Plot area calculator for rectangular and irregular four-sided sites
Enter measured boundary lengths and get the plot area. For irregular sites the calculation splits the plot into two triangles across a measured diagonal, which is the method a site survey actually uses.
Shape
Dimensions
Result — calculated from your inputs
Other units
Working
Continue to a full site assessment
A calculator answers one question. A DevPartner assessment carries planning capacity, programme, cost, revenue, deal structure and cash flow through one deterministic model and freezes a versioned decision report.
Nothing from this calculator maps into the assessment intake, so you will start from a blank site. No opportunity is created until you save one.
How this calculator works
Rectangular: area = length × width Irregular four-sided (two-triangle method): s = (a + b + d) / 2 T1 = √( s(s−a)(s−b)(s−d) ) ← Heron's formula T2 = √( s'(s'−c)(s'−e)(s'−d) ) area = T1 + T2 where d is the measured diagonal joining the two triangles.
- You enter
- A value you supply. Nothing is assumed for you.
- Calculated
- Arithmetic on your inputs, shown in the working.
- Assumption
- A factor or allowance you can change — mix ratios, wastage, dry-volume factors.
- Approximation
- Geometry or method that approximates reality; labelled wherever used.
- Verify locally
- Planning parameters, statutory rates and structural design must come from the authority or a qualified professional.
Why a diagonal is required for an irregular plot
Four side lengths alone do not define a shape. A four-sided figure with fixed sides can be flexed like a hinge into an infinite number of areas. One measured diagonal locks it, splitting the plot into two rigid triangles that Heron's formula solves exactly.
This is why a surveyor's field book records diagonals and offsets, not just boundary walks. If you only have four sides from a broker's sheet, the area you compute from them is not a measurement — it is a guess with an unstated error, often several percent.
What this calculator does not do
It does not handle plots with more than four boundary lines, curved boundaries, or boundaries following a watercourse. For those, split the plot into triangles yourself and add the results, or commission a total-station survey.
It also has no view on which area is legally yours. Encroachment, road widening lines, setback reservations and unrecorded pathways all reduce the area you can actually develop. Use the FSI calculator with a deduction figure to see the effect on buildable area.
Worked example
A plot measures 40 ft × 60 ft on a rectangular frontage: 2,400 sq ft exactly. An irregular plot with boundary walks of 40, 62, 41 and 58 ft and a measured diagonal of 72 ft splits into two triangles of roughly 1,240 and 1,180 sq ft, giving about 2,420 sq ft. The two results look similar; only the first is exact.
If the two triangles disagree wildly with your own pacing of the site, the diagonal is usually the input that was mis-measured.
Mistakes that distort a plot area
Estimating the diagonal instead of measuring it. Four sides alone do not fix a quadrilateral's area; the shape can flex. The diagonal is what pins it, and a guessed diagonal produces a confidently wrong number.
Measuring along a sloping boundary and treating it as a plan length. Planning area is the horizontal projection.
Including the road-side strip that is already notified for widening.
Where the approximation ends
The quadrilateral mode is an approximation by construction: it assumes straight boundaries between the corners you measured. Curved boundaries, splayed corners and irregular rear edges are not represented, and the page labels every quadrilateral result as approximate for that reason.
No result here substitutes for a licensed surveyor's plan. Use it to sanity-check a listing or a first site visit, then commission a survey before you price or commit.
Frequently asked questions
- How do I calculate the area of an irregular plot?
- Measure all four boundary sides and one diagonal, then split the plot into two triangles across that diagonal and apply Heron's formula to each. This calculator does that automatically. Without the diagonal the area cannot be determined from side lengths alone.
- The calculator says my sides cannot form a plot. What is wrong?
- A triangle only exists when each side is shorter than the sum of the other two. If your diagonal is longer than the two sides it joins, or much shorter, one of the five measurements is wrong. Re-measure the diagonal first — it is the one most often paced rather than taped.
- Should I use the title deed area or the measured area?
- Price on the smaller of the two and investigate the gap before you commit. Deed areas are frequently inherited from older records; measured areas reflect what you can fence today. Any shortfall becomes your loss after registration.
- Why does the calculator need a diagonal for an irregular plot?
- Four side lengths do not fix a quadrilateral: the shape can flex like a hinge and enclose very different areas. Measuring one diagonal splits the plot into two fixed triangles, each solvable by Heron's formula. Without a measured diagonal the answer would be a guess presented as a calculation.