What is the formula for the area of a triangle?

Area = ½ × b × h, where b is the base and h is the perpendicular height. The formula sits on the stage as ½ b h, and the first row under it is the same formula with your numbers in place, for example ½ × 4 × 3 = 6. The headline is shown to six significant figures and the rows below it to twelve, so a whole answer like 6 prints as 6.

Is the height the same as the slanted side?

No, and this is the mistake that costs most triangle answers their accuracy. The height is measured at a right angle to the base, from the base to the corner opposite it. A sloping side is always longer than that perpendicular distance, so using it in ½ b h gives an area that is too big. The hint under the fields repeats the rule while you type.

How do I get the area when I only know the three sides?

Switch to the Triangle, 3 sides shape in the flat row above the fields. It uses Heron's formula: s = ½(a + b + c), then area = √(s(s − a)(s − b)(s − c)). Sides of 3, 4 and 5 give 6. If the two shorter sides do not add up to more than the longest, they cannot meet to make a triangle, and the page says that rather than printing a number. Once you have the area you can recover the height on any side: h = 2 × area / that side.

Can any side be the base?

Yes. Any of the three sides can be the base, as long as the height you pair with it is the perpendicular distance from that side to the corner opposite. All three pairings give the same area, which is a useful check: measure a second base and its own height and the two answers should agree. In an obtuse triangle the perpendicular can land outside the triangle, on the base extended, and ½ b h still holds.

What if my base and height are in different units?

Convert one of them first. A single unit switch covers both fields here, so the labels read Base (cm) and Height (cm) together and the answer comes back in that unit squared. The rows under the answer then convert the area itself into six more units, using exact definitions: a square foot is exactly 0.09290304 square metres.

What does the drawing show?

The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Triangle Area Calculator

The area of a triangle is half the base times the perpendicular height: area = ½ × base × height. Type a base of 4 and a height of 3 and the answer is 6, with the working printed underneath as ½ × 4 × 3 = 6, so you can check it. The height is the straight distance from the base to the corner opposite, measured at a right angle to the base, and not the length of a sloping side: that one substitution is what makes a triangle come out too large, and the line under the fields says so. Sides go in millimetres, centimetres, metres, inches or feet, and the same area is listed in six more units below the answer. If you know three sides and no height at all, the Triangle, 3 sides shape beside this one uses Heron's formula instead.

Rectangle · l × w96cm²9,600 mm² · 0.0096 m² · 14.88 in²

Flat

Solid, surface area

Length (cm)

Width (cm)

Unit

Length times width. Any two sides that meet at a corner. The drawing is schematic. Choose the measured unit before typing; changing it sets the meaning of the numbers.

Result summary

Rectangle, length 12 cm, width 8 cm
96 cm²
12 × 8 = 96
9600 mm²
0.0096 m²
14.8800297601 in²
0.10333354 ft²
0.0114815044445 yd²
0.00000237221166208 ac

The workingarea, in cm²
12 × 8 = 96
Square millimetresmm²
9,600
Square metresm²
0.0096
Square inchesin²
14.8800297601
Square feetft²
0.10333354
Square yardsyd²
0.0114815044445
Acresac
0.00000237221166208

The formula is on the stage, and the working is the first row. Every answer here is one textbook formula with your numbers put in, printed so you can check it: a circle is π r², a three-sided triangle is Heron's formula, a regular polygon is n s² / (4 tan(π/n)). The solids give surface area for a closed shape, both ends of a cylinder and the base of a cone included. Conversions use exact definitions, so a square foot is 0.09290304 m² and 43,560 of them make an acre.

Common questions

What is the formula for the area of a triangle?: Area = ½ × b × h, where b is the base and h is the perpendicular height. The formula sits on the stage as ½ b h, and the first row under it is the same formula with your numbers in place, for example ½ × 4 × 3 = 6. The headline is shown to six significant figures and the rows below it to twelve, so a whole answer like 6 prints as 6.
Is the height the same as the slanted side?: No, and this is the mistake that costs most triangle answers their accuracy. The height is measured at a right angle to the base, from the base to the corner opposite it. A sloping side is always longer than that perpendicular distance, so using it in ½ b h gives an area that is too big. The hint under the fields repeats the rule while you type.
How do I get the area when I only know the three sides?: Switch to the Triangle, 3 sides shape in the flat row above the fields. It uses Heron's formula: s = ½(a + b + c), then area = √(s(s − a)(s − b)(s − c)). Sides of 3, 4 and 5 give 6. If the two shorter sides do not add up to more than the longest, they cannot meet to make a triangle, and the page says that rather than printing a number. Once you have the area you can recover the height on any side: h = 2 × area / that side.
Can any side be the base?: Yes. Any of the three sides can be the base, as long as the height you pair with it is the perpendicular distance from that side to the corner opposite. All three pairings give the same area, which is a useful check: measure a second base and its own height and the two answers should agree. In an obtuse triangle the perpendicular can land outside the triangle, on the base extended, and ½ b h still holds.
What if my base and height are in different units?: Convert one of them first. A single unit switch covers both fields here, so the labels read Base (cm) and Height (cm) together and the answer comes back in that unit squared. The rows under the answer then convert the area itself into six more units, using exact definitions: a square foot is exactly 0.09290304 square metres.
What does the drawing show?: The labelled drawing identifies the dimensions used by the formula; it is schematic rather than to scale. The working and conversion rows contain the numeric result. Saved inputs restore locally without overwriting their original record on load.

Supported formulas use rounded browser numbers and exact unit definitions. Results show up to twelve significant figures; measurements carry their own error.