,000 psi	3,500 psi

Notice the pattern. The same code asks for more strength, and air entrainment, where concrete freezes and thaws ⁷. Other states and code editions differ.

Sources: [7]

Where strength matters in a foundation

Footings, foundation walls, slabs, grade beams and piers are all concrete, and each may call for a different strength. Compression is the job concrete does well. A footing pressing down on soil and a wall carrying the framing above are both mostly in compression.

In a repair, the new concrete also has a strength spec. A new footing extension, a cast-in-place pier cap or a patch each carries one. Our concrete pier repair page covers one common method.

Strength also links to durability. Texas’ bulletin on concrete acceptance for foundations notes that the foundation design must give material specifications, including minimum compressive strength, and it shows an example spec with a strength, aggregate size, slump and air content together ⁶. A strength number never travels alone.

Diagram Where concrete carries load in a foundation

Illustrative, not to scale, not a design. The footing and stem wall are mostly squeezed by the load from above, which is the kind of load a psi rating describes.

What a strength spec gives you

What it is good for

A shared, testable target between designer, supplier and inspector ¹.
A way to check delivered concrete against the order using test cylinders ¹.
A check on early curing, because field-cured cylinders can show whether the concrete was protected ¹.
Something to compare across repair bids that name the same material in the same terms.

What it cannot tell you

It does not measure resistance to cracking. Concrete is weak in tension ⁵.
It does not say anything about soil, drainage or load, which drive most foundation movement.
It does not show how the concrete was placed, finished or cured.
A cylinder is a sample. It is not a test of the wall or slab itself ¹.

The limits of one number

Strength in compression is only one property. Concrete is weak in tension, and when drying shrinkage is held back by the subgrade or another part of the structure, tensile stress builds and the concrete cracks ⁵. So a high-psi slab can still crack. Joints and steel handle that, not psi.

Durability is another property. In freeze-thaw exposure, NRMCA says concrete needs entrained air, and a strength of 3,500 psi for freezing and thawing exposure and 4,000 psi where deicers are used should be adequate to prevent scaling when the other practices are followed ³.

Air content, curing and finishing still matter. See concrete spalling for what surface failure looks like.

Finally, a test result is a statistical thing. NRMCA says an individual test below f′c does not necessarily mean the concrete fails the spec, because the acceptance rule looks at the average of three consecutive tests and a cap on any single low result ¹.

Strength is not the same as durability

Strength tells you how the concrete behaves under squeezing load. Air content, water-to-cement ratio, curing and drainage decide how it ages in weather.

How strength affects cost

We do not print prices. A higher specified strength can change the mix, the supplier and the testing plan, so it can change what a job costs. It is one input among many: volume, reinforcement, access, testing and labor all matter more on most jobs.

On a repair quote, treat a psi figure as a line to compare, not a reason to pay more. Ask what the plans require and who set it. Our cost guide explains how repair quotes are built, and the cost calculator lets you test scope assumptions.

How concrete is checked on a job

A general outline of acceptance, not a procedure.

Specify: The designer sets f′c and related limits on the plans ¹⁶.
Order: The builder orders concrete to that spec. The delivery ticket shows the mix and delivery times ⁶.
Sample and cast: A trained technician makes standard-cured cylinders from the delivered concrete ¹.
Break: A lab tests cylinders at the specified age, usually 28 days ¹.
Judge: The average of three consecutive tests should equal or exceed f′c, and no single test should fall short by more than a set margin ¹².
Investigate if low: Check the testing first, then compare the measured strength with what the structure actually needs ².

Keeping concrete strong after it is built

Strength is set in the first weeks. Afterward, the job is to keep water and chemicals from wearing it down.

Keep gutters and downspouts discharging away from the foundation so concrete is not wet all the time.
Keep soil sloping away from the wall, with a few inches of surface regrading only.
Go easy on deicing salts near new concrete. NRMCA advises not using them in the first year after placement and using clean sand for traction instead ³.
Avoid fertilizers containing ammonium sulfate or ammonium nitrate on concrete. NRMCA says they can scale and chemically attack the surface ³.
Photograph and date any new surface damage or cracks so a professional can see change over time.

Risks and when to call a structural engineer

Low strength is rarely the first thing a homeowner can see. What shows up is cracking, crumbling, scaling, exposed rebar or floors and walls that move. Those are clues. They are not a diagnosis, and none points to low-strength concrete on its own. Movement history, water, construction details and soil all change what they mean.

If you suspect weak concrete, do not guess. NRMCA says that when test results look low, the first step is to check whether the testing itself was done to standard, and that a deficiency worth investigating needs a comparison of the structural requirement against the measured strength ². That is an engineer’s job.

Call a licensed structural engineer if movement is significant, widening or changing; if walls bow or are displaced; if doors and windows stick along with new cracks; if corners separate; or if you have any safety doubt. Our engineer guide explains who to call.

Do not drill, cut or core a foundation yourself

Taking a sample from a load-bearing wall, footing or slab can weaken it. Coring and strength evaluation of in-place concrete are done under a plan from an engineer.

Codes and standards behind psi

In states that use the International Residential Code, Section R402.2 covers concrete materials. A Texas bulletin on concrete acceptance for foundations lists IBC Chapter 19, IRC 402.2 and ACI 318 together ⁶.

California’s 2025 residential code, in its own Section R402.2, sets a table of minimum f′c values and requires air entrainment for concrete subject to moderate or severe weathering ⁷.

The test methods come from ASTM, as in C 31 for making and curing cylinders and C 39 for breaking them ¹. Acceptance rules come from the ACI building code, ACI 318 ².

Adoption differs by state and city, and local amendments exist. AWC notes that the number of codes, the method of enactment and their application vary considerably across the states ⁸. Ask your building department which edition applies.

Edition and adoption

The code table in this page is from one state’s 2025 code as reproduced by a third-party site. Other states and editions differ. Verify with the official publication and your local building department.

Compressive strength compared with nearby terms

Term	What it describes	How it differs
Compressive strength	Resistance to squeezing load, in psi ¹	This page
Tensile strength	Resistance to pulling apart; low in concrete ⁵	Why steel and joints are used
Drying shrinkage	Volume loss as concrete dries, which can crack restrained concrete ⁵	A reason for cracks that is separate from psi
Air content	Entrained air that helps concrete survive freezing and thawing ³	A durability property, tested on fresh concrete
Soil bearing capacity	The strength of the soil under a footing	See soil bearing capacity; concrete psi does not set it

Concrete strength myths

Myth Higher psi always means a better foundation.: Reality Strength is one property. Soil, drainage, reinforcement, joints and curing decide how a foundation performs.
Myth One low cylinder proves bad concrete.: Reality NRMCA says an individual test below the specified strength is not necessarily a failure, and that most low results trace to handling, curing or testing ²¹.
Myth Concrete is fully strong after one week.: Reality The usual specification is at 28 days. A 3- or 7-day test can flag problems but is not a basis for rejecting concrete ¹.
Myth Strong concrete will not crack.: Reality Concrete shrinks as it dries and is weak in tension. Cracks can form even in strong concrete, which is why joints are placed ⁵⁴.
Myth Every foundation uses the same psi.: Reality Code tables set different minimums by location and exposure ⁷, and engineers specify what each design needs.

Regional factors for concrete strength

Climate sets the exposure. In regions with many freeze-thaw cycles and deicing salts, codes ask for more strength and for air-entrained concrete; California’s table steps up for “moderate” and “severe” weathering and requires 5 to 7 percent air for concrete exposed to weathering ⁷.

NRMCA gives 6 to 7 percent air in fresh concrete for severe exposure with typical aggregate sizes ³.

In warm, dry regions the same weathering rules may not apply, but the soil rules might. Check your frost and weathering class with your building department.

What to do with this

If you are building or repairing, ask for the structural plans and the strength notes on them. Ask for the cylinder break reports after the pour. If a quote names a psi, ask what it applies to.

If you are looking at damage, record it before anything else. The crack identification tool helps you log what you see, and choosing a foundation contractor lists what to ask in writing.

Decision guide Concrete strength questions by situation

If A new build or addition: Then Ask for the plans, the f′c notes and the cylinder break reports
If A repair quote lists a psi: Then Ask what part it covers and who specified it
If A test report shows one low result: Then Ask whether the lab followed the standard, and what the average of consecutive tests shows
If Crumbling or scaling concrete: Then Photograph it, then see the spalling page and ask an engineer if it keeps changing

Questions about Concrete Compressive Strength

What does PSI mean for concrete?

PSI means pounds per square inch. For concrete it is the load a sample resists when squeezed, divided by its area. It is measured by crushing cylinders in a testing machine and is the most common performance number engineers use ¹. It describes squeezing strength, not crack resistance or watertightness.

What PSI should foundation concrete be?

It depends on the design, the exposure and the code your jurisdiction adopted.

NRMCA says residential concrete requirements can run from about 2,500 psi up to higher values in commercial work ¹.

One state’s 2025 code table lists 2,500 to 3,500 psi by location and weathering ⁷. Your plans give the number for your house.

What does f′c mean on plans?

The symbol f′c is the specified compressive strength. Designers use it to size structural elements, and it goes into the contract documents ¹. The supplier then designs a mix that averages somewhat above f′c so the job still meets the spec despite normal variation.

Why is concrete tested at 28 days?

Most specifications set the strength requirement at 28 days, so that is the age for acceptance cylinders ¹. Earlier breaks at 3 or 7 days can flag problems with the concrete or the lab, but NRMCA says they are not a basis for rejecting concrete ¹.

What if a concrete cylinder breaks low?

One low cylinder is not automatic failure. NRMCA says acceptance looks at the average of three consecutive tests and a cap on any single shortfall, and that the cause is often the testing, not the concrete ¹². An engineer decides what follow-up is needed.

Can I tell my concrete strength by looking at it?

No. Surface cracks, color or hardness do not tell you the strength. Only a properly made and tested sample, or an engineer-planned evaluation of the in-place concrete, can. Do not drill or core a foundation yourself, because that can weaken it.

Does higher strength mean fewer cracks?

Not necessarily. Concrete is weak in tension, and when drying shrinkage is restrained, tensile stresses can crack it ⁵. Joints are placed to control where cracks form ⁴. Soil, drainage and reinforcement also affect cracking.

Is a psi figure part of a good foundation repair quote?

It can be, where the work includes new concrete. A good quote says what the concrete is for and who specified it. The number alone does not make a repair suitable. Our contractor guide covers the questions to ask in writing.

Does concrete strength affect freeze-thaw damage?

Yes, but air content, curing and finishing matter as well. NRMCA says concrete exposed to freezing and thawing must be air-entrained, and that 3,500 psi for freeze-thaw exposure and 4,000 psi where deicers are used should be adequate when the other precautions are followed ³.

Sources

NRMCA, CIP 35 – Testing Compressive Strength of Concrete (2003), copy hosted by Prairie Material, retrieved October 14, 2026.
NRMCA, CIP 9 – Low Concrete Cylinder Strength (rev. 2014), retrieved October 14, 2026.
NRMCA, CIP 2 – Scaling Concrete Surfaces (1998), copy hosted by Prairie Material, retrieved October 14, 2026.
NRMCA, CIP 6 – Joints in Concrete Slabs on Grade (1998), copy hosted by Prairie Material, retrieved October 14, 2026.
NRMCA, TIP 17 – Drying Shrinkage of Concrete, retrieved October 14, 2026.
Texas Department of Licensing and Regulation, IHB Technical Bulletin TB 10-03, Concrete Acceptance for Foundations (Apr. 28, 2010), retrieved October 14, 2026.
2025 California Residential Code, Section R402.2 Concrete (Title 24, Part 2.5), as reproduced by GoCodeBook; third-party copy, verify against the official publication, retrieved October 14, 2026.
American Wood Council, Adoption of ICC codes by state, retrieved October 14, 2026.

Expert review

Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.

How the numbers were checked: Facts were checked against the NRMCA, Texas TDLR, AWC and California code documents listed above, retrieved 2026-10-14. Figures are quoted examples, not design values. No structural engineer has reviewed this page.

Suggest a correction. We fix errors and say what changed.