Calculator inputs
Enter slab geometry, loads, and material values. Results appear above this form after submission.
Example data table
Use this sample to understand the calculator fields and expected results.
| Case | lx (m) | ly (m) | Thickness (mm) | DL (kN/m²) | LL (kN/m²) | fck (MPa) | fy (MPa) | Main Bar | Typical Output |
|---|---|---|---|---|---|---|---|---|---|
| Office slab strip | 4.00 | 8.00 | 150 | 2.00 | 3.00 | 25 | 500 | 12 mm | Moderate steel demand with one-way action |
| Residential slab strip | 3.50 | 7.50 | 130 | 1.50 | 2.00 | 20 | 415 | 10 mm | Lighter reinforcement and lower moment |
| Storage slab strip | 4.50 | 9.50 | 180 | 2.50 | 5.00 | 30 | 500 | 12 mm | Higher steel need with stronger section |
Formula used
1) Self weight
Self Weight = Thickness × Concrete Unit Weight
Thickness is converted from millimetres to metres.
2) Factored load
Service Load = Self Weight + Dead Load + Live Load
Factored Load = Service Load × Load Factor
3) Effective depth and span
Effective Depth, d = Overall Thickness − Cover − Main Bar Diameter ÷ 2
Effective Span = lesser of (clear span + d) and (clear span + support width)
4) Ultimate moment
Simply Supported: Mu = wuL² ÷ 8
Continuous: Mu = wuL² ÷ 12
All moments are calculated per metre width strip.
5) Steel area
Mu = 0.87fyAs[d − 0.42xu]
xu = 0.87fyAs ÷ (0.36fckb)
The tool solves this relation and compares it with minimum steel limits.
6) Bar spacing
Spacing = 1000 × Bar Area ÷ Required Steel Area
Main bar spacing is limited to the lesser of 3d and 300 mm.
Distribution spacing is limited to the lesser of 5d and 450 mm.
7) Basic serviceability and shear checks
Basic span-depth ratio is compared with simple code-style limits.
Nominal shear stress = Vu ÷ (b × d)
How to use this calculator
Enter the short span and long span of the slab panel. Keep the long span equal to or greater than the short span.
Choose the support condition. Use simply supported for isolated spans and continuous for slabs extending over several supports.
Fill in slab thickness, cover, and support width. These values affect effective depth and effective span.
Enter superimposed dead load and live load. The calculator automatically adds slab self weight from thickness and concrete density.
Select concrete and steel grades. Then choose main and distribution bar diameters for practical spacing recommendations.
Press the calculate button. The results appear above the form with steel area, spacing, moment, shear, capacity, and a graph.
Use the CSV and PDF buttons to save the design summary. Review the graph to see how thickness affects steel demand.
Always confirm the final design with local code requirements, detailed load combinations, support fixity, crack control, and project-specific detailing rules.
FAQs
1) What type of slab does this calculator suit best?
It is best for preliminary one-way slab design per metre strip. When the long-to-short span ratio is below two, the tool warns that two-way slab behavior may govern final design.
2) Why does the calculator ask for both short and long span?
Both spans help identify likely slab action. The ratio ly/lx indicates whether the slab mainly behaves one-way or may need a two-way coefficient method for final engineering design.
3) Does the tool include slab self weight automatically?
Yes. The calculator computes self weight from slab thickness and concrete unit weight, then adds that value to superimposed dead load and live load.
4) How is required steel calculated?
It uses a reinforced concrete limit-state flexural relation for a one-metre strip. The tool also checks minimum reinforcement, so the final required area is never lower than code-style minimum steel.
5) Why do spacing values change when I change bar diameter?
Larger bars provide more area per bar. That usually allows wider spacing for the same required steel area, though maximum spacing limits still control the final recommendation.
6) Is the deflection check final?
No. It is a basic span-depth screening check. Final deflection evaluation should include code modification factors, support effects, reinforcement percentage, cracking behavior, and long-term serviceability.
7) Does passing the shear check mean the design is complete?
No. The shear result is a preliminary indicator only. Final slab design should still verify punching shear where relevant, local code provisions, openings, concentrated loads, and detailing around supports.
8) Can I use this output directly for construction drawings?
Use it as a strong starting point, not a final sealed design. Construction drawings need full engineering review, detailing, bar curtailment rules, code compliance, and project-specific loading checks.