Square Footage Calculator Guide — Rooms, Shapes and Area Totals

What This Calculator Does

Calculate square feet from dimensions and combine up to 10 added or subtracted areas. Each region can be a rectangle, circle or triangle and can use its own measurement unit. Results include square metres, square yards, square inches and acres, with an area-by-area formula breakdown.

When to Use It

Use it to estimate the flat area of rooms, floors, walls and other measured surfaces. Split an L-shaped footprint into non-overlapping shapes. For a rectangular wall, use its length and height as the two dimensions and subtract windows or doors separately. Curved surfaces, arbitrary boundaries and sloped-roof projection are not modeled.

Inputs Explained

• Shape: Rectangle takes length and width; Circle takes diameter; Triangle takes a base and the perpendicular height to that base, not two arbitrary sides.
• Operation: Add includes the region; Subtract removes a cutout. Do not subtract the same opening twice.
• Units: Feet & inches, Decimal feet, Inches, Metres or Centimetres. Dimensions within a region share a unit; separate regions may use different units.
• Feet & inches: Feet must be non-negative whole numbers. Inches may be decimal, from 0 to less than 12. The combined dimension must exceed zero. Blank inches count as zero, but blank feet do not.

Use ordinary decimals with a decimal point or comma and no thousands separators. For example, 1,200 is interpreted as 1.2, not twelve hundred. Fractions, scientific notation and text with unit suffixes are not accepted; enter half an inch as 0.5.

Adding a region creates blank dimensions to complete before calculation. Editing, removing, or changing a shape or unit clears the old result. Reset restores one empty rectangle in Add mode using feet and inches. Inputs remain in component state and are not saved to URLs or browser storage.

How the Calculation Works

Normalize dimensions to feet:

• Feet & inches = feet + inches ÷ 12.
• Inches ÷ 12.
• Metres ÷ 0.3048.
• Centimetres ÷ 30.48.

Rectangle area is length × width. Circle area is π × (diameter ÷ 2)². Triangle area is base × perpendicular height ÷ 2. Net area is the sum of added regions minus the sum of subtracted regions. The calculator has no spatial model and cannot detect overlaps.

One square foot is 0.09290304 m² or 144 in². Nine square feet make one square yard; 43,560 square feet make one acre. A length conversion factor must be squared for area: 0.3048 alone does not convert square feet to square metres.

Example

A 12 ft × 10 ft rectangle has 120 ft², or 11.1483648 m². Subtract a 2 ft × 3 ft cutout:

120 − 6 = 114 ft², approximately 10.591 m².

Try example fills both regions. For a single rectangle measuring 12 ft 6 in by 10 ft, the area is 12.5 × 10 = 125 ft².

A circle with a 10 ft diameter has an area of 25π, approximately 78.540 ft². A triangle with a 12 ft base and 10 ft perpendicular height has an area of 60 ft².

How to Understand the Result

The large number is net square footage. Added area and Subtracted area are separate subtotals. Each deducted region has a minus sign in the breakdown. A deduction greater than the added total produces an error; equal totals produce zero net area.

Ordinary numbers display up to three decimal places. Very large or small values use scientific display notation. Internally, calculations use unformatted floating-point numbers, so displayed rounded rows may not sum exactly to the rounded total. Copy result includes each shape, operation, formula in feet and the total.

Combined dimensions must be greater than zero and no larger than one billion feet. Each region must have 1e-9 to 1e15 square feet, and each subtotal must not exceed 1e15 square feet. These are computational bounds, not a claim of measurement precision.

Common Mistakes

• Entering 12.6 decimal feet for 12 feet 6 inches.
• Entering a circle's radius as its diameter.
• Treating a triangle's sloping side as its perpendicular height.
• Double-counting overlapping regions or subtracting the same cutout twice.
• Ordering material from net area without separately considering cutting waste.

FAQ

Can I include several identical rooms?

Yes, add each room as a separate region, up to 10 regions. There is no quantity multiplier in this calculator.

Can I enter 12 in the inches field?

Not in Feet & inches mode: carry that into one extra foot and zero inches. To enter a total number of inches directly, choose Inches as the unit.

Is this gross floor area or usable area?

It is a geometric calculation. Decide which walls, balconies or other features to include according to the measurement convention your project needs.

Notes

Conversions use the international foot definition; see the NIST conversion tables. This calculator does not estimate cost, specify structural capacity or certify property area.

Use the area converter for an area you already know, the length converter for individual dimensions, or the gravel calculator when you also need a layer's volume and weight.