One-Way Slab Calculator: Concrete Slab Design (ACI 318-19)

1 · Slab and concrete

Thickness h in.

f'_c psi

Rebar grade

Concrete λ

Unit weight w_c pcf

Clear cover in., top and bottom

2 · Reinforcement

Bars	Size	Spacing (in.)
Bottom (positive)
Top (over supports)
Transverse (S&T)

Main bars run in the span direction. Top bars are placed over every continuous or fixed support. Transverse bars resist shrinkage and temperature.

3 · Spans and supports

Support	Type	Span to next (ft)

Spans are center to center of the supporting beams or walls. Pinned supports also act as rollers; use a free end for a cantilever. Up to 5 spans.

4 · Service loads Include slab self-weight in D

Uniform loads (psf, downward positive)

Case	w start	w end	From (ft)	To (ft)

Leave From and To blank to load the full length. D superimposed dead (finishes, ceiling, MEP, partitions), L live, L_r roof live, S snow.

Line loads across the slab (plf, downward positive)

Case	P	x (ft)

A wall running across the slab, parallel to the supports, at x ft from the left end.

Pattern live load on continuous spans (ACI 318-19 §6.4.3)

5 · Deflection

Immediate live limit L/

Long-term limit L/

Sustained part of the live load %

ACI 318-19 Table 24.2.2: L/360 for live load on floors; after nonstructural elements are attached, L/480 if they are likely to be damaged by deflection, L/240 if not. Cantilevers use twice their length.

How the one-way slab calculator works

This free one-way slab calculator designs reinforced concrete floor and roof slabs that span in one direction between beams or walls, using ACI 318-19 (Building Code Requirements for Structural Concrete) with ASCE 7-22 load combinations. Enter the slab thickness, the concrete and rebar grade, the bottom, top and transverse bars, the spans and supports, and the service loads in psf. The calculator analyzes a 12 in. wide strip with pattern live load and checks thickness, flexure, reinforcement limits, shear and deflection.

What it checks

Minimum thickness from ACI Table 7.3.1.1 (l/20 simply supported, l/24 one end continuous, l/28 both ends continuous, l/10 cantilever), adjusted for rebar grade and lightweight concrete
Positive moment in each span and negative moment at each continuous or fixed support: φM_n with the rectangular stress block, φ from the net tensile strain, and the slab limit ε_t ≥ 0.004
The required steel area and the largest bar spacing for the bars you choose
Minimum and shrinkage and temperature reinforcement (0.0018A_g for Grade 60), maximum spacing of 3h or 18 in., and the crack-control spacing of §24.3.2
One-way shear at d from each support with the ACI 318-19 equation for members without stirrups, V_c = 8λ_sλ(ρ_w)^1/3√f′_c·bd, including the size effect factor λ_s
Immediate live load deflection and long-term deflection after nonstructural elements are attached, with the effective moment of inertia I_e of each span and the long-term factor λ_Δ

Method

The slab is analyzed as a continuous beam one foot wide with the stiffness method, the same engine as the Static Structures beam calculators. Live load is placed on every combination of spans, as ACI 318-19 §6.4.3 requires for continuous slabs, so the maximum positive and negative moments are found without the approximate ACI moment coefficients. Deflections are computed with a separate I_e for each span from ACI Eq. 24.2.3.5a, averaging the positive and negative moment sections of continuous spans.

Assumptions and limitations

One-way action: the slab spans between parallel supports, typically with a long-to-short side ratio of 2 or more. Two-way slabs, flat plates and punching shear are not covered.
Supports are knife edges at the span ends; moments are taken at the support centerlines, without reduction to the face of the support, and moment redistribution is not used.
Bar cutoff points, development lengths and splices are not checked; use the rebar development length calculator. Compression steel is ignored in the deflection calculation.

Frequently asked questions

How thick should a one-way slab be?

ACI 318-19 Table 7.3.1.1 gives the minimum thickness when deflections are not calculated: span/20 for a simply supported slab, span/24 with one end continuous, span/28 with both ends continuous and span/10 for a cantilever. A 12 ft end span of a continuous slab therefore needs 6 in. with Grade 60 bars.

What is the minimum reinforcement for a concrete slab?

For Grade 60 bars, ACI 318-19 requires at least 0.0018 times the gross area in each direction, which is 0.13 in² per foot for a 6 in. slab. The spacing may not exceed 3h or 18 in. for main bars and 5h or 18 in. for shrinkage and temperature bars.

Do one-way slabs need stirrups?

Usually not. The slab must carry the factored shear with the concrete alone, V_u ≤ φV_c. ACI 318-19 makes V_c depend on the reinforcement ratio and on the slab depth through the size effect factor λ_s, so thick, lightly reinforced slabs have less shear strength than older editions gave.

When is a slab one-way?

A slab supported on two opposite sides, or a panel whose long side is at least twice its short side, bends mainly in the short direction. It is designed as a series of one-foot strips spanning between the supports, with only shrinkage and temperature steel in the other direction.

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For educational and preliminary design purposes only. Always verify results with a licensed PE.