How to Calculate Square Footage of a Room (With Worked Examples)

Knowing how to calculate square footage of a room is the first step to any accurate material estimate. The math is simple. The mistakes happen when you measure the wrong thing, forget a space, or skip the closet.

This guide covers every room shape you’ll run into: plain rectangles, L-shaped rooms, rooms with bay windows or closet alcoves, and how to handle walls when estimating paint or drywall. Worked examples included.

To skip ahead and run the numbers, use our square footage calculator. But check the section on your room shape first so you know you’re measuring the right points.

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Quick Answer: How to Calculate Square Footage of a Room

For a rectangular room, the formula is:

Length (ft) x Width (ft) = Square Feet

Measure wall to wall at floor level. A room that is 12 feet long and 10 feet wide is 120 square feet.

For non-rectangular rooms, break the space into rectangles (and sometimes triangles), calculate each section, then add them together.

That covers 90% of rooms. The rest of this guide covers the 10% that trips people up.

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What You Need Before You Start

• A tape measure (25-foot minimum; 35-foot is better for larger rooms)
• A notepad or phone notes app to record each measurement
• A simple sketch of the room layout (does not need to be to scale)

The sketch is worth doing even for simple rooms. It gives you a place to record measurements as you go, and it catches forgotten spaces like closets and alcoves before they become ordering mistakes.

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Rectangular Rooms: The Standard Calculation

Most rooms are close to rectangular. Even if the corners are not perfectly square, measuring wall to wall and multiplying length by width gives you an accurate enough number for any material estimate.

Step-by-Step

1. Measure the longest wall in the room, wall to wall at floor level.
2. Measure the wall perpendicular to it.
3. Multiply: Length x Width = Square Feet.

Record inches as decimals: 6 inches = 0.5 ft, 3 inches = 0.25 ft, 9 inches = 0.75 ft.

Worked Example: Standard Bedroom

A bedroom measures 13 feet 6 inches by 11 feet:

• Length: 13.5 ft
• Width: 11 ft
• 13.5 x 11 = 148.5 square feet

For a flooring order, you would add waste factor on top of this number. For paint, you need wall area instead (covered below).

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L-Shaped Rooms: The Two-Rectangle Method

L-shaped rooms are the most common irregular shape in residential construction. The kitchen opens into a dining area, or a living room has a step-down section. The approach is always the same: split it into two rectangles.

Step-by-Step

1. Draw the L-shape on your notepad.
2. Draw a line dividing it into two rectangles (you choose where to split; pick the line that makes the measurement easiest).
3. Measure and calculate each rectangle separately.
4. Add the two areas together.

Worked Example: L-Shaped Living and Dining Room

A combined living/dining space is L-shaped. The main section runs 18 feet by 14 feet. The dining wing extends 9 feet by 10 feet beyond one corner.

Section	Dimensions	Square Feet
Living room main section	18 ft x 14 ft	252 SF
Dining wing	9 ft x 10 ft	90 SF
Total room		342 SF

18 x 14 = 252 SF. 9 x 10 = 90 SF. 252 + 90 = 342 SF.

Where you draw the dividing line does not change the total. Pick whichever split makes the measurement easiest to run.

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Rooms with a Closet Alcove or Bay Window

These are rectangles with a bump added to them. Measure the main rectangle first, then measure the bump-out separately, and add.

Worked Example: Bedroom with Closet Alcove

A bedroom measures 12 feet by 11 feet. There is a closet alcove at one end that adds 4 feet of depth across a 5-foot span of the back wall.

Section	Dimensions	Square Feet
Main bedroom	12 ft x 11 ft	132 SF
Closet alcove	4 ft x 5 ft	20 SF
Total		152 SF

12 x 11 = 132 SF. 4 x 5 = 20 SF. 132 + 20 = 152 SF.

The same math applies to a bay window: measure the main room rectangle, then measure the bay as a separate rectangle (or occasionally a shallow trapezoid, in which case treat it as a rectangle using the widest dimension and accept a slight overcount).

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Rooms with Angled Walls or Cut Corners

Less common, but they show up in rooms under rooflines, in older homes, and occasionally in master bedrooms and bonus rooms.

The Triangle Subtraction Method

If one corner is cut at a diagonal, calculate the full rectangle as if the corner were square, then subtract the triangle.

Triangle area = (Base x Height) / 2

Worked Example: Bonus Room with Angled Corner

A bonus room measures 14 feet by 12 feet overall, but one corner is cut at 45 degrees. The angled cut removes a triangle that is 3 feet along each wall.

• Full rectangle: 14 x 12 = 168 SF
• Triangle cut-out: (3 x 3) / 2 = 4.5 SF
• 168 - 4.5 = 163.5 SF

For material ordering, round to the nearest whole square foot. That room is 164 SF.

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Wall Square Footage: For Paint and Drywall

Floor square footage and wall square footage are two different numbers. If you are painting or hanging drywall, you need wall area, not floor area.

The Wall Area Formula

(Perimeter x Ceiling Height) - Door and Window Openings = Wall Square Feet

Perimeter is the total length of all four walls added together. Ceiling height is typically 8, 9, or 10 feet.

What to Deduct

• Standard interior door: approximately 20 SF (2.5 ft wide x 8 ft tall = 20 SF)
• Standard window: approximately 12-15 SF (estimate 3 ft x 4 ft = 12 SF per window)

Worked Example: Painting a 13 x 11 Bedroom

Room dimensions: 13 ft x 11 ft, 9-foot ceilings, one door, two windows.

Component	Calculation	Area
Perimeter	13 + 11 + 13 + 11 = 48 ft	-
Wall area (gross)	48 ft x 9 ft ceiling	432 SF
Subtract 1 door	2.5 ft x 8 ft	-20 SF
Subtract 2 windows	2 x (3 ft x 4 ft)	-24 SF
Net wall area		388 SF

One gallon of paint covers 350-400 SF. That room needs about one gallon per coat. Two coats means two gallons.

Use the paint calculator to run this automatically, or the drywall calculator if you need sheet counts for a new build or remodel.

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Common Mistakes When Measuring a Room

These are the mistakes that cause material shortages and reorders. Every one of them is avoidable.

Mistake 1: Measuring the Exterior Wall Instead of the Interior

You measure the floor area inside the room, not the outside of the building. Exterior wall measurements include wall thickness (roughly 6 inches per wall for wood frame, more for masonry), which adds area that does not exist inside.

Always measure wall to wall inside the room, at floor level.

Mistake 2: Forgetting the Closet

A standard closet (2 ft x 5 ft) is 10 SF. A master walk-in (6 ft x 8 ft) is 48 SF. If the closet is getting the same flooring, it goes in your total. Sketch the room first. Closets drawn on paper do not get forgotten. Closets you are “pretty sure are back there” do.

Mistake 3: Measuring to the Baseboard, Not the Wall

Baseboards sit proud of the wall by about 0.75 inches. For most rooms, this does not matter. For a precise measurement, set your tape at the wall surface behind the baseboard, not the baseboard face.

Mistake 4: Not Converting Inches to Decimal Feet

Multiplying 12 feet 6 inches by 10 feet 4 inches as written gives the wrong answer. Convert first: 12 feet 6 inches = 12.5 ft, 10 feet 4 inches = 10.33 ft. Then multiply: 12.5 x 10.33 = 129.1 SF.

A quick conversion chart:

Inches	Decimal Feet
1 in	0.08 ft
2 in	0.17 ft
3 in	0.25 ft
4 in	0.33 ft
6 in	0.50 ft
8 in	0.67 ft
9 in	0.75 ft
10 in	0.83 ft

Mistake 5: Treating One Number as Good for Every Purpose

Floor square footage is what you need for flooring. Wall square footage is what you need for paint or drywall. Ceiling square footage is floor area again (assuming a flat ceiling). These are not interchangeable. Run the right calculation for the right material.

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How Square Footage Connects to Material Estimates

Once you have accurate room square footage, it connects directly to your material calculations:

Flooring (LVP, hardwood, tile, carpet): Floor SF x waste factor = order quantity. Waste runs 7% for LVP, 10% for hardwood, 15% for tile. Use our flooring calculator to get the right quantity by material.

Paint (walls): Calculate wall SF as described above, divide by coverage per gallon (350-400 SF), and multiply by coats.

Tile (floor or wall): Same base formula as flooring for floor tile. For wall tile, calculate the tile field area separately. Our tile calculator handles both.

Drywall: Wall SF plus ceiling SF divided by 32 SF per sheet (standard 4x8) gives sheet count before waste. Add 10% for cuts.

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Quick Reference: Room Type Formulas

Room Shape	Formula
Rectangle	Length x Width
L-shape	Rectangle A + Rectangle B
Room with bump-out	Main rectangle + bump-out rectangle
Room with cut corner	Full rectangle - triangle (Base x Height / 2)
Wall area (for paint/drywall)	Perimeter x Ceiling Height - openings

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Frequently Asked Questions

How do I calculate the square footage of an odd-shaped room?

Break it into rectangles and triangles. Draw the room shape on paper, then draw lines to divide it into simple shapes. Calculate each shape separately and add the areas together. If you have a curved wall, treat it as a straight line at the widest point. You will be slightly over, which is the right side to be on for material ordering.

Do I include the closet in room square footage?

Yes, if you are ordering flooring or measuring for a remodel, include every closet that is getting the same treatment. A walk-in closet is its own room and should be measured as such. Small reach-in closets are just narrow rectangles. Measure width times depth and add to your room total.

How accurate do I need to be?

For material orders, within 5% is solid. You will add waste factor on top anyway, which creates a buffer. Being off by 3-4 SF on a single room will not cause a material shortage. Being off by 50-100 SF because you skipped three closets and a hallway will.

For contractor bids, accuracy matters more because the square footage drives both material cost and labor hours. Measure carefully and record every space.

What is the difference between floor square footage and room square footage?

They are the same thing. Room square footage is measured at floor level. The floor area equals the ceiling area in a room with a flat ceiling. Wall area is calculated separately using perimeter times height.

Can I use a laser measuring tool instead of a tape?

Yes. Laser distance meters are accurate to within 1/16 inch and are faster than a tape for large rooms. They work best when you have a flat wall surface to shoot at. They are less reliable in rooms with deep alcoves where the beam does not land cleanly on the far wall.

I measured my room but it does not match the floor plan. Which one is right?

Use your measurement. Floor plans show design intent, not as-built conditions. Walls shift during framing, and plans do not account for finished wall-to-wall dimensions. The number you take on site is the one you order from.

How do I calculate square footage for flooring if my room has a fireplace hearth?

Treat the hearth as a negative rectangle. Measure the main floor area, then measure the hearth footprint (the raised portion not getting flooring), and subtract. If the hearth is flush with the floor and getting the same material, include it.

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Measure Right the First Time

The math is not complicated. Length times width gets you there for any rectangular space. L-shapes, alcoves, and bump-outs just mean adding a second or third rectangle. The discipline is measuring every space, converting inches correctly, and using the right formula for the right material.

A tape measure, a notepad, and 15 minutes. Plug the numbers into the square footage calculator, then run the totals through the flooring calculator if you are ordering material. Get the measurement right first. Everything else follows.

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Measurements and material calculations in this guide are based on standard residential construction practices. Actual room dimensions, ceiling heights, and waste factors vary by project. Always verify your measurements on site before ordering materials.