Beam Calculator
Beam calculator works out Reaction at each support and Maximum shear from Beam and load, Units and Span or cantilever length (feet, or metres in metric). The default figures on this page give maximum deflection of 0.2069 in; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Case: Simply supported, uniform load, span 20 ft.
- Reaction = 5000 lb (22.241 kN); maximum shear the same.
- Maximum moment = 25000 lb-ft (33.895 kN m).
- Maximum deflection = 0.2069 in (5.255 mm), i.e. L/1160.
- No section modulus given, so the bending stress is not reported.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the beam calculator calculates
Computes support reactions, maximum shear, maximum bending moment, maximum deflection and the span-over-deflection ratio for a simply supported or cantilever beam under a uniform or point load, plus the bending stress and the section modulus a stated allowable stress would require.
How to use this beam calculator
Enter Beam and load, Units, Span or cantilever length (feet, or metres in metric) and Uniform load (lb per foot, or kN per metre in metric). Reaction at each support updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Reaction at each support, Maximum shear and Maximum bending moment.
The method follows AISC Steel Construction Manual, Table 3-23: Shears, Moments and Deflections, Roark’s Formulas for Stress and Strain, Table 8.1 (Warren C. Young, Richard G. Budynas) and ANSI/AISC 360-22, Specification for Structural Steel Buildings.
Simply supported 20 ft beam, 500 lb/ft, W-shape with I = 300 in^4 example
With Beam and load: simple-uniform, Span or cantilever length (feet, or metres in metric): 20, Uniform load (lb per foot, or kN per metre in metric): 500, Modulus of elasticity E (ksi, or GPa in metric): 29000, the calculator returns Maximum bending moment: 25000, Reaction at each support: 5000, Total load carried: 10000. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Reaction at each support, Maximum shear and Maximum bending moment. The inputs you can vary are Beam and load, Units, Span or cantilever length (feet, or metres in metric), Uniform load (lb per foot, or kN per metre in metric), Point load (pounds, or kilonewtons in metric) and Modulus of elasticity E (ksi, or GPa in metric).
Understanding Reaction at each support, Maximum shear and Maximum bending moment
This tool evaluates published elastic formulas for the beam and load you describe. It is NOT a beam design and does not replace a licensed engineer’s design and check. It does not apply load factors or load combinations, does not check shear, bearing, lateral-torsional buckling, web crippling, local buckling, connections, deflection limits, fire or serviceability, and takes no account of material grade, defects or duration of load. Do not size a beam, joist, header or lintel from this output.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I calculate what size beam I need?
Computes support reactions, maximum shear, maximum bending moment, maximum deflection and the span-over-deflection ratio for a simply supported or cantilever beam under a uniform or point load. Open “How this is worked out” beside the result to see each step for your own figures.
Is there a free online beam calculator?
The calculation is free and does not require an account. An account is only needed for optional features such as saving, comparing, exporting, or sharing supported scenarios.
What size beam for a 20 foot span calculator?
Computes support reactions, maximum shear, maximum bending moment, maximum deflection and the span-over-deflection ratio for a simply supported or cantilever beam under a uniform or point load. On this page it is computed from Beam and load, Units and Span or cantilever length (feet, or metres in metric) and shown as Reaction at each support, Maximum shear and Maximum bending moment.
What size beam for 25 foot span?
It needs Beam and load, Units, Span or cantilever length (feet, or metres in metric), Uniform load (lb per foot, or kN per metre in metric), Point load (pounds, or kilonewtons in metric) and Modulus of elasticity E (ksi, or GPa in metric). Nothing else is required, and the figures stay in your browser.
What information do I need for the beam calculator?
You need Beam and load, Units, Span or cantilever length (feet, or metres in metric), Uniform load (lb per foot, or kN per metre in metric), Point load (pounds, or kilonewtons in metric), Modulus of elasticity E (ksi, or GPa in metric), Moment of inertia I (in^4, or mm^4 in metric) and Section modulus S (in^3, or mm^3 in metric). The page opens with default figures that give maximum deflection of 0.2069 in, so you can see the scale before entering your own.
What does this beam calculator show?
It shows Reaction at each support, Maximum shear, Maximum bending moment, Maximum deflection, Span / deflection ratio (L/x), Total load carried, Bending stress at the maximum moment and Section modulus required at the allowable stress, with the working and any warnings for your entries.