Why does the estimated cost of concrete that corresponds to the plans tend to be underestimated at the time of pour? Concrete estimation involves three unique quantities and not just two or all of which are concrete. Cubic yards will determine the amount of ready-mix required, rebar is billed per pound, and the formwork is calculated per square feet. Overage is often caused by neglecting the rebar and formwork.
The amount of concrete is based on cubic yards: length x width x height in feet, then dividing by 27 plus waste. The rebar quantity is determined using weight; the total length of the bars with laps multiplied by weight per foot for the respective bar sizes. Formwork is quantified in square feet of contact area.
What Goes Into a Concrete Estimate?
Concrete Estimating Services combine several smaller estimates for concrete volume, reinforcement, formwork, placement, finishing, accessories, and testing. Every component, be it the footings, walls, columns, slab, or beams, receives the same treatment:
- Concrete volume. Concrete cubic yards based on mix design, because a 4,000 psi structural concrete mix and a 3,000 psi slab concrete mix have different prices.
- Reinforcement. Concrete rebar, including sizing and weights, and concrete welded wire reinforcement by area.
- Formwork. Square feet of contact area, priced by form type and number of reuses.
- Placement and finishing. Crew labor for placing, consolidation, finishing, and curing, plus pump and/or crane time.
- Accessories and testing. Chairs, ties, vapor barrier, jointing materials, embeds, and independent testing.
Take-off based on individual elements rather than total volume. Footings, walls, and slabs can require different concrete specifications even when they use the same basic material.
How Do You Calculate Cubic Yards of Concrete?
Change all measurements into feet, multiply, and divide by 27 because one cubic yard is equal to 27 cubic feet. The formula changes slightly by element:
Concrete Takeoff Services calculate each slab, footing, wall, column, and beam separately before converting the measured volume into cubic yards.
| Element | Formula | Example |
|---|---|---|
| Slab on grade | Length x width x thickness | 40 x 60 ft at 6 in. (0.5 ft) = 1,200 cu ft = 44.4 CY |
| Continuous footing | Length x width x depth | 200 ft x 2 ft x 1 ft = 400 cu ft = 14.8 CY |
| Wall | Length x height x thickness | 100 ft x 8 ft at 8 in. (0.667 ft) = 533 cu ft = 19.8 CY |
| Round column | 3.1416 x radius² x height | 18 in. diameter x 12 ft = 21.2 cu ft = 0.79 CY |
The geometric area is just the beginning. Prior to sending the quantity to the ready-mix contractor, make the adjustment for site conditions that can’t be easily reflected in the drawings:
- The geometric area is just the beginning. Prior to sending the quantity to the ready-mix contractor, make the adjustment for site conditions that can’t be easily reflected in the drawings:
- Subgrade variation. An uneven subgrade can push actual slab thickness above the drawing. Base the allowance on site conditions.
- Waste and over-order. Spillage, pump line priming, and a short final truck all add volume. Set the allowance from your own pour records rather than a flat rule.
- Openings. Subtract only large penetrations; the small ones are not worth the effort.
How Do You Estimate Rebar?
Rebar is taken off as linear feet by bar size, then converted to weight. Standard nominal weights under ASTM A615:
| Bar size | Diameter (in.) | Weight (lb per ft) |
|---|---|---|
| #3 | 0.375 | 0.376 |
| #4 | 0.500 | 0.668 |
| #5 | 0.625 | 1.043 |
| #6 | 0.750 | 1.502 |
| #7 | 0.875 | 2.044 |
| #8 | 1.000 | 2.670 |
- Count bars from spacing. For a slab mat, the number of bars in one direction = (width – 2 x cover) ÷ spacing + 1, rounding up. Then multiply by bar length, do the same for the other direction.
- Add laps and hooks. Bars longer than stock length need lap splices. Take lap length from the structural drawings or ACI 318, not a rule of thumb. Hooks and bends add length at every end. Rebar Detailing Services help confirm bar sizes, lap locations, hooks, bends, and reinforcement geometry before fabrication quantities are finalized.
- Convert to weight. Multiply total linear feet by weight per foot for each bar size.
- Add accessories. Chairs, bolsters, and tie wire are separate line items.
Example: 40 x 60 feet slab with #4 bars at 12 inches spacing both ways with 3-inch cover. The 59.5 feet bars require 41 pieces (2,439.5 feet), while the 39.5 feet bars require 61 pieces (2,409.5 feet). This equals 4,849 feet * 0.668 = 3,239 pounds excluding laps, and the long bars are longer than standard lengths, thus laps will increase the amount.
Bar counts are where takeoffs and the field most often disagree. Rebar shop drawings will provide the estimate with bar marking, lengths, and bends schedule, providing a quantity that will be cut by the fabricator.
How Is Formwork Measured?
Formwork is measured in square feet of contact area (SFCA): the concrete surface the form touches. Each element is measured differently:
- Walls. Both faces, so 2 x length x height. The 100 x 8 ft wall above needs 1,600 SFCA.
- Footings. Edge forms on both sides, unless the footing is earth-formed.
- Columns. Perimeter x height.
- Slab edges. Perimeter x slab thickness. For the 40 x 60 ft slab which has 200 ft perimeter, 200 x 0.5 = 100 SFCA.
- Elevated slabs and beams. Soffit area plus sides, with shoring and reshoring priced separately.
Formwork cost depends heavily on reuse. The cost of material for a form that will be used on multiple pourings is spread out, but for the single curved form, the material cost is borne entirely by it. Formwork labor costs are larger compared to the material cost.
What Factors Cause the Increase in the Cost of Labor?
Labor costs to lay and finish concrete vary from project to project, unlike the concrete itself. The main factors:
| Factor | Why it changes labor |
|---|---|
| Placement method | Pump, crane and bucket, or direct chute each need a different crew and equipment |
| Finish requirement | Tight floor flatness specs and burnished finishes take far longer than a broom finish |
| Pour size and sequence | Many small pours repeat setup, cleanup, and inspections |
| Access and height | Elevated decks and tight sites slow every step |
| Weather | Hot- and cold-weather requirements add blankets, heaters, retarders, or night pours |
| Curing and protection | Curing compound, wet curing, and protection before the slab can be loaded |
| Testing | Cylinder testing and special inspections required by the specification |
Footings excavation and backfill normally constitute earthwork takeoff and not the concrete takeoff. Do not confuse between the two so that no task is done twice.
What Mistakes Throw Off a Concrete Estimate?
There are some mistakes that tend to happen in concrete estimates and each is much cheaper to correct in the takeoff than in post award period:
- Pricing one mix for the whole job. Different strengths and admixtures may be needed for footings, structures, and slabs.
- Absent piers, grade beams, and edge thickening. These hide in sections and details rather than plan views.
- Leaving out laps. Laps and hooks can add significant weight on long runs and heavily reinforced elements.
- Pricing forms without a reuse plan. Form material and labor change with the pour sequence.
- Forgetting pumps and testing. Both are easy to leave off and rarely covered by the ready-mix price.
Pricing a concrete package on a tight bid schedule? Optimar Precon’s estimators produce concrete takeoffs by element, with rebar weight and formwork SFCA broken out, so every quantity can be checked before the bid goes out.
FAQs
1.23 cubic yards. This is a 10 x 10 x 0.333 feet, which is 33.3 cubic feet and 33.3/27 = 1.23 CY without waste considerations.
0.668 lb per foot under ASTM A615. For comparison, #3 weighs 0.376 lb and #5 weighs 1.043 lb per foot.
SFCA refers to square feet of contact area, which is the concrete surface that will be in contact with the formwork. SFCA is the unit used when estimating the cost of formwork.
It is used for spillage, pump priming, and subgrade variation. The exact amount will depend on the nature of the element and the construction site. The allowance should be based on past pour records and not a percentage.
Not always. Rebar may be supplied and erected by a separate subcontractor, depending on how the specification divides the work between the two materials.
