Quick rule-of-thumb starting dimensions to kick off a concept design — then run a full check.
Preliminary sizing only — always follow with a full design. These rules of thumb give a starting member size for concept and estimating; they do not check strength, deflection, shear or detailing. Take the suggested size into Section Design for a code-compliant design before issuing anything.
How the Preliminary Sizing Calculator works
At concept stage you need a starting size for each member before any analysis. This calculator gives one for five elements using common thumb rules tied to IS 456:2000: beams, one-way slabs, two-way slabs, square columns and square isolated footings.
Beam depth is taken as span/12 when simply supported and span/15 when continuous, rounded up to 25 mm, with width equal to half the depth (rounded up to 25 mm, at least 230 mm). One-way slabs take the Cl 23.2.1 basic ratio (20 or 26) on the effective depth with an assumed modification factor of 1.4, plus 25 mm for cover, rounded up to 10 mm with a minimum of 100 mm. For cantilevers the basic ratio of 7 from Cl 23.2.1 is applied to the effective depth, and 50 mm (beam) or 25 mm (slab) is added for cover and bars. Two-way slabs with a shorter span up to 3.5 m use Cl 24.1 note 2 (span/thickness of 35 or 40, multiplied by 0.8 for Fe 415); longer spans use the Cl 23.2.1 basic ratio with an assumed modification factor of 1.4.
Columns are sized from the service axial load P using the short axially loaded column formula of Cl 39.3 with a load factor of 1.5, 1 % steel and Fe 415, rounded up to the next 25 mm (minimum 230 mm). Footings are sized as 1.1P divided by the safe bearing capacity, rounded up to the next 0.05 m. These are starting sizes only: no strength, deflection, shear or detailing check is made.
- Beam depth
- D ≈ L / 12 (simply supported), L / 15 (continuous); b ≈ D / 2, b ≥ 230 mm
- One-way slab thickness
- d ≈ L / (20 × 1.4) or L / (26 × 1.4); t = d + 25 mm, t ≥ 100 mm
- Cantilever
- d ≈ L / 7; D = d + 50 mm (beam), t = d + 25 mm (slab)
- Two-way slab, Lx ≤ 3.5 m
- t ≈ Lx / 28 (simply supported), Lx / 32 (continuous)
- Two-way slab, Lx > 3.5 m
- d ≈ Lx / (20 × 1.4) or Lx / (26 × 1.4); t = d + 25 mm
- Column area
- Ag = 1.5P / (0.4 fck (1 − p) + 0.67 fy p), p = 0.01, fy = 415
- Footing area
- A ≈ 1.1P / q
Follows: IS 456:2000 Cl 23.2.1 (basic span/effective depth ratios 7, 20, 26) · IS 456:2000 Cl 24.1 note 2 (two-way slabs up to 3.5 m) · IS 456:2000 Cl 39.3 (short axially loaded columns)
Worked example: starting size of a square column carrying 1200 kN
Given
- Service axial load P = 1200 kN
- Concrete M25, fck = 25 N/mm²
- Assumed by the tool: 1 % longitudinal steel (p = 0.01), Fe 415
Steps
- Factored load: Pu = 1.5 × 1200 = 1800 kN.
- Capacity per unit area: 0.4 × 25 × (1 − 0.01) + 0.67 × 415 × 0.01 = 9.900 + 2.781 = 12.681 N/mm².
- Gross area: Ag = 1,800,000 / 12.681 = 141,950 mm².
- Side of a square column: √141,950 = 377 mm.
- Round up to the next 25 mm: 400 mm, which is above the 230 mm minimum.
Result: Start with a 400 × 400 mm column.
Enter the same values in the calculator above to check each step.
Frequently asked questions
What is the thumb rule for beam depth?
A common starting point is overall depth = span/12 for a simply supported beam and span/15 for a continuous one, with width about half the depth. For a 6 m span this gives 250 × 500 mm simply supported and 230 × 400 mm continuous.
What is the span to depth ratio as per IS 456?
IS 456 Cl 23.2.1 gives basic span to effective depth ratios of 7 for cantilevers, 20 for simply supported and 26 for continuous members, for spans up to 10 m. These apply to the effective depth d, not the overall depth, and are then modified for the tension and compression steel provided.
How thick should a two-way slab be as per IS 456?
For two-way slabs with a shorter span up to 3.5 m and live load up to 3 kN/m², Cl 24.1 note 2 allows span/overall depth of 35 (simply supported) or 40 (continuous), multiplied by 0.8 for Fe 415. A continuous slab with a 3.5 m short span therefore needs about 3500/32 = 109 mm, rounded to 110 mm. For a 4.5 m continuous slab the tool gives 150 mm.
How do I find footing size from column load and SBC?
Divide 1.1 times the service load by the safe bearing capacity to get the plan area, then take the square root for a square footing. For 1200 kN on soil with an SBC of 150 kN/m², A = 1.1 × 1200 / 150 = 8.80 m², √8.80 = 2.97 m, so start with 3.00 × 3.00 m.
Can I use these preliminary sizes in the final design?
No. The sizes are starting points only; the tool does not check flexure, shear, deflection, punching, slenderness, biaxial bending or detailing. The column rule assumes a short, axially loaded column with 1 % Fe 415 steel, so columns with significant moment or slenderness will need a larger section once designed.
Step-by-step guides
- Thumb Rules for Column, Beam and Slab Size (RCC Preliminary Design)Starting sizes for RCC beams, slabs, columns and footings from IS 456-based thumb rules, with a worked house-frame example and a ready table. Not a design.
- Bihar Building Bye-laws: Setback & FAR Rules — Patna ExampleBihar building bye-laws setback and FAR rules from the 2014 bye-laws as amended in 2022 and 2025: Tables 8, 9, 15 and 16, the 1.5 × (road + setback) height rule, the 800 m² high-rise limit and Table 19 parking, worked for a 12 × 18 m plot.
- RCC Building Estimate: Concrete, Steel & Cost — ExampleRCC building estimate calculation step by step: concrete volume, shuttering, bar bending schedule cutting lengths, steel kg (d²/162), cement bags, excavation and a priced BOQ, worked for a G+1 frame with every number checked.
Related tools
- Section DesignDesign the beam or column properly from this starting size.
- Slab DesignCheck the slab thickness for bending, shear and deflection.
- Foundation DesignDesign the footing for bearing, punching and reinforcement.
- Load Combination GeneratorList the factored combinations to check the members against.