How to use the concrete beam deflection calculator
A reinforced concrete beam does not stop deflecting once the load is applied. Creep and drying shrinkage keep the sag growing for years, which is why service checks look at total deflection rather than the elastic value alone. Enter the span, sustained uniform load, section dimensions, specified strength, and compression steel ratio to get both parts at once.
Immediate deflection uses the simply supported uniform load formula d = 5wLā“/(384EcIe) with the concrete modulus Ec = 57,000āf'c in psi. Additional long-term deflection uses the ACI 318 multiplier lambda = xi / (1 + 50 rho'), where xi is 1.0 at 3 months, 1.2 at 6 months, 1.4 at 12 months, and 2.0 at 5 years or more.
For the moment of inertia you can pick the cracked value 0.35Ig from ACI 318 Table 6.6.3.1.1 or the uncracked gross section Ig. Beams that crack under service load are closer to the cracked value; a detailed check should compute the effective moment of inertia Ie from the cracking moment and the service load moment instead.
The allowable limit shown is L/240, which ACI 318 Table 24.2.2 applies to total deflection of members supporting nonstructural elements not likely to be damaged. Members carrying brittle partitions or glazing often need the tighter L/480 limit. Confirm the governing serviceability criterion with the structural engineer of record before finalizing the design.
Frequently asked questions
Concrete under sustained load keeps deforming through creep and drying shrinkage. After five years or more, a beam with no compression steel can deflect an additional 2.0 times its immediate deflection.
Yes. The ACI 318 multiplier is lambda = xi / (1 + 50 rho'), so a larger compression reinforcement ratio lowers the multiplier because the compression bars restrain creep strain.
Beams that crack under service load should use a cracked section. This tool offers the 0.35Ig value from ACI 318 Table 6.6.3.1.1 for quick checks; detailed work needs the effective moment of inertia Ie.