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Describe the shape before entering dimensions
This example estimates a rectangular slab 20 feet long, 10 feet wide, and 4 inches thick. The dimensions are chosen to demonstrate arithmetic; they are not a recommendation for slab thickness or structural design. Assume a uniform depth, straight sides, and no thickened edges, footings, openings, or embedded objects.
Length and width describe area. Multiplying that area by thickness produces volume only after the three dimensions use the same length unit. Entering 4 as feet instead of inches would make this estimate twelve times too large.
Convert the thickness before multiplying
Four inches is 4 ÷ 12 = one-third of a foot. The slab therefore has a net geometric volume of 20 × 10 × (4 ÷ 12) = 66.6667 cubic feet. One cubic yard contains 3 × 3 × 3 = 27 cubic feet, so divide by 27 to obtain 2.4691 cubic yards.
In Slab or rectangular footing, enter Length 20, Width 10, Thickness 4, and Extra allowance 0 to check the net volume. For dimensions measured in different systems, use Mixed-unit shapes, quantity & cost and select the unit beside each measurement. Convert using the unrounded dimensions and round the result at the end.
| Step | Calculation | Result |
|---|---|---|
| Area | 20 ft × 10 ft | 200 ft² |
| Thickness | 4 in ÷ 12 | 0.333333… ft |
| Net volume | 200 ft² × (4/12) ft | 66.6667 ft³ |
| Cubic yards | 66.6667 ft³ ÷ 27 | 2.4691 yd³ |
Keep an allowance separate from net geometry
An allowance increases the estimated quantity without changing the shape’s net volume. With an illustrative 10% allowance, multiply 2.4691358 by 1.10 to obtain 2.7160 cubic yards. The added amount is 0.2469 cubic yards. Do not add ten cubic yards or multiply by ten; the percentage represents a fraction of the net volume.
The table compares assumptions, not recommended ordering margins. Actual needs depend on measured forms, base irregularities, placement losses, and supplier practices. Confirm an appropriate quantity and delivery increment with the people responsible for the job.
| Allowance assumption | Net volume | Volume with allowance |
|---|---|---|
| 0% | 2.4691 yd³ | 2.4691 yd³ |
| 5% | 2.4691 yd³ | 2.5926 yd³ |
| 10% | 2.4691 yd³ | 2.7160 yd³ |
A small depth change can exceed the allowance
If the average depth is 4.5 inches rather than 4 inches, net volume rises to 20 × 10 × (4.5 ÷ 12) ÷ 27 = 2.7778 cubic yards. That is a 12.5% increase because volume scales directly with thickness when area stays constant.
Notice that 2.7778 cubic yards is already greater than the 2.7160 cubic yards produced by adding 10% to the original 4-inch estimate. An allowance cannot reliably correct a systematically understated measurement. Measure the form and base, split irregular shapes into appropriate sections, and avoid double-counting overlapping areas.
To compare the two depths, calculate the 4-inch slab with zero allowance, use the calculator’s scenario comparison to keep that result, then change thickness to 4.5 and calculate again. Keeping length, width, and allowance fixed reveals the effect of depth alone.
Use the quoted volume unit for material cost
For an illustrative price of $160 per cubic yard, the 4-inch slab with a 10% allowance has a volume-based material estimate of 2.716049… × 160 = $434.57. Use the unrounded volume in the multiplication. The $160 rate is an invented example, not a current supplier quote.
In Mixed-unit shapes, quantity & cost, select Slab; enter 20 ft, 10 ft, and 4 in; set quantity to 1 and allowance to 10%; enter price 160 with volume price unit yd3 and price volume Including allowance. Selecting Net geometry instead would price 2.469135… cubic yards, or $395.06, and omit the allowance from the cost.
A delivery invoice may also include a minimum order, quantity rounding, delivery, short-load charges, waiting time, taxes, or other services. This worksheet multiplies volume by a rate. It cannot confirm availability, a delivered price, mix specifications, or the suitability of the design.
Check units again when the result looks wrong
A square-foot result measures surface area, while cubic feet and cubic yards measure volume. Divide cubic feet by 27, not by 3, to convert to cubic yards. For more than one identical item, apply quantity once. If you already combined all areas, do not also multiply by the number of items.
The examples here use rectangular geometry and explicit assumptions so each step can be reproduced. NIST’s reference below supports the unit conversions; it is not a structural design guide or an ordering recommendation.