What is 'nominal' moment capacity?

It's the theoretical maximum moment the section can resist from its materials and geometry alone, before any safety factor is applied. Design capacity is φMn, and φ is not a fixed 0.9: ACI 318 gives the full 0.9 only to a tension-controlled section (net tensile strain εt of 0.005 or more) and interpolates φ down towards 0.65 as εt falls through the transition zone, so the least ductile sections are penalised hardest. This calculator does that interpolation and prints both the φ it used and the resulting φMn in the breakdown — read them there rather than assuming 0.9. Factored load effects must stay below that φMn, not below the nominal headline.

Does this check for over-reinforcement?

Yes. It derives β₁, the neutral axis depth c and the net tensile strain εt, then compares εt against ACI 318's strain limits: below 0.004 it labels the section OVER-REINFORCED, drops confidence to low and replaces the headline with the largest moment the section can reach while staying at or above that floor; between 0.004 and 0.005 it flags the transition zone and the reduced φ. That 0.004 line is a ductility floor rather than a yield check, so it can fire on a section whose bars would still have yielded — the note tells you which of the two cases you are in. It is still not a design check: minimum reinforcement, shear, deflection, detailing and the load demand all sit outside this calculator.

Does this apply to T-beams or doubly-reinforced sections?

No — this is for a singly-reinforced rectangular section only. T-beams and sections with compression steel need a different, more involved formula.

Reinforced Concrete Beam Moment Capacity Calculator

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Tension Steel Area (As)

The cross-sectional area of tension reinforcement.

Sum the area of all tension-side rebar in the beam's cross-section.

Steel Yield Strength (fy)

The rebar's yield strength.

420 MPa (Grade 60) is standard US construction rebar.

Concrete Strength (f'c)

The concrete's specified compressive strength.

28 MPa (4,000 psi) is common for structural concrete.

Beam Width (b)

The beam's cross-sectional width.

The width of the concrete section resisting compression — for a rectangular beam the beam width, and for a T-beam or a beam cast into a slab the effective flange width, which is wider and is set by rules about span and spacing rather than by measurement. Using the web width for a beam that acts as a T under-states capacity substantially, and using the full slab width over-states it.

Effective Depth (d)

The distance from the compression face to the centroid of tension steel.

This is slightly less than the beam's total depth, accounting for cover and half the bar diameter.

Nominal moment capacity

132 kip·ft

Medium confidence

This is the nominal (unfactored) moment capacity for a singly-reinforced rectangular section. The section is tension-controlled, so φ is the full 0.9 and the failure would be ductile — the steel yields and the beam deflects visibly before the concrete crushes. A real design still checks minimum reinforcement, shear, deflection and detailing, and must be verified by a licensed structural engineer.

Stress block depth (a): 2.33 in
Neutral axis depth (c): 2.74 in
Net tensile strain at nominal strength (εt): 0.02
Design moment φMn at φ = 0.90: 119.22 kip·ft
Most steel this section can use singly-reinforced: 4.37 in²

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

ACI 318 rectangular stress block method for a singly-reinforced beam: a = As x fy / (0.85 x f'c x b); Mn = As x fy x (d - a/2)

Inputs used

Tension Steel Area (As): 1.55 in²
Steel Yield Strength (fy): 60915.85 psi
Concrete Strength (f'c): 4061.06 psi
Beam Width (b): 11.75 in
Effective Depth (d): 18 in

Intermediate steps

Stress block depth (a): 2.33 in
Neutral axis depth (c): 2.74 in
Net tensile strain at nominal strength (εt): 0.02
Design moment φMn at φ = 0.90: 119.22 kip·ft
Most steel this section can use singly-reinforced: 4.37 in²

Final result132.47 kip·ft

Confidence note: This is the nominal (unfactored) moment capacity for a singly-reinforced rectangular section. The section is tension-controlled, so φ is the full 0.9 and the failure would be ductile — the steel yields and the beam deflects visibly before the concrete crushes. A real design still checks minimum reinforcement, shear, deflection and detailing, and must be verified by a licensed structural engineer.

What this calculation does not cover

This is a flexural capacity figure only. It does not check shear, torsion, bar development and lap lengths, bar spacing, cover, deflection or minimum reinforcement, and it does not verify that the steel area you entered physically fits inside the width you entered.
Singly-reinforced rectangular sections only. Compression steel, T-beam and L-beam flanges, and any axial load on the member all move the neutral axis and change the capacity, and none of them are modelled here.
It gives capacity, never adequacy. There is no load input, so nothing is compared against factored moment demand, load combinations or the beam's own self-weight. The result cannot tell you the beam is strong enough for what you intend to put on it.
The ductility limits are the fixed 0.004 and 0.005 strain values rather than limits tied to the bar grade. That follows older ACI 318 practice written around Grade 60 rebar; with higher-grade bars the phi factor and the tension-controlled verdict shown here read more favourably than the current edition of the code allows.
This is not a structural design and carries no engineer's stamp. A beam that will carry real load needs a licensed structural engineer working from the full drawing set, the governing code edition and the actual load case.

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This result is a specification — 132 kip·ft — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

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Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

ACI 318 rectangular stress block method for a singly-reinforced beam: a = As x fy / (0.85 x f'c x b); Mn = As x fy x (d - a/2)

Which documents these citations point at

Standards referenced: ACI 318 (American Concrete Institute, United States).

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Tension Steel Area (As)	Nominal moment capacity (kip·ft)
1 in²	86.1
1.5 in²	126
2 in²	165
2.5 in²	201
3 in²	236

Reinforced Concrete Beam Moment Capacity Calculator (ACI 318)

How this was calculated

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Which documents these citations point at

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How to calculate reinforced concrete beam moment capacity (ACI 318) in 6 steps

Nominal moment capacity by tension steel area (As)

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