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About Joist Deflection Calculator
Calculate midspan bending deflection, moment of inertia, tributary line load, and span-to-deflection ratios for simply supported floor joists.
Inputs and calculations stay in your browser and require no signup.
How the calculation works
Planning calculation
Tributary line load w = uniform load (psf) × spacing (ft); Moment of inertia I = b × d³ ÷ 12; Maximum deflection delta = 5wL⁴ ÷ (384EI); Allowable limit = L ÷ 360; Ratio = L ÷ delta.
How to use this calculator
- 1Enter measured project dimensions and quantities.
- 2Adjust material, spacing, waste or density assumptions to match the project.
- 3Review the live planning quantities.
- 4Round purchasing quantities appropriately and confirm field conditions.
Understanding the result
Higher span-to-deflection ratios (e.g. L/480 or L/720) indicate stiffer floors with less noticeable bounce, vibration, and finish cracking.
Results use the units shown beside each field and output.
Important limitations
- Educational engineering estimate for simply supported, prismatic rectangular members with static uniform loads.
- Does not evaluate shear deformation, vibration/bounce dynamics, composite subfloor action, duration of load factors, holes, notches, or connection stiffness.
- Does not certify structural safety or building code compliance; consult a licensed structural engineer for actual building design.
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Frequently asked questions
What is the L/360 deflection standard?
L/360 is the traditional building code deflection limit for floor live loads (span in inches divided by 360), designed to prevent plaster cracking and excessive sag.
Should I use nominal or actual lumber dimensions?
Always use actual dimensions (e.g. 1.5 inches by 9.25 inches for a nominal 2x10 joist).
What modulus of elasticity (E) should I use?
Standard dimension framing lumber (such as No. 2 Douglas Fir or Southern Pine) typically has an E between 1,400,000 psi and 1,700,000 psi. Engineered lumber (LVL) is often 1,900,000 to 2,000,000 psi.
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