Irregular and triangular area calculator
Calculate triangular sections and combine them with rectangles without hiding the working.
20 LAUNCH TOOLS
Choose a calculator
When to use it
Use for one triangular section. For a complex footprint, make a labelled sketch, split it into non-overlapping rectangles and triangles, calculate every section and add the net areas.
How it works
Triangle area = base × perpendicular height ÷ 2; waste is applied afterwards.
Worked example
Assume one triangular section with a 6 m base, a 3 m perpendicular height and a 10% allowance. Net area = 6 × 3 ÷ 2 = 9 m². Buying area = 9 × 1.10 = 9.9 m² before product coverage and pack rounding are checked.
Measurement guidance
Triangle base and perpendicular height in metres, plus a separate allowance percentage. The height must meet the chosen base at a right angle; a sloping side is not the perpendicular height.
Buying and specification checks
The calculator represents one flat triangle. Subdivision gives a transparent planning approximation whose accuracy depends on the measurements and chosen shapes; it is not a boundary survey or a substitute for a site survey where position, ownership or high-consequence quantities matter.
Break the plan into shapes without gaps or overlaps
A clear sketch is part of the calculation. It makes the chosen subdivision, measurements and approximation visible before quantities are ordered.
- Draw the boundary and fixed features, then choose straight section lines that create rectangles and triangles without overlap. Give every section a reference so its working can be checked.
- For each triangle, mark the base and measure the perpendicular distance from that base to the opposite point. Use a square, set-out line or another suitable method to establish the right-angle direction rather than measuring a sloping edge.
- Record all dimensions in metres. Calculate the net area of each section, add the section totals, and apply one justified allowance to the combined area unless the material or layout requires section-specific treatment.
- Treat curved and uneven boundaries as approximations: use more, smaller sections where that improves the model, and obtain a suitable survey where the boundary, legal position, fit or cost consequence requires more certainty.
Common mistakes
- Using a sloping side as the height
- Counting overlapping shapes twice
- Adding waste twice
FREE TOOL AND PROJECT PACK
What the free calculator provides
The free calculator gives the area of one triangular section and a separately stated allowance. A complex footprint must be subdivided and totalled by the user.
What the dedicated pack adds
If the measured footprint is for a patio or paved outdoor project, the Patio Project Pack adds project controls around the quantity work.
- Measurement and site-readiness checklists.
- Quote comparison, ordering and delivery controls.
- Work-control, completion and retained project records.
19-page printable digital workbook (PDF). Immediate digital delivery with 90-day access; no subscription.
Practical FAQs
Which height should I use for a triangle?
Use the perpendicular distance to the selected base: the measurement that meets the base at 90 degrees. Do not substitute a sloping side.
How do I combine several rectangles and triangles?
Label non-overlapping sections, calculate every net area and add them. Apply the allowance once to the combined net area unless there is a documented reason to treat sections differently.
How should I handle a curved boundary?
Approximate it with several smaller measurable sections and state that assumption. Smaller sections can improve the approximation, but the result is still not a surveyed measurement.
Can this establish a legal boundary or survey area?
No. The result depends on your sketch and measurements. Use appropriately qualified surveying or boundary advice where ownership, legal position, setting-out accuracy or a high-value commitment depends on it.
Planning guidance, not a quotation or a substitute for project-specific design and competent advice.