Formula
Triangle area = base × perpendicular height ÷ 2. Optional planning area = area × (1 + allowance percent ÷ 100). Perimeter check = base + side A + side B when side lengths are entered.
Triangle Area Calculator
Calculate triangle area from base and height, with side-length cross-checks, optional planning allowance and a printable geometry record.
Worked example
For base 12 and height 8: triangle area = 12 × 8 ÷ 2 = 48 square units. If a 10% allowance is used for material planning, planning area = 48 × 1.10 = 52.8 square units. With side lengths 10 and 10, the perimeter check is 12 + 10 + 10 = 32 units.
Professional note
Master’s Tip: check that the height is perpendicular before trusting the area. A tape measure along a sloping side can look like height, but it gives the wrong area unless it meets the base at a right angle.
Regional and unit assumptions
Standard or basis: Euclidean plane geometry using base and perpendicular height. Display is rounded for readability; the calculation keeps the raw entered values until the final result is shown.
Assumptions and limitations
- Base and height must use the same linear unit, such as metres, feet, centimetres or inches.
- Height means the perpendicular distance from the base to the opposite vertex, not a sloping side unless the triangle is right-angled in that orientation.
- The area result is in square units matching the input unit: m², ft², cm² or another stated square unit.
- The optional allowance is a planning buffer and is kept separate from the exact geometric area for cleaner worksheets, quotes and cut notes.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
Triangle area = base × perpendicular height ÷ 2. Optional planning area = area × (1 + allowance percent ÷ 100). Perimeter check = base + side A + side B when side lengths are entered.
Standard or basis
Standard or basis: Euclidean plane geometry using base and perpendicular height. Display is rounded for readability; the calculation keeps the raw entered values until the final result is shown.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: check that the height is perpendicular before trusting the area. A tape measure along a sloping side can look like height, but it gives the wrong area unless it meets the base at a right angle.
Related calculators
Questions
How do you calculate the area of a triangle?
Multiply the base by the perpendicular height, then divide by 2. The formula is area = base × height ÷ 2.
Does triangle height mean the sloping side?
No. Height means the perpendicular distance from the base to the opposite vertex. A sloping side is only the height in special right-triangle cases.
What units does triangle area use?
Triangle area uses square units based on the inputs. Metres give square metres, feet give square feet and centimetres give square centimetres.
Why is the formula divided by 2?
Two identical triangles can form a rectangle or parallelogram with area base × height, so one triangle has half that area.
What is the allowance field for?
It keeps material waste, cut tolerance or planning margin separate from the true geometric area so the printed report remains traceable.
Calculation note
Triangle area is a core geometry calculation because many real shapes can be split into triangles: roof sections, garden beds, land sketches, bracing, signs, fabric panels and classroom diagrams. Showing the formula beside the report makes the measurement easier to check later.
A triangle is half a matching rectangle or parallelogram
The base-times-height relationship is easy to see by pairing two equal triangles. Together they make a rectangle or parallelogram with area base × height, so one triangle takes half that area.
Perpendicular height is the key measurement
The most common mistake is measuring a sloping edge and calling it height. The formula needs the shortest straight distance from the base line to the opposite vertex, meeting the base at a right angle.
Printable triangle records help in school and trade notes
A one-page report with base, height, side checks, formula, allowance, date and notes can serve as a classroom worksheet, roof/garden sketch note, fabric panel estimate or quote attachment.
Sources and further readingEncyclopaedia Britannica: TriangleNIST: SI units and prefixes