A thickened slab edge extends below the uniform slab body. The quantity record should show the slab volume and the additional edge volume on separate lines so the same concrete does not enter both.
Use the accepted plan and section detail. Record whether the edge runs around the complete rectangle, follows selected sides, slopes across its width, or changes section along the route.
How do you calculate thickened edge slab volume?
Calculate the uniform slab first. Then measure only the edge section below the uniform slab and multiply that extra area by its accepted plan length or footprint.
Uniform slab volume = plan area × slab thickness
Additional edge volume = extra edge footprint × extra depth
Additional sloped-edge volume = extra cross-sectional area × approved run length
Use one compatible unit before multiplying. Metres produce cubic metres. Feet produce cubic feet, and 27 ft³ equals 1 yd³. The Concrete Slab Calculator handles a complete rectangular square-edged band, as well as uniform rectangular, circular and known-area slab bodies.
Which part of the edge is additional concrete?
The additional section starts at the underside of the uniform slab and ends at the accepted bottom of the thickening. Subtract the slab thickness from the total edge depth.
Extra depth = total edge depth from slab top − uniform slab thickness
A 300 mm total edge depth below a slab surface with a 120 mm uniform slab gives 180 mm of extra depth. Multiplying by the full 300 mm depth would count the top 120 mm twice.
| Dimension | Measurement | Quantity use |
|---|---|---|
| Slab thickness | Top of slab to uniform underside | Uniform slab body |
| Total edge depth | Top of slab to bottom of edge | Defines extra depth |
| Edge width | Horizontal width shown in section | Square or sloped cross-section |
| Edge route | Accepted plan path or band footprint | Run length and corner check |
| Section change | Station, grid or side where geometry changes | Starts a separate quantity line |
How does the exact full-perimeter square-edge formula work?
A constant inward band around a complete rectangular slab forms a plan ring. Subtract the inner rectangle from the outer slab area, then multiply the band area by the extra depth.
Inner length = slab length − 2 × band width
Inner width = slab width − 2 × band width
Band area = outer slab area − inner area
Extra edge volume = band area × (total edge depth − slab thickness)
This method counts each corner once. It applies when the band has one width, one total depth and a square vertical section around all 4 sides of one rectangle. The band width must remain less than half of each slab plan dimension.
Worked metric example: full rectangular thickened edge
An accepted slab measures 6 m by 4 m with a 120 mm uniform thickness. A 300 mm wide square-edged band runs around the complete perimeter and reaches a total depth of 300 mm.
- Uniform slab volume: 6 × 4 × 0.12 = 2.8800 m³.
- Inner dimensions: (6 − 0.60) × (4 − 0.60) = 5.4 × 3.4 m.
- Band area: 24 − 18.36 = 5.64 m².
- Extra depth: 0.30 − 0.12 = 0.18 m.
- Additional edge volume: 5.64 × 0.18 = 1.0152 m³, or 1,015.2 L.
- Measured slab and edge volume: 2.8800 + 1.0152 = 3.8952 m³.
The result has no automatic allowance. Keep a later planning adjustment beside its reason and authority.
Does perimeter × width count corners twice?
Yes, when you multiply all 4 full outside side lengths by the full inward band width. Each corner square then appears on 2 adjoining side rectangles.
The example perimeter is 20 m. The shortcut gives 20 × 0.30 × 0.18 = 1.0800 m³. The exact ring contains 1.0152 m³, so the shortcut adds 0.0648 m³, or 64.8 L, of duplicate corner volume.
Four duplicate corners = 4 × band width² × extra depth
A centreline, clear-run or mitered length convention can remove the overlap before multiplication. State that convention in the takeoff. Perimeter length without a convention does not prove how the corners were handled.
How should partial edge runs and L-corners be measured?
Draw the thickened footprint in plan and split it into non-overlapping rectangles. Use a separate line for each width, depth or detail change.
Example: one 6 m edge and one adjoining 4 m edge each have a 0.30 m inward width and 0.18 m extra depth. Full side rectangles give 0.5400 m³. Their shared corner contains 0.30 × 0.30 × 0.18 = 0.0162 m³, so the net edge volume is 0.5238 m³.
| Method | Corner treatment | Record needed |
|---|---|---|
| Plan rectangles | Deduct each shared overlap once | Marked plan with non-overlapping boundaries |
| Clear runs | One adjoining run stops at the inner face | Clear start and end points |
| Centreline path | Valid only for the defined constant band geometry | Centreline convention and closed-path check |
| Model quantity | Model controls included solids | Model revision, object filters and checker |
A thickening along 3 sides, around a recess, or beside an opening needs its actual plan footprint. The complete rectangular ring formula cannot describe that boundary.
How do you calculate a sloped thickened edge?
Use the extra cross-section below the slab. For a uniform trapezoid, multiply its vertical extra depth by the average of its horizontal top and bottom widths.
Extra trapezoid area = extra depth × (top width + bottom width) ÷ 2
Extra sloped-edge volume = extra trapezoid area × run length
The run length must describe the path of the same perpendicular cross-section. Split the route where top width, bottom width, depth or slope changes. A sloping-face length measured along the diagonal cannot replace the vertical depth.
A triangular extra section is the special case where one horizontal width is 0. A square section has equal top and bottom widths. NASA's area formulas support those geometric relationships; the approved project detail supplies the dimensions.
Worked imperial example: partial sloped edge
An approved sloped extra section continues for 24 ft. It extends 8 in below the uniform slab, measures 12 in across the top and 18 in across the bottom.
- Extra depth: 8 ÷ 12 = 0.666667 ft.
- Top width: 12 ÷ 12 = 1 ft.
- Bottom width: 18 ÷ 12 = 1.5 ft.
- Cross-section: 0.666667 × (1 + 1.5) ÷ 2 = 0.833333 ft².
- Extra volume: 24 × 0.833333 = 20.0000 ft³.
- Cubic yards: 20 ÷ 27 = 0.740741 yd³.
A bounding rectangle at the 18 in maximum width contains 24.0000 ft³. It exceeds the stated trapezoid by 4.0000 ft³, or 0.148148 yd³. Use a bounding rectangle only when the takeoff method calls for that conservative envelope and labels it as such.
How are stepped or changing edge sections scheduled?
Break a stepped section into rectangles or triangles that share boundaries without overlapping. Multiply each area by its matching run length and sum the lines.
Create another line where the section changes along the route. A 10 m run at one profile and a 6 m run at another profile need 2 calculations even when both sit on the same slab side.
A profile that tapers along its length needs a suitable 3-dimensional method from the accepted model or detail. The average of 2 end volumes may fail when the shape changes in more than one direction.
Which thickening conditions need a different method?
| Condition | Quantity method |
|---|---|
| Complete inward square band around a rectangle | Outer area minus inner area, multiplied by extra depth |
| Selected straight square-edge sides | Non-overlapping plan strips multiplied by extra depth |
| Uniform sloped cross-section on a straight run | Trapezoid area multiplied by run length |
| Outside projection beyond the slab plan | Add the actual outside footprint or section; the inward ring formula does not apply |
| Internal thickened strip or beam | Add only the section below the slab along its accepted route |
| Curved edge | Use the accepted curved plan area or model quantity |
| Changing width, depth or slope | Separate sections or an approved model-based volume |
Keep thickened-edge concrete separate from a strip footing that has its own boundary or pour. The Concrete Slab Volume guide explains how to schedule several slab solids without duplicate interfaces.
How do openings and blockouts affect the edge?
Deduct only the part of an opening that overlaps concrete included in the gross quantity. A full-depth opening through the slab body can stop above the deeper edge, cross part of it, or remove the complete edge section.
Record the opening footprint and vertical limits beside the edge footprint and vertical limits. Their shared solid is the deduction. A plan-area deduction multiplied by uniform slab thickness cannot remove deeper concrete unless the opening also passes through that depth.
An opening already excluded from an entered known area needs no second slab deduction. Check the Concrete Slab Thickness guide when the opening, recess or changed zone uses another accepted thickness.
How does measured edge volume become an order quantity?
Sum the uniform slab and edge sections at full precision. Add a documented allowance after the measured geometry, then use the producer's current increment, minimum delivery, mixture, access and schedule information.
ACI's CCS-1(10) preview explains that concrete is sold by fresh volume and lists information a producer needs for an order. Its example percentages do not establish one allowance for this slab. Site dimensions, form accuracy, subgrade, placement conditions and the accepted quantity plan control that decision.
Use the Ready-Mix Concrete Calculator after the geometric total and planning basis are checked. Keep product yield and bag rounding in the separate concrete-bag workflow.
Common thickened-edge quantity mistakes
- Multiplying the edge width by the full total depth after counting the slab body.
- Using all 4 outside side lengths without removing corner overlap.
- Applying the complete ring formula to selected sides or an irregular slab.
- Entering extra depth where a calculator asks for total edge depth.
- Using a diagonal sloping-face length as vertical depth.
- Using the maximum sloped width as a rectangle without labelling the envelope.
- Carrying one section through a change in width, depth or slope.
- Counting an internal thickening and the perimeter band over the same shared area.
- Deducting an opening through a deeper edge that the opening does not remove.
- Rounding each run before the final sum.
- Adding a fixed waste percentage without a recorded basis.
- Treating measured concrete volume as approval of the slab or foundation design.
What design and safety limits apply?
The project designer, drawings, specifications, permits, geotechnical information and local authority determine the edge purpose, width, depth, slope, reinforcement, concrete properties, frost treatment, drainage and support conditions. This page checks quantity geometry from accepted dimensions.
Formwork, excavation, subgrade preparation, reinforcement and placement each need their own plan. Use the Concrete Slab Formwork Area guide for vertical edge and stop-end contact surfaces. A sloped concrete face and its supporting form can have a different area from the volume cross-section.
Wet portland cement can cause cement burns and dermatitis. Follow the producer's safety information and applicable project controls for skin and eye contact, access, equipment, placing and washout. OSHA supports the cited hazard context for covered United States work.
What should the edge takeoff record contain?
- Project, slab mark, drawing, section, revision, takeoff date and checker.
- Uniform slab boundary, thickness and separately calculated volume.
- Edge route, start and end points, length convention and section IDs.
- Total depth, extra depth, top width, bottom width and units.
- Plan overlaps, corner deductions, openings and interfaces with other solids.
- Measured edge volume, full slab total and display precision.
- Allowance, reason, authority, supplier increment and order basis when those steps begin.
- Unresolved geometry, field changes and approval references.
Return to the Concrete Planning hub for related volume, bag, ready-mix, footing and reinforcement resources.
Sources and scope
- NIST Circumference, Area and Volume: compatible units and basic area and volume geometry.
- NIST Handbook 44, 2026, Appendix C: volume-unit relationships used in the fixtures.
- NASA Glenn Area formulas: rectangle, triangle and trapezoid cross-sectional areas.
- ACI Concrete Craftsman Series CCS-1(10) preview: slab volume and producer-order context.
- OSHA concrete-products hazard guidance: wet-cement skin-hazard context for covered United States work.
Source scope: NIST and NASA support the stated geometry and unit relationships. ACI supports slab volume and supplier-order context. OSHA supports the named United States hazard context. These sources do not choose project edge dimensions, structure, reinforcement, concrete mixture, allowance, price or supplier quantity.
Review note: Saleem Sial owns the research and editorial record. Formula fixtures, source checks, build validation and rendered QA form the internal publication gate. Waseem Sial, External Reviewer and Engineer, is listed for ongoing external review; no completed review date is claimed.
