A rectangle’s area is one of the first formulas most people learn, and one of the most useful in everyday life — flooring, paint, sod, fencing, and furniture layouts all start with a length-times-width calculation. This calculator handles two closely related shapes at once: a rectangle (from length and width) and a triangle (from base and height), since both come up constantly in the same kinds of home and classroom projects.
Pick the shape you need below, enter the two measurements, and the calculator shows the area along with the exact formula used.
Rectangle Area Formula
Area = Length × Width
Where:
Length = the longer side of the rectangle
Width = the shorter side of the rectangle
Multiplying the two sides works because a rectangle is made of a uniform grid of unit squares — length rows of width columns (or vice versa), so the total count of unit squares is simply the product of the two.
Rectangle & Triangle Area Calculator
Calculate the area of a rectangle or a triangle in a few seconds.
Worked Example: A 12-ft by 9-ft Room
Area = 12 × 9 = 108 square feet. This is the calculation behind ordering flooring, carpet, or tile for a rectangular room — though the material you actually buy should add a waste allowance on top of this raw floor area, since cutting and pattern-matching always uses a bit more than the bare minimum.
Triangle Area Formula (Base and Height)
Area = 0.5 × Base × Height
Where:
Base = any one side of the triangle, chosen as the reference side
Height = the perpendicular distance from that base to the opposite vertex (not the length of one of the other sides)
Why the Formula Includes 0.5
A triangle is exactly half of a parallelogram built from the same base and height — draw a matching triangle rotated 180 degrees against the first one, and the two together form a parallelogram with area Base × Height. Since the triangle is half of that parallelogram, its area is half the product.
Worked Example: A Garden Bed Triangle
A triangular garden bed with an 8-ft base and a 5-ft height: Area = 0.5 × 8 × 5 = 20 square feet.
Common Mistakes When Measuring for These Formulas
Using a Slanted Side Instead of the Height
For the triangle formula, height must be measured perpendicular (at a right angle) to the base — not along one of the triangle’s other two sides. If the triangle isn’t a right triangle, the height often falls inside the shape rather than along an edge, so it may need to be measured or calculated separately rather than read directly off a tape measure.
Mixing Units
Both measurements need to be in the same unit before multiplying. A room measured partly in feet and partly in inches will give a meaningless result unless one is converted to match the other first.
Rectangle and Triangle Area in Real Projects
Flooring, Tile, and Carpet
Rectangular room area is the starting point for estimating flooring materials, though most installers add roughly 10% for waste, cuts, and pattern matching on top of the bare floor area calculated here.
Paint Coverage
Wall area (a rectangle, in most rooms) divided by a paint can’s stated coverage rate gives the number of cans needed — though coverage rates vary by paint brand and surface texture, so always check the specific product’s label rather than assuming a fixed number.
Land and Garden Beds
Triangular garden beds, plot corners, and roof-adjacent yard sections are common places the triangle formula shows up outside a math classroom.
Frequently Asked Questions
How do I find the area of a rectangle?
Multiply its length by its width: Area = Length × Width. Both measurements must be in the same unit.
How do I find the height of a triangle if I only know the three side lengths?
This calculator’s Triangle mode takes base and height directly. If you only know three side lengths, you’d need Heron’s formula (base and height aren’t required in that case) — a different calculation than the one used here.
Does the base and height formula work for any triangle, not just right triangles?
Yes — as long as height is measured as the perpendicular distance from the chosen base to the opposite vertex, the 0.5 × Base × Height formula works for any triangle, right or not.
Why do flooring and carpet estimates end up higher than the raw square footage?
The area calculated here is the bare floor area. Actual material purchases typically add a waste allowance for cuts, pattern matching, and irregular room edges, which isn’t part of the geometric area itself.
Can I use this calculator for a square?
Yes — a square is just a rectangle with equal length and width, so entering the same value for both gives the correct area.