Square Feet Calculator: How to Work It Out

This calculator works out the area of a rectangular space in square feet when you enter its length and width in feet, inches, or a combination of both. You get the total square footage instantly, which is the standard unit for flooring, painting, tiling, and real estate listings in the US. Simply type in the dimensions, pick your unit, and the calculator returns the area in square feet.

Formula

The formula is simple: Area (sq ft) = Length (ft) × Width (ft). If you measure in inches, convert them to feet first by dividing by 12.

• Length – the longer side of the rectangle, in feet (ft) or inches (in).
• Width – the shorter side, in feet (ft) or inches (in).
• Area – the result, in square feet (sq ft or ft²).

For example, a room that is 15 feet long and 12 feet wide has an area of 15 × 12 = 180 sq ft. If you measure in inches, say 180 inches by 144 inches, convert each to feet (180/12 = 15 ft, 144/12 = 12 ft) and multiply.

Worked Example

Using the default values from the calculator: Length = 12 ft 0 in, Width = 10 ft 0 in.

1. Convert inches to feet: 0 in ÷ 12 = 0 ft.
2. Total length = 12 + 0 = 12 ft.
3. Total width = 10 + 0 = 10 ft.
4. Multiply: 12 × 10 = 120 sq ft.

The calculator shows 120.00 sq ft as the area, matching the manual calculation.

When to Use Square Footage

Square footage is the standard measure for many home improvement and real estate tasks. You need it when:

• Buying flooring, carpet, or tile – you need the area to estimate how much material to purchase.
• Painting walls or ceilings – you need the surface area to estimate paint coverage.
• Listing or renting a property – real estate ads quote price per square foot.
• Installing sod, gravel, or mulch – you need the area to calculate volume or weight.
• Calculating HVAC load – room size affects heating and cooling requirements.

For irregularly shaped rooms, break the space into rectangles, calculate each area, and add them together.

How to Measure Correctly

Accurate measurement is key. Here are practical steps:

1. Clear the area of furniture or obstructions.
2. Use a tape measure or laser distance measurer.
3. Measure the longest length and widest width, ignoring alcoves or closets for a rough estimate.
4. Record measurements in feet and inches; for example, 12 ft 6 in.
5. Convert inches to decimal feet (6 in ÷ 12 = 0.5 ft) before multiplying.
6. Multiply length by width to get square feet.

If the room is L-shaped, divide it into two rectangles, measure each, and add the areas.

Common Mistakes and How to Avoid Them

Even with a simple formula, errors happen. The most frequent are:

• Mixing units – multiplying feet by inches without converting. Always convert to the same unit first.
• Rounding too early – if you round 12 ft 6 in to 13 ft, you overestimate by 0.5 ft. Keep the decimal until the final answer.
• Forgetting to multiply – some people add length and width instead of multiplying. Area is always multiplication.
• Measuring the wrong dimension – for a rectangle, you need the two perpendicular sides, not the diagonal.

Double-check your measurements and use the calculator to verify your manual math.

Limitations of the Formula

The length × width formula only works for rectangles (including squares). It does not apply to:

• Triangular rooms – use (base × height) / 2.
• Circular areas – use π × radius².
• Irregular shapes – break into smaller rectangles, triangles, or circles.

Also, the formula assumes a flat, two-dimensional surface. If you are measuring walls, you need the height as well, and for volume you need a third dimension (cubic feet).

Square Feet vs. Other Units

Square feet is common in the US, but other countries use square meters. To convert:

• 1 sq ft = 0.092903 sq m
• 1 sq m = 10.7639 sq ft
• 1 sq ft = 144 sq in
• 1 sq yd = 9 sq ft

If you have measurements in meters, multiply length × width to get square meters, then multiply by 10.7639 to get square feet. Conversely, divide square feet by 10.7639 to get square meters.

Frequently Asked Questions

How do I calculate square feet from inches?

Convert each inch measurement to feet by dividing by 12, then multiply length × width. For example, 120 in × 60 in = (120/12) × (60/12) = 10 × 5 = 50 sq ft.

What is the difference between square feet and feet?

Feet measure length (one dimension), while square feet measure area (two dimensions). A room that is 10 ft by 10 ft has an area of 100 sq ft, but its perimeter is 40 ft.

How many square feet is a 12×12 room?

A 12 ft by 12 ft room is 144 sq ft. This is a common size for a small bedroom or office.

Do I include closets in square footage?

For real estate listings, usually yes, if the closet is inside the heated living area. For flooring purchases, measure the actual floor area you need to cover, including closets if you plan to floor them.

How do I calculate square feet for a triangle?

Measure the base and the height (the perpendicular distance from the base to the opposite corner). Multiply base × height, then divide by 2. For example, a triangle with base 10 ft and height 6 ft is 10 × 6 / 2 = 30 sq ft.

What if my room is not a perfect rectangle?

Break the space into rectangles, triangles, or other shapes, calculate each area, and add them together. For a more precise result, use a floor plan and divide it into regular shapes.

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in len_ft | Length (feet) | ft | 12 | 0
in len_in | Length (inches) | in | 0 | 0
in wid_ft | Width (feet) | ft | 10 | 0
in wid_in | Width (inches) | in | 0 | 0
out area | Area | sq ft | (len_ft + len_in/12) * (wid_ft + wid_in/12) | 2