Calculating the rebar involved in a construction project is done to provide a means to be able to bill for labour and materials. The overall weight of the materials is typically how the project price is determined and placed under contract. While much rides on this calculation, the formula is very easy to learn and perform. Key to your success will be your attention to detail to ensure you have accurate numbers to use.
Select one simple section, such as a basic rectangular slab, of your building plans to practice the formula. Once you learn and practice the formula, you will be able to use it for any section of your building that contains rebar.
Subtract 0.5 from the overall length of your slab. This gives you your "placement length." For example, if your slab is 20 feet long, your placement length would be 19.5. The placement length assumes that the required clearance of the lengthwise bar at each end of the slab is three inches from the edge of the concrete. Clearance is the space between the end of the rebar and the edge of the concrete to ensure the rebar is encased in the concrete and not exposed.
Multiply the sum you found in Step 2 by the number of bars listed on the print details to be installed in the full length of the slab. For example, if the plans said that 42 bars must be placed down the length of the slab then 42*19.5 = 819.
Multiply the sum from Step 3 by the weight of the bar. For example, if the 42 bars are #7 bars then the weight of a #7 bar is 0.00102 tn lf or ton per linear foot. You would multiply 819 by 0.00102 to find the total of 0.83538 tn lf.
Repeat these steps substituting the width for the length and then add the total weight of the width and length together to calculate the total rebar for that particular slab.
Check the general notes in the front of your plans under the section titled "Concrete" to find out the specific clearance required of the rebar from edge of concrete. Convert that measurement to decimals and use it in your formula in place of 0.5 if it differs from the example of a three-inch clearance.
Doublecheck your calculations because they are the basis for your bid and contract amounts. An error could result in a serious loss of money.