Square Footage Calculator: Area of Shapes & Rooms

Q: What's the formula for a ring's area?

π × (R² − r²) — outer radius R, inner radius r. Same as the area of the outer circle minus the area of the inner circle.

Q: How do I get area from three sides of a triangle?

Heron's formula: semi-perimeter s = (a + b + c) / 2, then area = √[s(s−a)(s−b)(s−c)]. The three sides have to form a valid triangle (any two sides must add up to more than the third). We check that for you here.

Q: What's the difference between area and surface area?

Area is the size of a flat 2D shape (floor, wall, lot). Surface area is the total of all faces on a 3D object (box, ball). This tool does 2D area; for 3D you'd add up each face.

Q: How do I calculate the area of a sector?

Sector (pie slice): (θ/360) × πr² for area, (θ/360) × 2πr for the curved arc length. θ is the angle in degrees, r is the radius. We show both.

Q: What's the formula for parallelogram area?

A = b × h — base times perpendicular height (not the slanted side). Same as a rectangle: a parallelogram can be cut and rearranged into one.

Square Footage Calculator: Area, Perimeter & Units

Area and perimeter for nine shapes—rectangles, circles, triangles, trapezoids, sectors, rings, and more. Change the unit and everything converts.

What this calculator does

Nine shapes

From rectangles and circles to trapezoids, sectors, and rings. Each shape uses the right formula; for triangles we check that your three sides actually form a valid triangle.

One unit, all inputs

Pick feet, meters, or another unit and every field updates. No need to convert by hand.

Perimeter and arc length

Handy for trim, fencing, and edging. Every shape gives you perimeter or circumference; sectors also show arc length.

Labeled diagrams

Each shape has a diagram so you can see which dimension goes where.

Square Footage Calculator: Area of Rectangle, Circle, Triangle & More

Free square footage calculator for nine shapes. Area, perimeter, and arc length in feet, meters, or any unit—conversions update as you go.

Area vs. surface area

2D vs 3D

Area (2D):

Floors, walls, lawns, patios—any flat surface. We do this here.
Surface area (3D):

Every face of a box, ball, or cylinder. You have to find each face and add them up.

Area is for flat shapes; surface area is for 3D objects.

Formulas at a glance

Rectangle:

A = L × W
L length, W width.
Rectangle border (hollow):

A = (L_outer × W_outer) − (L_inner × W_inner)
Outer minus inner rectangle.
Parallelogram:

A = b × h
Base × perpendicular height. Same as rectangle (parallelogram can be rearranged into one).
Trapezoid:

A = ½(a + b) × h
a, b parallel bases; h height.
Circle:

A = πr²
Circumference C = 2πr.
Ring (annulus):

A = π(R² − r²)
R outer, r inner radius.
Sector (pie slice):

A = (θ/360) × πr²
Arc length (θ/360) × 2πr. θ in degrees.
Triangle (base × height):

A = ½b × h
Perpendicular height.
Triangle (Heron):

s = (a+b+c)/2
A = √[s(s−a)(s−b)(s−c)]
Sides a, b, c. Need: any two sides sum > third.

Where you'll use it

Common uses

Flooring and carpet:

Get square footage, then add 5–10% for cuts and waste.
Paint:

Wall area for coverage. Most paint is 350–400 sq ft per gallon—check the can.
Landscaping:

Beds, patios, paths. Use circles and sectors for curves; add shapes together for odd layouts.
Ordering materials:

Suppliers use sq ft, sq yd, or sq m. Convert once you have your area.

FAQ

How do I calculate the area of an L-shaped room?

Split it into two rectangles, find each area (length × width), then add them. Example: 10 ft × 8 ft = 80 sq ft and 6 ft × 4 ft = 24 sq ft → 104 sq ft total.

What's the formula for a ring's area?

π × (R² − r²) — outer radius R, inner radius r. Same as the area of the outer circle minus the area of the inner circle.

How do I get area from three sides of a triangle?

Heron's formula: semi-perimeter s = (a + b + c) / 2, then area = √[s(s−a)(s−b)(s−c)]. The three sides have to form a valid triangle (any two sides must add up to more than the third). We check that for you here.

What's the difference between area and surface area?

Area is the size of a flat 2D shape (floor, wall, lot). Surface area is the total of all faces on a 3D object (box, ball). This tool does 2D area; for 3D you'd add up each face.

How do I calculate the area of a sector?

Sector (pie slice): (θ/360) × πr² for area, (θ/360) × 2πr for the curved arc length. θ is the angle in degrees, r is the radius. We show both.

What's the formula for parallelogram area?

A = b × h — base times perpendicular height (not the slanted side). Same as a rectangle: a parallelogram can be cut and rearranged into one.

DIY Estimation Note

Estimates Only: These calculators provide theoretical estimates based on standard dimensions. Material density, waste factors, and specific project conditions vary significantly.

Verify Locally: Always verify measurements and material requirements with a professional contractor or local building codes before purchasing supplies or starting work.

Project Accuracy: CalcRegistry is not responsible for material shortages, overages, or structural issues resulting from the use of these general estimates.

Square Footage Calculator

Shape & dimensions

Cost estimator