Concrete Slab Load Capacity Calculator

1. Enter the slab thickness and the concrete strength (f prime c).
2. Enter the subgrade modulus k for the ground under the slab, and the contact radius of the load.
3. Set a factor of safety, around 2 for an industrial floor.
4. Read the result: the allowable single interior load, with the stiffness and strength figures behind it.

Here is what each result line means:

A concrete slab load capacity calculator estimates how much concentrated load a slab on ground can carry before the concrete reaches its flexural strength. It is used to sanity-check a slab for a forklift wheel, a post, a rack leg or machinery, away from the edges.

It uses the simplified Westergaard interior-loading method, which treats the slab as a plate on an elastic subgrade. The result is a planning estimate, not a structural design: it covers a single interior load only, not edges, corners, joints, repeated loads or fatigue. For a load-bearing slab, an engineer must do the design. To size the concrete itself, use the concrete slab calculator, and to set the mix water for strength the water cement ratio calculator.

The method first finds the radius of relative stiffness, which measures how widely the slab spreads a load into the subgrade. A stiffer slab or a softer subgrade spreads the load further.

1. Find the concrete modulus from the strength, E = 57000 times the square root of f prime c.
2. Work out the radius of relative stiffness from the modulus, thickness and subgrade k.
3. Find the equivalent contact radius b from the load radius and thickness.
4. Set the Westergaard interior stress equal to the modulus of rupture divided by the safety factor, and solve for the load.

Suppose you have a 6 inch slab at 4000 psi on a compacted base with k of 150, a 5 inch load contact radius, and a factor of safety of 2.

Calculation: the concrete modulus is about 3,605,000 psi, giving a radius of relative stiffness of about 25.8 in. The modulus of rupture is 7.5 times the square root of 4000, about 474 psi, so the allowable stress at a safety factor of 2 is 237 psi. With an equivalent contact radius of 4.67 in, the interior load works out to about 6,690 pounds.

The lesson: a 6 inch slab can carry a few thousand pounds on a single interior wheel, but the same load at an edge or corner is much less, which is why an engineer checks the real layout.

Thickness

Thickness has the biggest effect, because the capacity grows with the thickness squared. Going from 5 to 6 inches lifts the allowable load by roughly half again.

Concrete Strength

Higher strength raises the modulus of rupture, so the slab bends further before it cracks. The gain is smaller than from thickness because strength enters through a square root.

Subgrade Support

A stiffer subgrade, a higher k, carries more of the load and raises capacity. A soft or wet subgrade lowers it, which is why the base matters as much as the concrete.

Load Footprint and Safety Factor

A larger contact area spreads the load and raises the allowable value, while a larger safety factor lowers it. The factor accounts for repeated loading and uncertainty.

Westergaard gives three load positions, and they are not equal. This tool calculates the interior case, which has the highest capacity. The same slab carries far less at an edge or a corner.

Because edges and corners govern many real layouts, an engineer checks those positions and the joint detailing, not just the interior load this tool estimates.

Sanity-checking a Slab

Get a quick sense of whether an existing slab is in the right range for a forklift, a rack or a machine, before paying for a full assessment.

Comparing Thicknesses

See how much more a 6 inch slab carries than a 5 inch one for the same load, to guide an early design conversation.

Learning the Method

Follow how thickness, strength and subgrade combine in the Westergaard method, with the intermediate values shown.

1. Treating the Result as a Safe Working Load

This is a simplified interior estimate, not a design. Do not load a slab to this figure without an engineer confirming the full layout.

2. Ignoring Edges and Corners

The interior case is the most favourable. A load near an edge, corner or joint can need a far thicker slab.

3. Guessing the Subgrade K

The subgrade modulus swings the answer. Use a tested or well-justified value, not a hopeful one, and remember a wet subgrade is weaker.

4. Forgetting Repeated Loading

A slab that holds a load once can fatigue under thousands of passes. The safety factor only partly covers this; fatigue is a design check.

5. Using It for Structural Slabs

Suspended slabs, slabs on piles and structural floors are not slabs on grade and are outside this method entirely.

The method is a recognised simplification; the result is a planning estimate, not a design, and depends heavily on the subgrade value and the assumptions you enter.

It depends on the thickness, the concrete strength, the subgrade and where the load sits. As a planning estimate, a 6 inch slab at 4000 psi on a firm base can carry roughly 6,700 pounds under a single interior wheel with a safety factor of 2. Edge and corner loads are much lower, and only an engineer can confirm a safe working load.

It is a classic way to estimate stresses in a concrete slab on ground, treating the slab as a plate resting on an elastic subgrade. It gives separate results for a load in the interior, at an edge, and at a corner. This tool uses the simplified interior case.

It measures how stiffly the ground pushes back under the slab, in pounds per cubic inch. A weak subgrade might be around 100, while a well-compacted base is often 150 to 200. A higher k supports more load, so the base matters as much as the concrete.

Yes. In this method the capacity grows roughly with the thickness squared, so thickness is the strongest lever. Going from a 5 inch to a 6 inch slab raises the interior capacity by around half again, more than a strength increase would.

No. This is a simplified interior estimate for learning and sanity-checking, not a design. Forklift floors involve edge and joint loading, repeated passes and fatigue, which need a licensed engineer and a full design method.

The method is a recognised simplification, and the arithmetic is exact, but the answer swings with the subgrade value and the assumptions. Treat it as a ballpark for the interior case, not a safe working load.

Enter the specified 28-day strength, f prime c, in psi. A slab on ground is commonly 4000 psi, sometimes 3500 to 4500. The tool turns it into the modulus and the modulus of rupture used in the method.

No. It calculates the interior case only, which has the highest capacity. Edge and corner loads are lower and often govern the real design, so an engineer checks those positions separately.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Westergaard rigid pavement theory (interior stress) (Westergaard interior stress, radius of relative stiffness, equivalent radius b).
• American Concrete Institute (ACI; concrete modulus E and modulus of rupture from f prime c).
• American Cement Association (American Cement Association; concrete slabs on ground).