Thumb Rule is the method used for quick and approximate calculations by multiplying the desired entities (i.e. area, volume etc.) with some constant values derived from our regular jobs and previous experience etc. Many times, we deal with such jobs, which need instant calculations in order to be more productive. Hence, it becomes more important for professional involved in construction and estimation works to learn about basic Thumb Rules.
One can create their own set of Thumb Rules to make their calculations faster. However, always remember that these calculations are not accurate.
What are the most common used thumb rules in construction?
Followings are some Thumb Rule constants;
- Number of bricks(9″x4″x3″) required per square meter in 4″ partition wall = 57nos/m2
- Number of bricks(9″x4″x3″) required per cubic meter in 9″ wall = 498nos/m3
- Number of Concrete Blocks(16″x8″x4″) required per square meter = 12.5nos/m2
- Number of Cement bags required per square meter in;
- 4″ brick work, ratio 1:4: 0.252bags/m2
- 9″ brick work, ratio 1:4: 0.428bags/m2
Thumb Rules for Plastering
|#||Area of Plastering||Ratio of Mortar||Plastering Thickness (mm)||Cement (bags)||Sand (cum)|
Thumb Rule for steel rebar in;
- Footings: 0.8-1% of the concrete volume x Density of steel (7850kg/m3)
- Floor Slab: 1-1.5% of the concrete volume x Density of steel (7850kg/m3)
- Columns: 2-3% of the concrete volume x Density of steel (7850kg/m3)
- Beams: 1.5-2% of the concrete volume x Density of steel (7850kg/m3)
or use Quantity of steel rebar in Kg/m3 of concrete
- Footings: 50-60 kg/m3
- Floor Slab: 80 kg/m3
- Columns: 150-200kg/m3
- Beams: 120kg/m3
Thumb Rules for Stripping of Formwork
|1||Sides of foundation, beams, columns and walls||2 days|
|2||Sides of span larger than 4.5m||14 days|
|3||Sides of span shorter than 4.5m||7 days|
|4||Sides of beams and arches upto 6m span||14 days|
|5||Sides of beams and arches from 6m to 9m span||21 days|
|6||Sides of beams and arches above 9m span||28 days|
Formwork Estimation Thumb Rules
Estimating formwork requires a more detailed approach and is typically based on the specific shape, size, and complexity of the formwork required for a particular structure. However, here is a general thumb rule that can be used as a rough estimate for formwork:
1. Wall Formwork: For wall formwork, a general thumb rule is to assume approximately 1.2 to 1.5 square feet of formwork per square foot of wall surface area. This estimate includes both the vertical and horizontal formwork surfaces.
2. Slab Formwork: For slab formwork, a general thumb rule is to assume approximately 2.5 to 3 square feet of formwork per square foot of slab area. This estimate includes the formwork required for both the top and bottom surfaces of the slab.
3. Column Formwork: For column formwork, a general thumb rule is to assume approximately 0.15 to 0.2 square feet of formwork per square foot of column cross-sectional area. This estimate includes the formwork required for all sides of the column.
4. Beam Formwork: For beam formwork, a general thumb rule is to assume approximately 0.5 to 0.7 square feet of formwork per linear foot of beam. This estimate includes the formwork required for both the top and bottom surfaces of the beam.
It’s important to note that these thumb rules are rough estimates and can vary depending on the specific project requirements, complexity of the formwork, and other factors. It’s always recommended to consult with a qualified engineer or experienced contractor to get more accurate and detailed estimates for formwork.