Construction Takeoff Example: Concrete Slab on Grade

What are we taking off?

The example is one detail off a small commercial job: a 60 ft by 40 ft slab on grade, 5 in thick, with a continuous thickened edge 16 in wide and 24 in deep measured from the top of slab. Slab steel is #4 at 18 in on center each way. The turndown carries three #5 bars continuous. The drawings call for 2 ft laps on #4, 2 ft 6 in on #5, and rebar arrives in 20 ft stock.

Every number below comes from those dimensions and nothing else. If your detail is different, the arithmetic is the same and the answers are not — that is the point of working an example rather than reading a formula.

Two simplifying assumptions are worth naming because real jobs break them: the site is flat, and there are no depressions, equipment pads, or interior footings. A single 4 in depression for a cooler slab changes three lines on the sheet, not one.

The sheet below is organized the way a quantity sheet should be — one line per material, neat quantity and ordered quantity shown separately, units on every line. That structure is what makes it reviewable by someone other than the person who wrote it.

How do you measure the slab and convert to cubic yards?

Area first, then thickness. 60 ft x 40 ft = 2,400 SF. Convert the thickness to feet before multiplying: 5 in ÷ 12 = 0.4167 ft. 2,400 x 0.4167 = 1,000 CF. Working in feet throughout avoids a common unit error: multiplying square feet by inches and getting a number 12 times too large that still looks plausible.

Convert once, at the end. There are 27 cubic feet in a cubic yard. 1,000 ÷ 27 = 37.04 CY. Do the conversion once, on the final cubic-foot figure. Rounding each intermediate value to two decimals and then converting compounds the error across a long sheet, and on a big pour it compounds in the direction you do not want.

Keep the slab and the turndown on separate lines. They behave differently. The slab sits on a graded base and its overrun is driven by grading tolerance. The turndown is trenched in earth and its overrun is driven by over-dig. They take different waste factors and they get placed by the same truck, so a combined line hides which one moved when the yardage comes back high.

Where does the thickened edge double-count?

This is the error that survives review, because it does not look like an error. It looks like waste.

The 16 x 24 trap. The obvious move is to run the full turndown section: 16 in x 24 in = 2.667 SF of cross-section, times the perimeter, = 519 CF = 19.2 CY. But the top 5 in of that section is slab, and the slab line already counted it. You have paid for the same concrete twice over a 16 in wide band around the whole building.

Deduct the slab thickness. The volume below the slab is 16 in x 19 in = 2.111 SF. Times a centerline perimeter of 194.67 LF = 411 CF = 15.2 CY. The difference between the right number and the wrong one is 4.0 CY on this detail — 7.7% of the total pour, which is the same size as a waste factor. It never shows up as a mistake. It shows up as a crew that always seems to have a little extra.

Three perimeters, one detail. The turndown centerline is 194.67 LF, because the 16 in wide beam sits 8 in inside the slab edge on each side. The outside perimeter, which is what you form, is 200 LF. The slab is neither — it is an area. Using the outside perimeter for the concrete overstates the turndown line by about 2.7% on this geometry — under 1% of the total pour, but a separate mistake from the one above. They stack.

What should the waste factor actually cover?

A waste factor is an estimate of a specific, nameable loss. When it stops being that and becomes a flat percentage added to everything, it is a markup with a technical-sounding label.

Subgrade tolerance drives the slab. If the stone base finishes an average of 1/4 in low across 2,400 SF, that is 50 CF — 1.85 CY, or 5% of the slab, from one quarter of an inch. That is not spillage and it is not your crew. It is the grading contractor's tolerance, and whether you eat it depends entirely on what the subcontract says. Put it in the scope in writing or assume you are paying for it.

Earth-formed turndowns run higher. Only the outside face of the turndown is formed; the inside face is the trench wall. Trenches over-dig at the corners, in soft spots, and wherever the operator was moving fast. Something in the range of 10% is what many contractors carry here in firm soil, but the right number comes from your own soil conditions and your history on similar trenches, not from a default. In sand or after rain it is optimistic, and a wet-weather pour is the one where the extra truck arrives too late to matter.

What it does not cover. Waste is not contingency. It does not cover a mix change, a re-pour, a section rejected on slump, or drawings that turn out to be at 90%. Those are risks and they belong somewhere a reviewer can see them, not buried inside a percentage on a concrete line.

The applied numbers. Slab 37.04 x 1.07 = 39.6 CY — 5% for the subgrade tolerance named above, plus 2 points for chute spillage and end-of-pour washout. Build your own number from components you can name rather than adopting this one. Turndown 15.22 x 1.10 = 16.7 CY. Total 56.4, ordered at 56.5 CY. Ready-mix is typically ordered in quarter- or half-yard increments, and mixer trucks commonly haul somewhere in the range of 8 to 11 CY — both vary by plant and by local load limits. At that range this is roughly six or seven loads. Confirm the truck size and the short-load minimum with your supplier before you split the last load.

How do you count rebar, laps and drops?

Count bars, do not scale them. Bar quantity is dimension ÷ spacing, plus one, rounded up — you need a bar at both ends. Across the 40 ft dimension: 40 ÷ 1.5 = 26.67, round to 27, plus one = 28 bars running the long way. Across the 60 ft: 60 ÷ 1.5 = 40, plus one = 41 bars. Dropping the plus-one is short by about 3% on a mat this size — and by far more on a small one. The shorter the member, the bigger the error.

Net length is not what you buy. At 3 in cover the long bars are 59.5 ft and the short bars 39.5 ft. Net steel is 3,286 LF. But stock is 20 ft and every splice eats 2 ft, so each added bar only extends a run by 18 ft. Three bars reach 56 ft on a 59.5 ft run, so you need a fourth piece 5.5 ft long: 65.5 ft of stock consumed for 59.5 ft of bar in place.

Where the 10% rule comes from, and where it breaks. Both runs here land at about 10.1% over net, provided every drop gets used somewhere else on the job — close to where the common 10% rule of thumb sits. That agreement is a coincidence of this geometry, not a derivation of the rule. It works on long runs in a large mat. It fails badly on short members — a 12 ft footing cut from a 20 ft bar leaves an 8 ft drop — 40% waste if nothing else on the job uses an 8 ft piece — and the rule under-buys. Check the rule against the longest and shortest run on the job before you trust it.

Convert linear feet to weight. Steel is bought and priced by weight. Nominal weights are 0.668 lb/ft for #4 and 1.043 lb/ft for #5. Slab steel: 3,617 LF of #4 = 2,417 lb. Turndown: three continuous #5 on the 194.67 LF centerline. A closed loop is different from a straight run: there is a lap at every joint including the one that closes the loop, so no bar gets credit for its full 20 ft and you divide straight through by the 17.5 ft effective length. 194.67 ÷ 17.5 = 11.1, round up to 12 bars per run, three runs = 36 bars at 20 ft = 720 LF, plus 12 corner bars — assuming a 4 ft leg each way, 8 ft per bar, ≈ 96 LF. That leg length is an assumption, not a dimension off this detail; read it off your corner bar schedule before you price the steel., totaling 816 LF = 851 lb. Together, 3,268 lb — about 1.6 tons.

What else goes on the sheet?

The concrete and steel are the money. The rest of the lines are where scope gaps live — and the first gap is usually the dirt. Before any of the lines below, the turndown trench has to be excavated (194.67 LF at 16 in x 19 in is 15.2 CY neat, more with over-dig and working room), the spoil stockpiled or hauled, and the outside face backfilled and compacted if it was formed rather than trenched. Fine grading and subgrade compaction under the slab are separate lines again.

Vapor barrier. 2,400 SF net, roughly 2,640 SF with side and end laps, plus the turn-up at the slab edge that gets left off more often than not. It is sold by the roll, so the ordered quantity is whole rolls, not square feet. Round up and say so on the line. Add the accessories underneath it: seam tape in linear feet for every lap (6 in minimum lap, taped) and pipe boots by the each for every penetration. Those are the items that get left off and then argued about on site. Round up and say so on the line.

Base course. 4 in compacted under the slab, but only where there is slab over base. The 16 in turndown band takes 194.67 LF x 1.333 ft = 260 SF out of the 2,400 SF, leaving 2,140 SF. 2,140 x 0.333 = 713 CF = 26.4 CY in place. Same deduction logic as the turndown concrete, applied one line down. That is the compacted volume. Base is sold and hauled by the ton, so the in-place cubic yards are an intermediate figure, not an ordered quantity. Convert using the compacted density of the specific material — commonly in the range of roughly 1.7 to 1.9 tons per compacted cubic yard for crushed aggregate base, but confirm it with the supplier rather than assuming, because the spread between materials is wide enough to change the truck count. On 26.4 CY compacted that is roughly 45 to 50 tons.

Edge forms. On a trenched turndown both faces of the beam are earth and only the slab edge is formed: 200 LF at the 5 in slab thickness, about 85 SF of contact area. If the detail or the soil requires the outside face to be formed full depth, say so explicitly and add the outside over-excavation and the backfill that follows it — a formed 24 in outside face is a different means and methods, not a bigger form number. That single fact is the reason the turndown carries a higher waste factor than the slab, and writing the two lines next to each other is what makes the connection obvious to whoever reviews the sheet.

Finish, cure and saw cuts. 2,400 SF of trowel finish and the same area of cure. For joints, the common rule of thumb is panel spacing of 24 to 36 times the slab thickness in inches — put another way, 2 to 3 feet of spacing per inch of slab, so 10 to 15 ft for a 5 in slab — with panels kept near square, no worse than about 1.5:1. A 12 ft by 10 ft grid gives four cuts at 40 ft and three at 60 ft: 340 LF. Put the depth on the line as well as the length — a control joint has to go 1/4 to 1/3 of the slab thickness, so 1.25 to 1.67 in here, and the price per foot moves with depth and blade. Note the timing too: conventional sawing has a window of roughly 4 to 12 hours after finishing depending on mix and weather, early entry is much sooner, and whoever prices the line needs to know which one is specified. Draw the layout before you measure it; an assumed grid is usually short.

The finished quantity sheet

Neat quantity and ordered quantity shown separately, units on every line.

What does this sheet not tell you?

A quantity sheet is an input to a bid, not a bid. Treating it as one is how estimators get surprised.

It carries no price. Quantities times unit costs gives you a number, but the unit costs swing on placement method (chute versus pump), crew size, time of year, and finish specification — and placement method moves the quantity too, not just the price. A pump takes priming grout and leaves concrete in the line, typically a quarter to a half yard that never reaches the slab. If the job is pumped, that comes off the order, not out of the waste factor.. The same 56.5 CY can cost meaningfully different amounts on two jobs a mile apart.

It does not say who eats the overrun. The supplier bills what leaves the plant. A placing crew may bill placed yardage. The waste sits with whoever the contract says it sits with. If the contract is silent, it becomes an argument — and one you should not assume you will win. Get the allocation in writing before the pour.

It does not resolve scope between trades. Who compacts the base and to what, who sets and tapes the vapor barrier, who cuts the joints and how soon after the pour — those are three lines on this sheet and three arguments on site if nobody assigned them.

And it says nothing about mix design. Fiber, air entrainment, strength, and admixtures change the price of a cubic yard without changing how many you need. Keep the quantity question and the specification question on separate pages or they contaminate each other.

Checks before the sheet leaves your desk

• Every dimension traced to a printed dimension, not scaled off the sheet, unless the drawings are marked as scalable
• Slab thickness deducted from the turndown depth — check this one first, every time
• The correct perimeter used on each line: outside for forms, centerline for concrete and turndown steel
• Spaces taken as dimension ÷ spacing and rounded UP, then bars = spaces + 1 — round the spaces, not the total, and measure between end bars at cover rather than edge to edge
• Lap footage added in linear feet before converting to weight, not after
• Stock length checked against the longest and shortest run, with drops counted rather than assumed
• Neat and ordered quantities shown in separate columns so the waste is visible to a reviewer
• Units labeled on every line — CY, SF, LF, LB; an unlabeled number is a phone call later
• Control joint layout drawn before the linear feet are measured off it

Do you need to send this out?

Honestly, for one detail like this, no.

A 2,400 SF slab with a turndown is an hour with a scale, a calculator and a clean sheet of paper. Do it yourself. You will likely find the turndown double-count the hard way once, and it tends to stay found after that — which is why it is the first line on the check list below. That is worth more than a delivered PDF.

Outsourced estimating earns its keep when the problem is volume rather than arithmetic: four bids due the same Thursday, a 200-page set where the concrete is spread across fifteen details, an unfamiliar trade, or a scope where you want a second count on the deducts before you commit. That is a capacity decision, not a skill decision, and it is worth being clear with yourself about which one you are making.

The limitation is worth stating plainly. An outside estimator works from the documents you send. An incomplete set produces a complete-looking sheet built on the same holes, and it will look authoritative doing it. The judgment calls — which waste factor, which subgrade tolerance you are willing to accept, what you are assuming about the grading subcontract — still have to come from you, or be written down clearly enough for someone else to apply them. Quantities can be delegated. Assumptions cannot.