How to Estimate Concrete Quantities From Your Phone: A Field Guide for Contractors

Most concrete takeoffs still get done twice, once with a tape and a notepad at the jobsite, and again that night at a kitchen table with a calculator. Here is the math behind each phase, the waste factors that keep you from coming up short, and how to run the whole thing from a phone before you leave the site.

Why mobile takeoffs matter

The gap between walking a job and sending a number is where bids get lost. A general contractor asks three subs for a price. The one who answers in a day usually gets the call back. The one who answers in a week is bidding against a decision that has already been made.

The work itself has not changed. Slabs are still length times width times thickness. What changed is that you no longer need to be sitting at a desk to do it. Every calculation below can be done standing on the pad with your phone in one hand.

Slabs: getting to cubic yards

Concrete is sold by the cubic yard, and a cubic yard is 27 cubic feet. That is the one number to memorize.

Take your length in feet, times your width in feet, times your thickness in feet, divided by 27. Thickness is where most mistakes happen, because slabs get specified in inches. A 4 inch slab is 0.333 feet. A 5 inch slab is 0.417 feet. A 6 inch slab is 0.5 feet.

A 40 by 60 slab at 4 inches: 40 x 60 x 0.333 = 799 cubic feet, divided by 27 = 29.6 cubic yards.

Now add waste. Subgrade is never perfectly flat, forms bow, and the last truck is the one you do not want to come up short on. Most contractors carry 5 to 10 percent on a slab. On that same 40 by 60, 8 percent puts you at 32 yards, which is what you would actually order.

Thickened edges and turndowns get calculated separately and added. A 12 inch wide by 12 inch deep turndown around that slab perimeter, 200 linear feet, is 200 x 1 x 1 divided by 27, or 7.4 yards before waste.

Footings: linear feet drives everything

Footings are the phase where quantities multiply. One footing run produces concrete, rebar, chairs, and sometimes stirrups and dowels, and they all key off the same linear footage.

Concrete works the same way as a slab: width in feet times depth in feet times length in feet, divided by 27. A 20 inch wide by 10 inch deep footing running 240 linear feet is 1.667 x 0.833 x 240 = 333 cubic feet, or 12.3 cubic yards.

Rebar is where estimates drift. If that footing carries three number 5 bars continuous, you have 720 linear feet of bar before laps. Laps are not optional and they are not small. A common lap is 48 bar diameters, so a number 5 bar, which is 0.625 inches, laps at 30 inches, two and a half feet added at every splice. With 20 foot stock you are splicing roughly every 20 feet, which adds about 12 percent to your total bar length. Leaving laps out of a rebar takeoff is one of the most common ways a concrete bid comes in short.

Corner bars, dowels and hooks get counted on top of that. A hooked dowel is not just its vertical length, the hook adds bend length that has to be in the total.

Stem walls and block: count the block, then the grout

Masonry stem walls key off wall area. Take wall length times wall height to get square feet, then divide by the face area of the block. A standard 8 by 8 by 16 block covers 0.888 square feet of wall face, which works out to about 1.125 blocks per square foot. A 100 foot wall at 4 feet tall is 400 square feet, or roughly 450 blocks before waste. Add 3 to 5 percent for breakage.

Mortar runs off the raw block count, not the square footage. Filled cells and bond beams are separate grout quantities, and this is where double counting creeps in: if your vertical filled cells run the full wall height and your bond beam sits in the top course, you are paying for the same grout twice unless the vertical cell height stops at the bottom of the bond beam.

Ladder wire, or joint reinforcement, is calculated by the course. Laid every other course on a 4 foot wall of 8 inch block, that is six courses, so three runs of wire at the wall length each.

Columns and piers

Columns are straightforward volume but easy to miscount. A square column is width times depth times height divided by 27. A round column is pi times the radius squared times the height, divided by 27. An 18 inch diameter column 10 feet tall is 3.1416 x 0.75 x 0.75 x 10 = 17.7 cubic feet, or 0.65 cubic yards. The mistake is almost never the math. It is missing four columns on sheet S-3 because they are drawn at a different scale than the ones on S-2.

The waste factors worth arguing about

Waste is not padding. It is the difference between what the drawing says and what the truck has to deliver.

Ready mix concrete: 5 to 10 percent on slabs, more on rough subgrade or hand dug footings. Rebar: laps first, then 3 to 5 percent on top for cutting. Block: 3 to 5 percent for breakage and cuts. Grout: 10 percent, because cells are never as clean as the section detail.

Carry them consistently. An estimator who uses 5 percent on one job and 15 percent on the next has no way to learn from what the job actually cost.

From quantities to a number a GC can accept

Quantities are half the job. A bid is quantities times unit pricing, split across material, labor, equipment and subcontract, with your markup applied on top.

Material is the easiest: yards times your ready mix price, tons of steel times your supplier price, block times unit cost. Labor is where experience shows, crew size times day rate divided by production, or a straight unit rate per yard placed or per block laid if you have the history to back it. Equipment covers the pump, the laser screed, the forms. Subcontract is anything you are buying complete, like a pump truck or a finishing crew.

Then markup. The number you hand the GC has to carry your overhead and your profit, and it has to be arrived at the same way every time, so that when a job goes sideways you can find out where.

Sending the proposal before you leave

The last step is the one that actually wins the work. A takeoff that lives in a notepad is not a bid. A proposal is a document with your scope, your inclusions and exclusions, your number and your name on it.

Doing that from a phone is the whole point of a mobile estimating workflow: walk the job, enter the dimensions on the pad, let the calculators return the quantities, apply your saved unit pricing, and send a PDF proposal from the jobsite parking lot.

That is what we built Cornerstone Bids to do. It covers footings, slabs, stem walls, block walls, columns, stairs and more, each phase with its own calculator, feeding one cost summary broken out by material, labor, equipment and subcontract, and a polished PDF proposal at the end. You set your own unit pricing once and it carries to every project. It is free to try, on iPhone, Android and the web at cornerstonebids.com.

Quick reference

Cubic yard = 27 cubic feet. 4 inch = 0.333 ft, 5 inch = 0.417 ft, 6 inch = 0.5 ft, 8 inch = 0.667 ft, 10 inch = 0.833 ft, 12 inch = 1.0 ft. Slab yards = length x width x thickness in feet, divided by 27. Round column cubic feet = 3.1416 x radius x radius x height. Blocks = wall square feet x 1.125. Number 5 bar lap at 48 diameters = 30 inches.