Strip Foundation Calculator

Length and layout
Length input mode
Depth, height, and thickness
Concrete waste
Blinding layer
Concrete class
Reinforcement
Formwork

A 5% concrete waste allowance was applied.

Lap length uses the simplified 40 x bar diameter figure (480 mm for a 12 mm bar) for good bond conditions, not the full Eurocode 2 calculation.

A 160 mm stirrup hook allowance (two 10 x diameter hooks) was assumed -- trade practice, not a cited standard.

Ground bearing pressure covers the footing's own weight alone, excluding blinding and any building load -- indicative only, not a bearing-capacity check.

A 10% formwork waste allowance was applied.

Diagram
Cross-section
Plan
Total concrete volume (with waste)12.10m³
Longitudinal reinforcement mass166.20kg
Formwork boards required80
Length and layout
Total strip length (centreline)36 m
Total foundation height800 mm
Concrete
Net volume11.52 m³
Waste volume576.00 L
Strip concrete mass28,800.00 kg
Blinding
Blinding volume2.16 m³
Blinding mass5,184.00 kg
Surfaces and bearing
Footing base area (waterproofing)14.40 m²
Side surface area (insulation, both faces)57.60 m²
Ground bearing pressure (indicative)19.62 kPa
Sitework
Excavation volume10.80 m³
Longitudinal reinforcement
Bar segments per row4
Laps per row3
Total bar length (incl. laps)187.2 m
Longitudinal bar mass166.20 kg
Stirrups
Single stirrup length2,240 mm
Stirrup count121
Total stirrup length271.04 m
Stirrup mass106.95 kg
Formwork
Formwork boards (before waste)72
Formwork boards required80
Formwork offcut length32 m
Parts list
Longitudinal barCount: 16
Length
11,700 mm
Diameter
12 mm
StirrupCount: 121
Length
2,240 mm
Diameter
8 mm
Formwork boardCount: 80
Length
4,000 mm
Width
200 mm
Thickness
25 mm

About this calculator

This calculator takes a strip foundation from a single cross-section to a full take-off: concrete and blinding volume, the waterproofing and insulation areas the foundation hands off to neighbouring trades, an indicative bearing pressure, and quantities for the longitudinal reinforcement, stirrups, and formwork a real strip footing also needs.

Length comes from one of two modes. Footprint derives the total strip length from the building's own length and width plus an internal wall layout (none, a single cross wall, a single lengthwise wall, or both) — the common case, where you would otherwise have to add up the perimeter yourself. Manual accepts a single total length directly, for an irregular footprint or a layout the four built-in options don't cover.

This calculator does not design your reinforcement. Bar diameter, row count, stirrup diameter and spacing, and concrete cover are all your own inputs — quantities only, computed from what you enter. The actual reinforcement for a real foundation must be designed by a qualified structural engineer against the specific soil, building load, and local code, not read off this page.

Formula

Length (centreline convention). buildingLength/buildingWidth are themselves measured centreline-to-centreline, so the perimeter counts every corner exactly once:

perimeter = 2 x (buildingLength + buildingWidth)
internal wall length = 0 (none) | buildingWidth (cross) | buildingLength (lengthwise)
                      | buildingWidth + buildingLength (both)
totalStripLength = perimeter + internal wall length

An internal wall's own centreline reaches exactly to the perimeter's centreline at both ends, meeting it at a point rather than overlapping it — the same reason a plain rectangle's own corners are never double-counted. This assumes one uniform thickness across the perimeter and every internal wall.

Concrete and blinding.

totalHeight = depthBelowGrade + heightAboveGrade
crossSectionArea = thickness x totalHeight
volumeNet = crossSectionArea x totalStripLength
volumeWaste = round(volumeNet x waste% / 100); volumeTotal = volumeNet + volumeWaste
massStrip = volumeNet x 2500 kg/m3 (reinforced concrete density, see Assumptions)

blindingWidth = thickness + 2 x blindingOverhang
blindingVolume = totalStripLength x blindingWidth x blindingThickness
blindingMass = blindingVolume x 2400 kg/m3 (plain concrete density)

Surfaces for neighbouring trades and bearing pressure.

footingBaseArea (waterproofing) = totalStripLength x thickness
sideSurfaceArea (insulation, both faces) = 2 x totalStripLength x totalHeight
groundBearingPressure = totalHeight x 2500 kg/m3 x 9.81 m/s2   (indicative only)

The bearing-pressure formula simplifies to depend only on totalHeight: the footing's own weight (crossSectionArea x length x density x g) divided by its own base area (thickness x length) cancels both the thickness and the length. It covers the footing's own weight alone — it excludes the blinding layer and any load the building above transmits into the foundation, and is not a substitute for an actual bearing-capacity check against your soil.

Excavation widens to fit the blinding layer when one is used:

trenchWidth = blindingWidth (blinding enabled) | thickness (no blinding)
excavationVolume = totalStripLength x trenchWidth x depthBelowGrade

Longitudinal reinforcement. The lap length uses the common 40 x barDiameter simplification for good bond conditions — see the FAQ for what this leaves out:

lapLength = 40 x barDiameter
segmentsPerRow = 1                                                  if totalStripLength <= stockBarLength
              = 1 + ceil((totalStripLength - stockBarLength) / (stockBarLength - lapLength))   otherwise
lapsPerRow = segmentsPerRow - 1
totalBarLength = segmentsPerRow x stockBarLength x barRows
barMass = totalBarLength x (pi x (barDiameter/2)^2) x 7850 kg/m3

Each splice's lap consumes length rather than adding covered run — two stock bars overlapped by lapLength cover 2 x stockBarLength - lapLength, not 2 x stockBarLength — which is why segmentsPerRow can come out one higher than simply ceil(totalStripLength / stockBarLength) once a run needs several splices (see the worked example).

Stirrups. The stirrup's own rectangle is the section inset by cover on all four sides, plus a hook allowance:

stirrupLength = 2 x ((thickness - 2 x cover) + (totalHeight - 2 x cover)) + 20 x stirrupDiameter
stirrupCount = floor(totalStripLength / stirrupSpacing) + 1
totalStirrupLength = stirrupCount x stirrupLength
stirrupMass = totalStirrupLength x (pi x (stirrupDiameter/2)^2) x 7850 kg/m3

The 20 x stirrupDiameter hook allowance is two hook bends of 10 x diameter each — trade practice, not a cited figure (see Assumptions).

Formwork. Boards run horizontally: their own width stacks up the total height in courses, their own length is cut to fit each wall run:

courseCount = ceil(totalHeight / boardWidth)

For every course, every wall segment (each side of the footprint, plus any internal wall — footprint mode; the single manual length, in manual mode), and both faces, one run of that segment's own length is needed. A run longer than one stock board is chunked into board-length pieces first (boards are simply butted end to end along a straight run — unlike a rigid trim board, nothing stops two boards meeting mid-wall). Every resulting piece across the whole job, plus a waste allowance chunked the same way, is then packed into the fewest stock boards with first-fit decreasing bin packing — sort pieces longest first, place each into the first board with enough spare length, open a new board only when none fits:

boardsRequired = packBoards(every chunked run + chunked waste allowance, boardLength)

This is not the same as dividing total area (or total length) by one board's own area (or length): several runs that are each more than half a stock board's length can never share a board with each other, however the totals divide — a 6 m board cannot yield two 4 m runs, no matter how much spare length the arithmetic implies is "left over" elsewhere. Dividing area/length instead of packing pieces can significantly under-count the boards a real job needs (see the worked example).

Worked example

A 10 x 8 m building, no internal walls, 400 mm thick strip, 300 mm below grade + 300 mm above grade (600 mm total height), with every other input left at its default.

totalStripLength = 2 x (10 + 8) = 36 m
crossSectionArea = 0.4 x 0.6 = 0.24 m2
volumeNet = 0.24 x 36 = 8.64 m3; +5% waste = 9.072 m3 total
massStrip = 8.64 x 2500 = 21,600 kg

blindingWidth = 0.4 + 2x0.1 = 0.6 m; blindingVolume = 36 x 0.6 x 0.1 = 2.16 m3

footingBaseArea = 36 x 0.4 = 14.4 m2; sideSurfaceArea = 2 x 36 x 0.6 = 43.2 m2
groundBearingPressure = 0.6 x 2500 x 9.81 / 1000 = 14.715 kPa

reinforcement (12mm bars, 4 rows, 11.7m stock): lap = 40x12 = 480mm
  segmentsPerRow = 1 + ceil((36,000-11,700)/(11,700-480)) = 1 + ceil(2.166) = 4
  totalBarLength = 4 x 11.7 x 4 rows = 187.2 m

stirrups (8mm, 300mm spacing, 40mm cover): inner rect = 320 x 520mm
  stirrupLength = 2x(320+520) + 20x8 = 1,840mm; count = floor(36,000/300)+1 = 121

formwork (200mm boards, 4m stock, 10% waste): 3 courses x 4 segments x 2 faces,
  chunked and packed -> 60 boards required

For these inputs the calculator returns exactly this: net concrete volume 8.64 m³ (9.072 m³ with waste), total bar length 187.2 m (16 segments, 3 laps per row), 121 stirrups, and 60 formwork boards.

FAQ

Why does the calculator offer both a footprint and a manual length mode? Deriving the length from length/width/internal-walls avoids making you compute your own perimeter, which is the more common case and the more useful drawing. Manual mode exists for anything the four-option internal-wall list can't describe — an irregular footprint, more than one internal wall, a shape with no straight rectangle to describe it. Manual mode is treated as a single run for formwork packing and the plan drawing, since the calculator has no individual wall segments to work with in that mode.

Is 40 x barDiameter the actual Eurocode 2 lap length? No — it's a common simplification for good bond conditions. Eurocode 2 (EN 1992-1-1, Cl. 8.7) derives lap length as alpha1 x alpha2 x alpha3 x alpha5 x alpha6 x lb,rqd, a product of several coefficients covering bond conditions, confinement, and the percentage of bars lapped at one section — not a flat multiple of the bar diameter. This calculator uses the simplified figure because it does not know your bond conditions, confinement, or lap percentage; a real design should run the full Eurocode 2 calculation.

Where does the stirrup hook allowance come from? Trade practice, not a standard: two hook bends of 10 x stirrupDiameter each, 20 x stirrupDiameter total. The cross-section draws both legs at the stirrup's top-left closing corner so the drawing shows what the length counts; real hook geometry depends on the specific bend and anchorage detail your engineer specifies.

Does concrete class affect the volume or mass? No. concreteClass (C12/15 through C30/37, the LST EN 206 strength-class designations) is for the specification only — this calculator uses one constant concrete density regardless of which class you pick.

Does the calculator check whether my reinforcement is adequate? No. Bar diameter, row count, stirrup diameter and spacing are your own inputs. This calculator computes quantities from them; it does not check minimum reinforcement, crack control, or capacity against any load. A qualified structural engineer must design the actual reinforcement.

Why does a longer run sometimes need one more bar segment than totalLength / stockLength suggests? Because each splice's lap consumes length rather than adding it — see the Formula section. Once a run needs several splices, the lost length can push the segment count one higher than a naive division would suggest.

Assumptions and limits

Standards cited, and what each one actually covers:

  • LST EN 206 covers the concrete strength-class designations (C12/15 ... C30/37) offered in concreteClass — nothing else this calculator computes.
  • LST EN 1992-1-1 (Eurocode 2) covers reinforcement lap length in general (Cl. 8.7). This calculator does not run that calculation — it uses the common 40 x barDiameter simplification for good bond conditions instead (see the FAQ).
  • LST EN 1991-1-1 Annex A (informative) is the source of both concrete densities used: 2400 kg/m³ for the blinding layer (plain, unreinforced) and 2500 kg/m³ for the strip itself (reinforced, normal percentage) — the Annex's own footnote for the 100 kg/m³ step between them.

Trade practice, not a standard — disclosed here rather than in meta.standards: the 11.7 m default stock bar length (a common commercial length in this market, not universal), the stirrup hook allowance (20 x stirrupDiameter), the 100 mm default blinding thickness and overhang, and the formwork board sizes.

Ground bearing pressure is indicative only. It covers the footing's own weight alone, excludes blinding and any building load, and is not a substitute for an actual bearing-capacity check by a geotechnical/structural engineer.

Excavation assumes the trench is dug exactly as wide as the blinding layer (or the strip itself, with no blinding), with no extra working clearance — a real trench is often dug wider.

Formwork board packing chunks a long wall run into stock-length pieces before packing them. This is a specific, disclosed cutting strategy (always cut full stock-length pieces first, with one shorter remainder piece per run), not a claim that it is the only or the most material-efficient way to cut a given run.

This calculator does not model two different thicknesses for perimeter and internal strips. One thickness input applies to the whole take-off.

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