Beam Deflection Check (Span / Depth)
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Enter the support type, span and effective depth, and get the span-to-depth ratio against the IS 456 limit — the deemed-to-satisfy check that controls deflection without calculating it.
Beam Deflection Check — IS 456 span/depth
Allowable L/d = basic (7 / 20 / 26) × tension-steel modification factor
Simply supported
L/d 12.5 vs allowable 28
IS 456 Fig 4 — ~1.4 for pt≈0.5% Fe415, up to ~2.0 for lightly-stressed steel
Span / depth ratio
L/d = 12.5
allowable 20 × 1.4 = 28 · deflection OK ✓
Actual L/d
12.5
Allowable L/d
28
Min depth
179 mm
Basic (no mod.)
250 mm
Basic span/depth ratios (IS 456 cl 23.2.1)
Cantilever
7
d ≥ 714 mm
Simply supported
20
d ≥ 250 mm
Continuous
26
d ≥ 192 mm
This is the deemed-to-satisfy check, not a deflection value. A basic L/d of 20 before modification means a bare simply supported beam needs d ≥ 250 mm; the tension-steel factor from IS 456 Fig 4 (here 1.4) relaxes that to 179 mm. Spans over 10 m need a further span/10 factor, and a real deflection calculation governs where the check is close.
How it works (IS 456, cl 23.2.1)
Instead of computing a deflection, IS 456 lets you keep the span / effective-depth ratio below a limit:
- Basic L/d = 7 cantilever, 20 simply supported, 26 continuous (for spans up to 10 m).
- Multiply by a modification factor for the tension steel (IS 456 Fig 4) — about 1.4 for 0.5% steel of Fe415, rising toward 2.0 for lightly-stressed steel.
- If the actual L/d ≤ basic × modification factor, deflection is deemed satisfied.
Worked example
A 5 m simply supported beam:
- Basic L/d = 20, so before modification d ≥ 5,000 ÷ 20 = 250 mm
- With a modification factor of 1.4, the limit becomes L/d = 28, so d ≥ 5,000 ÷ 28 = 179 mm
- A 400 mm effective depth gives L/d = 12.5 — well within 28 ✓
Basic span/depth ratios
| Support | Basic L/d |
|---|---|
| Cantilever | 7 |
| Simply supported | 20 |
| Continuous | 26 |
Frequently asked questions
What is the span to depth ratio for a beam? For deflection control, keep L/d below 7 (cantilever), 20 (simply supported) or 26 (continuous), multiplied by the tension-steel modification factor. A simply supported beam with ~0.5% Fe415 steel can reach about L/d = 28.
What is the minimum depth of a beam? Roughly span ÷ (basic ratio × modification factor). A 5 m simply supported beam needs about 179 mm effective depth for deflection; add cover and half the bar diameter for the overall depth.
What is the modification factor in IS 456? A factor from Fig 4 that increases the allowable span/depth ratio based on the tension-steel percentage and its stress — lightly-stressed, lightly-reinforced beams deflect less, so they are allowed a higher L/d.
Does passing this mean the beam won't deflect? It means deflection is within the serviceability limit without a separate calculation. Where the check is close, or for long or heavily-loaded spans, calculate the actual deflection.
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CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.