Free Square Footage Calculator Online

Square Footage Calculator: Measure the Area of a Rectangle, Circle, or Triangle

A square footage calculator turns a handful of tape-measure numbers into the one figure that almost every home project actually runs on: total area. Flooring is sold by the square foot. Sod and mulch are priced by area before anyone converts to volume. Paint coverage, tile counts, and even a rough materials budget all start from the same starting line — how many square feet (or square meters) is this space? This tool takes the dimensions you already have, applies the right geometric formula for the shape you're measuring, and returns the area instantly, entirely in your browser.

What makes the calculator useful beyond a basic length-times-width multiplication is that most real rooms and yards aren't clean rectangles. A dining nook might be a rectangle plus a triangular bay window. A patio might be circular. A finished basement is frequently an irregular collection of rectangular rooms that need to be summed into one total. This tool handles three base shapes — rectangle, circle, and triangle — and a multi-room mode that adds up several rectangular spaces at once, so you get one accurate number instead of stitching together several manual calculations.

How to Calculate Square Footage with This Tool

The layout keeps every step visible so you can double-check the inputs before trusting the result:

1. Pick the shape you're measuring — Rectangle, Circle, Triangle, or Multiple Rooms — from the buttons at the top.
2. Choose the unit your tape measure or blueprint uses: feet, meters, or inches. Whatever unit you pick applies to every length field in that shape, so there's no need to convert anything by hand first.
3. Enter the dimensions for the shape you selected: length and width for a rectangle, radius for a circle, or base and perpendicular height for a triangle. In Multiple Rooms mode, add a row per room with its own length and width, and remove rows you don't need.
4. Optionally enter a cost per square foot. If you supply one, the tool multiplies it by the total area to give you a ballpark materials or labor estimate alongside the area itself.
5. Read the result card: the area appears in both square feet and square meters at once, so you don't need a second lookup if you're working from metric plans or comparing to a spec sheet in the other unit.

Everything recalculates the instant you change a number — there's no submit button, no page reload, and no file to upload, because the whole computation is arithmetic your device finishes long before you notice a delay.

Why People Need to Measure Square Footage

Square footage shows up as the first number in a surprising range of home and yard projects:

• Buying flooring, carpet, or laminate. Retailers price and sell these materials by the square foot or square meter, and getting the area wrong by even a small margin means either an awkward mid-project run back to the store or an expensive pile of unused planks.
• Estimating a paint or drywall job. Wall and ceiling area feeds directly into how many gallons of paint or sheets of drywall a room requires, and the room's footprint is usually the first measurement a contractor asks for on the phone.
• Pricing landscaping and lawn work. Sod, seed, and fertilizer are typically sold per thousand square feet, and an oddly-shaped yard often has to be broken into a rectangle plus a triangular or circular section to get an accurate total.
• Real estate and rental listings. Buyers and renters compare properties on square footage, and a quick independent check against a floor plan can catch a listing error before it becomes a bigger misunderstanding.
• Planning a tile, deck, or patio layout. Before pricing out tile or decking boards, you need the base area — this calculator's output is usually the very first number that gets fed into a materials calculator further down the project.

The Formulas Behind Each Shape

Each shape uses a standard geometry formula, applied after every length is converted to feet internally so the math stays consistent regardless of which unit you typed in:

Shape	Formula	Inputs needed
Rectangle	length × width	Length, width
Circle	π × radius²	Radius (not diameter)
Triangle	0.5 × base × height	Base, and height measured perpendicular to that base
Multiple rooms	Sum of (length × width) for each room	Length and width per room

A worked rectangle: a 10-foot by 12-foot room comes out to 120 square feet, or about 11.15 square meters. A worked circle: a round patio with a 5-foot radius covers roughly 78.54 square feet (about 7.30 square meters) — note that's radius, not the 10-foot span across the patio, which is a common mix-up covered below. A worked triangle: a gable end with a 6-foot base and a 4-foot perpendicular height covers 12 square feet (about 1.11 square meters). Scale those same formulas up and an 18-foot by 24-foot garage floor works out to 432 square feet (roughly 40.13 square meters), and a circular fire pit patio with a 7-foot radius covers about 153.94 square feet.

Multi-room mode doesn't introduce a new formula — it just applies the rectangle formula once per room you add and totals the results, which is the same thing you'd do with a calculator and a scratchpad, minus the risk of a transposed digit partway through adding up five or six rooms.

This is also how the tool handles an L-shaped or otherwise irregular room, even though "L-shaped" isn't a shape option on its own: split the room into two or three rectangles on paper first, then enter each piece as its own row in Multiple Rooms mode. A living room with a bumped-out reading nook, for instance, becomes a main rectangle plus a smaller rectangle for the nook, and the tool sums both into one accurate total instead of forcing you to approximate the whole footprint as a single rectangle and either overstate or understate the real area. The same trick works for a wraparound porch, a garage with a storage bump-out, or basically any floor plan that isn't a clean box.

Common Mistakes When Measuring Area

A short list of errors accounts for most of the wrong totals people bring into a materials estimate. Confusing radius and diameter on a circle is probably the single most frequent one — if you measured across the full width of a round table, patio, or planting bed, that's the diameter, and you need to halve it before entering the radius field, or the calculator will return an area four times too large. For triangles, the height has to be the perpendicular distance from the base to the opposite point, not the length of one of the slanted sides; using a slanted edge by mistake inflates the area. Mixing units within a single shape is another trap — if you measured one wall in feet and another in inches, convert everything to the same unit before entering it, since the calculator applies one unit to every field in the shape you've selected. Finally, remember that a raw room measurement usually includes wall thickness and sometimes closets or nooks that a flooring or paint estimate needs to exclude, so treat the calculator's output as the geometric area of the shape you entered, not automatically the exact usable floor space.

Manual Math vs. Using a Calculator

You can absolutely work these formulas out with a phone calculator and a notepad, and plenty of people do:

• Doing it by hand. Length times width is trivial for one rectangle, but a project with several rooms, a circular section, and a cost estimate layered on top means several separate calculations to track and add — a single transcription error anywhere in that chain throws off the final total.
• Construction estimating apps. Dedicated contractor software often bundles square footage into a larger takeoff workflow, which is worth it for a professional running dozens of estimates a week but is overkill for a single home project.
• Graph paper and a scale drawing. Sketching a floor plan to scale and counting grid squares works for irregular shapes, though it's slower and less precise than plugging exact measurements into a formula.
• Spreadsheet formulas. A quick =LENGTH*WIDTH column handles a list of rooms well enough, but you still have to write and check the circle and triangle formulas yourself if the project needs them.
• This tool's metered API. If you're generating estimates programmatically — inside a quoting tool, a contractor's internal app, or a batch job that processes a list of room dimensions — the same rectangle/circle/triangle math is available as a metered API endpoint that returns area in square feet and square meters as JSON; see the API documentation for the request format and credit cost.

Does the circle calculation want the radius or the diameter?

Radius — the distance from the center of the circle to its edge, which is half the distance measured straight across. If you measured across the full circle, divide that number by two before entering it, or the resulting area will be four times too large.

Can I mix units, like measuring one wall in feet and another in inches?

Not within a single calculation. The unit you select applies to every length field for the shape you're working on, so convert all your measurements to one unit — feet, meters, or inches — before entering them.

Does the multi-room total account for hallways, closets, or wall thickness?

Only if you measure and enter them as their own rows. The tool adds up exactly the rectangles you give it; it has no way to know about a closet or hallway you didn't include as a separate room, or to subtract wall thickness from a measurement you made wall-to-wall.

Is the optional cost estimate a real quote?

No — it's the area multiplied by whatever cost-per-square-foot figure you enter, so it's only as accurate as that number. Treat it as a quick back-of-envelope figure to compare scenarios, not a substitute for an actual contractor or supplier quote.

Why does the result show both square feet and square meters?

Different suppliers, countries, and spec sheets default to different units, so showing both at once saves you from running the area back through a separate unit converter when you need to compare against a metric plan or product listing.

How do I handle a room that isn't a rectangle, circle, or triangle?

Break it into simpler pieces first. Sketch the room and divide it into two or more rectangles (or a rectangle plus a triangle for an angled wall), then either add those pieces individually in Multiple Rooms mode or run each shape through the calculator separately and add the totals by hand.

Related Construction Tools

Once you know the area, most projects need a second calculator to turn that number into an actual materials list. Feed the same square footage into the concrete calculator to size a slab or footing, the tile calculator to figure out how many tiles to order, or the mulch and gravel calculator to estimate loose material for a bed or path. For an interior room, the paint calculator takes the same kind of wall dimensions and converts them into gallons needed.