Enter your slab span, thickness, concrete strength, and applied load to calculate maximum deflection and instantly check compliance with ACI 318 limits (L/360, L/480, L/240).
ACI 318 compliant
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No sign-up required
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Checks L/360, L/480, and L/240
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Imperial & metric supported
✓ Simply-supported slab formula✓ Gross section Ie (conservative)✓ Three ACI 318 limits checked✓ Last verified May 2026
Measure center-to-center of supports. Use the clear span for simply-supported slabs.Please enter a valid span greater than 0.
Overall slab depth from top to bottom surface.Please enter a valid thickness greater than 0.
28-day compressive strength. Typical residential: 3,000–4,000 psi.Please enter a valid concrete strength greater than 0.
Enter live load only for L/360 check, or total load for L/240. Typical office: 50–100 psf.Please enter a valid load greater than 0.
Results appear instantly. No sign-up required.
Deflection Results
Maximum Midspan Deflection
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Deflection (inches)
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Deflection (mm)
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Span Ratio (L / δ)
ACI 318 Deflection Limit Checks
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L/360 Limit (live load, floors)
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L/480 Limit (sensitive partitions)
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L/240 Limit (roofs, non-sensitive)
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—Span (L)
—Thickness (h)
—Ec (psi)
—Ig (in⁴/ft)
Step 1: Convert span L to inches; thickness h to inches
Step 2: w (lb/in per 12" strip) = load (psf) × 12 in ÷ 144 = load ÷ 12
Step 3: Ec (psi) = 57,000 × √f'c (psi) [ACI 318-19 §19.2.2]
Step 4: Ig (in⁴ per 12" strip) = 12 × h³ ÷ 12 = h³
Step 5: δ_max = (5 × w × L⁴) ÷ (384 × Ec × Ig) [simply-supported, UDL]
Step 6: ACI 318 limits — L/360 (live load floors), L/480 (sensitive partitions), L/240 (roofs)
Note: Uses gross section Ig (conservative). For cracked sections, apply Ie per ACI 318 §24.2.3.
How to Use This Concrete Slab Deflection Calculator
Identify your slab span and boundary conditions.
Measure the clear span between support faces — not centerline to centerline. This calculator uses a simply-supported boundary condition, which is the most conservative and the most common assumption for one-way slabs in typical residential and light commercial construction. If your slab is continuous over multiple spans or has fixed ends, the actual deflection will be lower than what this tool calculates.
Enter the slab thickness (overall depth h).
Use the overall depth from the top surface to the bottom, not the depth to the rebar centroid. The calculator uses the gross moment of inertia (Ig) based on the full cross-section, which is the conservative approach prescribed by ACI 318 for initial design checks. A cracked-section analysis will yield lower Ig and higher deflection — consult a structural engineer for final verification on critical members.
Enter concrete compressive strength and applied load.
Use the specified 28-day strength (f'c) from your mix design. For the load, enter the live load only if you're checking against the L/360 or L/480 limit. Enter the total (dead + live) load if checking against L/240 for roof slabs or non-sensitive applications. Mixing load types with the wrong limit is one of the most common errors when doing deflection checks by hand.
Read the compliance results and act on them.
A green status means your slab satisfies that ACI 318 limit. A red status means it does not — you'll need to increase thickness, reduce span, increase concrete strength, or reduce the load. The span-to-deflection ratio in the primary result is the quickest way to compare against any custom limit your project specifications may require. Always have results reviewed by a licensed structural engineer before finalizing a structural design.
⚠ Pro Tip: The L/360 limit in ACI 318 Table 24.2.2 applies to live-load deflection only, not total deflection. If you run this calculator with the total load and compare to L/360, you will incorrectly flag compliant slabs as failures. Separate your live and total loads and use the right limit for each check.
Concrete Slab Deflection Formula
This calculator uses the elastic beam deflection formula for a simply-supported member under uniformly distributed load, applied to a one-foot-wide strip of the slab. The modulus of elasticity is computed per ACI 318-19 §19.2.2.1 using normal-weight concrete (density 145 pcf).
Step
Formula
Example (L=20 ft, h=6 in, f'c=3000 psi, w=100 psf)
1. Convert span to inches
L (in) = L (ft) × 12
20 × 12 = 240 in
2. Load per inch of strip width
w (lb/in) = w (psf) ÷ 12
100 ÷ 12 = 8.333 lb/in
3. Modulus of elasticity
Ec = 57,000 × √f'c
57,000 × √3000 = 3,122,019 psi
4. Gross moment of inertia
Ig = 12 × h³ ÷ 12 = h³
6³ = 216 in⁴ per ft width
5. Max deflection (midspan)
δ = 5wL⁴ ÷ (384 × Ec × Ig)
5×8.333×240⁴ ÷ (384×3,122,019×216) = 0.534 in
6. ACI 318 L/360 limit
δ_allow = L (in) ÷ 360
240 ÷ 360 = 0.667 in → PASS
Common Slab Deflection Reference Table
Calculated midspan deflection for common slab configurations (normal-weight concrete, 100 psf uniform load). Green = passes L/360; Red = exceeds L/360.
Span
Thickness
f'c (psi)
Deflection (in)
L/360 Limit (in)
Status
10 ft
4 in
3,000
0.045
0.333
✓ PASS
12 ft
5 in
3,000
0.060
0.400
✓ PASS
15 ft
6 in
3,500
0.075
0.500
✓ PASS
20 ft
6 in
3,000
0.534
0.667
✓ PASS
20 ft
8 in
4,000
0.195
0.667
✓ PASS
25 ft
8 in
4,000
0.476
0.833
✓ PASS
30 ft
10 in
5,000
0.452
1.000
✓ PASS
25 ft
6 in
3,000
2.083
0.833
✗ FAIL
30 ft
8 in
3,000
2.292
1.000
✗ FAIL
All values computed using gross section Ig and 100 psf uniform load. Actual deflection with cracked section Ie will be higher.
Which ACI 318 Deflection Limit Applies to Your Slab?
ACI 318 Table 24.2.2 sets maximum computed deflection limits based on the member type, the load considered, and what the slab is supporting below it. Using the wrong limit for your situation is the single most common mistake in deflection checking.
ACI 318-19 Table 24.2.2 — Maximum permissible calculated deflections.
Member Type
Condition
Load Considered
Limit
Flat roofs
Not supporting or attached to non-structural elements likely to be damaged by deflection
Live load only
L/180
Floors
Not supporting or attached to non-structural elements likely to be damaged by deflection
Live load only
L/360
Roofs or floors
Supporting or attached to non-structural elements likely to be damaged by deflection
Total load that occurs after attachment of non-structural elements
L/480
Roofs or floors
Supporting or attached to non-structural elements NOT likely to be damaged by deflection
Total load that occurs after attachment of non-structural elements
L/240
The most commonly cited limit — L/360 — is the floor live-load limit, not a universal standard. It does not account for long-term creep deflection. ACI 318 §24.2.4 requires an additional check for long-term deflection (due to shrinkage and creep) using a multiplier λΔ. This calculator gives you the elastic check only. For slabs supporting brittle finishes like tile or stone, the long-term check is often the controlling one.
5 Common Mistakes in Concrete Slab Deflection Checks
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Using total load against the L/360 limit.
L/360 in ACI 318 Table 24.2.2 is explicitly for live load only on floors not supporting damage-sensitive elements. Checking total (dead + live) load against L/360 makes the check artificially conservative and will cause you to oversize the slab for no structural reason. Use L/240 or L/480 for total load checks, depending on what the slab supports.
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Ignoring long-term (creep and shrinkage) deflection.
The elastic deflection this calculator produces is just the instantaneous result. Over time, concrete creeps under sustained load and shrinks as it cures. ACI 318 §24.2.4 applies a multiplier of 1.2–2.0 to long-term deflections depending on the reinforcement ratio. For slabs supporting tile, stone, rigid partitions, or elevator equipment, the long-term check routinely controls over the elastic check.
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Using gross section Ig when the slab is cracked.
This calculator uses gross section Ig, which is correct for uncracked concrete. Once the applied moment exceeds the cracking moment, the slab cracks and the effective moment of inertia (Ie) drops — sometimes to 40–60% of Ig. If your slab has significant loading relative to its thickness, a cracked-section analysis per ACI 318 §24.2.3 will give a more accurate (and typically higher) deflection result.
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Applying a one-way formula to a two-way slab.
This calculator solves a one-way slab strip. If your slab spans in two directions (aspect ratio shorter side to longer side > 0.5), two-way plate bending behavior kicks in and the actual deflection will be less than what a one-way strip analysis shows — but the calculation is far more involved. Applying the beam formula to a two-way slab is conservative, but using it to claim a two-way slab passes the limit when a one-way slab doesn't is incorrect.
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Checking deflection at the wrong load stage.
ACI 318 requires the deflection check to be performed under the load that occurs after the non-structural elements are attached — not at the time of pour, and not at ultimate load. Running the check under the total factored load (1.2D + 1.6L) instead of the unfactored service load produces a misleading result and has no basis in ACI 318 deflection provisions.
Frequently Asked Questions
ACI 318 Table 24.2.2 sets four limits depending on member type and loading condition. For floors not supporting damage-sensitive elements: L/360 for live load. For members supporting brittle partitions, cladding, or tile: L/480 for the incremental load occurring after attachment. For members supporting non-sensitive elements: L/240. For flat roofs not supporting sensitive elements: L/180. The most commonly cited limit — L/360 — applies to live load on typical floor slabs. Using the wrong limit for your situation leads to either over-design or a non-compliant slab.
The calculator applies the standard elastic beam deflection formula for a simply-supported member under uniformly distributed load: δ = 5wL⁴ / (384EI). It analyzes a one-foot-wide strip of the slab. The modulus of elasticity is computed as Ec = 57,000√f'c per ACI 318-19 §19.2.2.1, which applies to normal-weight concrete (145 pcf). The moment of inertia is the gross uncracked section, Ig = bh³/12, where b = 12 inches per unit strip width. This is the appropriate conservative approach for initial design and quick compliance screening.
Elastic (instantaneous) deflection is what happens immediately when load is applied — it's what this calculator computes. Long-term deflection accounts for concrete creep under sustained load and drying shrinkage over months and years. ACI 318 §24.2.4 requires adding long-term deflection to the instantaneous value for the sustained-load portion. The long-term multiplier λΔ ranges from 1.0 to 2.0 depending on the compression reinforcement ratio. For slabs supporting brittle finishes like ceramic tile, long-term total deflection routinely governs design and is often 2–3x the elastic value alone.
Because the moment of inertia (Ig) is proportional to the cube of the thickness (h³). Double the thickness and Ig increases by a factor of 8, cutting deflection to one-eighth. This is why going from a 6-inch slab to an 8-inch slab drastically reduces deflection. Span length has an even stronger effect — it appears to the fourth power (L⁴) in the deflection formula. A small increase in span can easily overwhelm the benefit of a thicker slab. In practice, the fastest way to control deflection is always to reduce the span first, then increase thickness second.
Tension rebar does not significantly change the pre-cracking elastic deflection (which this calculator computes using gross Ig). However, once the slab cracks, rebar governs post-cracking stiffness through the effective moment of inertia (Ie). Compression reinforcement (top bars) has a direct effect on long-term creep deflection — it reduces the ACI 318 long-term multiplier λΔ from 2.0 (with no compression steel) down to 1.2 (with ρ' ≥ 0.01). Adding top bars at midspan is one of the most cost-effective ways to limit long-term deflection in slabs supporting sensitive finishes.
This calculator is specifically for one-way slab strips. For two-way slabs (aspect ratio of short span to long span greater than 0.5), plate bending distributes load in both directions and actual midspan deflection is significantly less than the one-way strip analysis predicts. Using this tool for a two-way slab gives a conservative result (over-predicts deflection), which is acceptable for preliminary screening. For accurate results on two-way slabs, use plate-theory methods or finite element analysis, or refer to the ACI 318 waffle slab and flat plate guidelines.
In order of typical effectiveness and cost: (1) Reduce the span — even a small reduction has a massive impact because deflection scales with L⁴. (2) Increase slab thickness — the most common fix, with deflection scaling with 1/h³. Going from 6 to 8 inches cuts deflection by more than half. (3) Increase concrete strength — f'c affects Ec, but only as a square root, so doubling f'c reduces deflection by just 29%. It's the least effective lever. (4) Use prestressed (post-tensioned) concrete — creates an upward camber that counteracts applied-load deflection. Common in parking structures and long-span commercial slabs. (5) Recheck the boundary conditions — a continuous slab has significantly less deflection than a simply-supported one. If the real slab is continuous, the simply-supported analysis you ran is over-conservative.
ACI 318 Table 7.3.1.1 provides minimum slab thickness values that allow the engineer to skip the deflection calculation, provided the slab is not supporting or attached to partitions or other construction that would be damaged by large deflections. For one-way slabs with one end continuous, the minimum is L/24 of span. For simply-supported slabs, L/20. For cantilevers, L/10. These limits apply when normal-weight concrete and Grade 60 rebar are used — there are correction factors for other densities or rebar grades. The span-over-depth ratios are conservative and frequently lead to thicker slabs than a deflection check would actually require, so the check is worthwhile when span or load pushes the boundaries.
Stronger concrete has a higher modulus of elasticity (Ec), which means it deforms less under the same load. In the ACI formula, Ec = 57,000√f'c. Increasing f'c from 3,000 to 4,000 psi improves Ec by about 15%, reducing deflection by roughly 13%. From 3,000 to 5,000 psi gives a 29% improvement. This is a real but modest effect — it takes a 4x increase in f'c to cut deflection in half. In contrast, a 26% increase in slab thickness (say 6 to 7.5 inches) achieves the same reduction through the h³ relationship. Specifying higher-strength concrete is rarely the most efficient path to deflection control.
They're related but opposite concepts. Deflection is the downward displacement of a structural member under load. Camber is an intentional upward bow built into a member before loading, designed to offset anticipated deflection so that the member is approximately flat under the design load. Prestressed concrete slabs and precast members are often cambered. In cast-in-place concrete, camber is not typically used — deflection is controlled by choosing adequate thickness and span. The net deflection under service loads is what ACI 318 limits regulate.
