Knowing how to calculate the square metres of a room isn’t just about maths—it’s the foundation of any interior design, renovation, or property sale project. Whether you’re planning to renovate your home, lay a new floor, or simply paint the walls, knowing the exact surface area is essential.

What are square metres?

Square metres (m²) are a unit of measurement used to calculate the surface area of a space. In architecture and interior design, this unit is crucial. One square metre represents a square that is one metre long on each side. For example, if a room is 4 metres wide and 5 metres long, its surface area will be 20 m².

Sounds simple, right? And it is. But truly understanding how square metres are applied allows you to make smart decisions when buying materials, reorganising space, or planning a refurbishment.

Square metres aren’t just used for floors—they’re also used for walls, ceilings, and even façades. You’ll find them in blueprints, legal documents, quotes, and land registry records. So knowing how to calculate them accurately gives you a real advantage.

Why is it important to know the square metres of a room?

Imagine buying paint and running out. Or hiring a flooring installer who overcharges you because you didn’t measure correctly. Knowing how to calculate the square metres of a room helps you avoid all that. It gives you control, clarity, and the ability to make informed decisions.

Here are some of the main reasons to master this calculation:

  • Accurately budget for materials

  • Avoid waste or unnecessary purchases

  • Optimise space—especially in small renovations

  • Apply for permits correctly

  • Assess a property’s value in the housing market

  • Comply with minimum living space regulations

Even if you’re not a professional, knowing how many square metres your room has can be useful when renting or buying a property.

Tools you need to calculate square metres

Before you start, make sure you have these basic tools:

  • Tape measure (preferably long and metal)

  • Notepad or phone to record measurements

  • Calculator (or your phone’s calculator app)

  • Laser level (optional, but useful for more precise measurements)

  • Ruler or measuring tape for smaller details

You can also use mobile apps that do the calculation for you, although it’s a good idea to take a manual measurement first to get a clear sense of the space.

Step by step: How to calculate the square metres of a room

The easiest way to do this is by using the basic formula:

Area = Length × width

  1. Measure the length of the room (in metres)

  2. Measure the width of the room (in metres)

  3. Multiply both measurements: that’s the total number of square metres

Practical example:
A room measures 4 metres in length and 3 metres in width.
4 × 3 = 12 m²

And that’s it. Easy as that. However, not all rooms are that simple…

How to calculate if the room is square or rectangular

A square or rectangular room is the easiest to work with. Just make sure to measure from opposite walls and keep the tape measure straight and taut. If you apply the formula correctly, there’s little room for error.

To check if the room is rectangular:

  • Ensure the opposite walls are the same length

  • Measure the diagonals—if they are equal, it’s a perfect rectangle

Rooms with irregular shapes: What to do?

Rooms often come in L-shapes, with awkward corners or protruding columns. In these cases, it’s best to break the space down into simpler shapes:

  • Visually divide the floor plan into rectangles and squares

  • Calculate the area of each section

  • Add all the areas together to get the total

Example:
A room with a small extension measures 5 m × 3 m in one part and 2 m × 1 m in another.
Total area = (5 × 3) + (2 × 1) = 15 + 2 = 17 m²

Common mistakes when measuring a room

Avoid these errors to ensure an accurate measurement:

  • Not measuring in a straight line

  • Ignoring skirting boards or uneven surfaces

  • Forgetting small spaces like niches or corners

  • Rounding figures incorrectly

  • Measuring in inches or feet without converting

Tips for more accurate measurements

  • Always measure twice

  • Use a long tape measure to avoid joining segments

  • Measure at the widest and longest points

  • Move furniture if possible

  • Use a pencil to mark measurement points

Apps to easily calculate square metres

These apps can help you:

  • RoomScan Pro

  • MagicPlan

  • Measure (by Apple)

  • Google Measure (Android)

  • Houzz Pro (for professionals)

Most of them allow you to take a photo or scan the room to get automatic measurements.

When is it better to hire a professional?

When:

  • The room has a complex shape

  • There are uneven floors or sloped ceilings

  • A certified floor plan is required

  • It’s for an official property valuation

  • You need to apply for building permits

Calculating the square metres of a room for painting

When you're planning to paint a room, it's not just the floor you're interested in. You need to know the square metres of the walls and ceiling. For this, a different type of calculation is required:

Walls:

  • Measure the height and width of each wall

  • Multiply height × width for each wall

  • Add all the surface areas together

  • Subtract the areas of doors and windows

Example:

Two walls measuring 4 m × 2.5 m → 4 × 2.5 × 2 = 20 m²
Two walls measuring 3 m × 2.5 m → 3 × 2.5 × 2 = 15 m²
Total: 35 m²

Door: 2 m²
Final result: 33 m² to paint

Ceiling:

Simply calculate length × width, just as you would for the floor.

Calculating the square metres of walls for wallpapering or cladding

This is very useful for:

  • Wallpaper

  • Decorative panels

  • PVC or wood cladding

Step by step:

  • Measure each wall (height × width)

  • Add the total area

  • Subtract doors and windows

And don’t forget to add an extra 10% for overlaps or mistakes!

Calculating the square metres of a room for new flooring

When installing new flooring (wood, tiles, carpet, vinyl…), knowing the exact square metres is essential to buy the right amount of material. But also:

  • Add 10% extra for cuts and waste

  • If there are columns or curved areas, waste may reach up to 15%

Practical example: A room of 18 m² → you should buy at least 19.8 m² of material

Calculating square metres for furniture and layout

When planning furniture placement, you need to consider the actual space it will occupy.

  • Mark out the furniture dimensions with tape

  • Calculate its area (length × width)

  • Subtract this from the total room size to determine the free space

This is extremely helpful for 3D designs, space optimisation, and avoiding mistakes like “the sofa doesn’t fit.”

Factors that can affect the calculation

  • Columns

  • Mezzanines

  • Stairwells

  • Bay windows

  • Uneven levels

All of these can alter the calculation. You’ll need to decide whether to include or exclude them depending on the purpose of the measurement.

Difference between usable and built area

This concept is important when buying or renting property.

Usable area: the space you can actually use. It does not include walls or structural elements.
Built area: includes walls, pillars, installation shafts, etc.

Example: A flat with 90 m² built may have only 72 m² of usable space.

How to record square metres in legal documents

If you need to register measurements for the land registry, notary, or property deeds, it’s essential that:

  • An accredited professional (architect or technical surveyor) carries out an official measurement

  • The report includes a sketch or floor plan

  • The calculation follows local regulations

What if the measurements aren’t exact?

No house is perfect. There may be:

  • Walls that aren’t completely straight

  • Differences between floor and ceiling dimensions

  • Internal columns

In these cases, it’s best to:

  • Measure in several places and take an average

  • Round to the nearest tenth

  • Consult a professional if legal accuracy is required

How to use a floor plan to calculate square metres

If you have access to a floor plan:

  • Use the graphic scale shown on the plan

  • Measure with a ruler (e.g. 1 cm = 1 m)

  • Apply area calculations

Floor plans allow you to take measurements even without being physically present in the space.

Converting other units to square metres

 
Unit Conversion to m²
Square feet 1 ft² = 0.0929 m²
Square yards 1 yd² = 0.8361 m²
Hectare 1 ha = 10,000 m²

Use these conversions when working with foreign plans or furniture purchased from abroad.

Measurements during renovations: Before and after

Measure the room before starting any renovation. Once the work is complete, measure again. This allows you to:

  • Compare surface area gained or lost

  • Check that the builder has complied with the plans

  • Update floor plans or property deeds

Recommendations for renting or selling a property

  • Ensure the listing matches the actual surface area

  • Specify whether the square metres are built or usable

  • Include floor plans to give buyers greater confidence

Measuring outdoor spaces

If you want to measure terraces, balconies, or patios:

  • Use the same formula: length × width

  • Be aware of local regulations: these areas are not always counted as 100% usable

Some covered terraces may count as only 50% of usable space in certain urban areas.

FAQs on how to calculate the square metres of a room

What if the room is L-shaped?
Divide the room into two rectangles, calculate each one separately, and add them together.

Should I include the space under the bed or wardrobe?
Yes, if it’s part of the free floor area. No, if it’s a fixed or built-in structure.

Can I use my phone to measure?
Yes. There are augmented reality apps that are fairly accurate, although it’s always best to double-check with a tape measure.

Which area counts when selling my home?
The built area, although it’s advisable to also state the usable square metres.

Are walls included in the square metre calculation?
Yes, in the built area. No, in the usable area.

What margin of error is acceptable in measurements?
A 2–3% margin is acceptable for manual measurements. For official plans, professional accuracy is required.

Opinion

Now that you know how to calculate the square metres of a room, you're one step closer to making smarter decisions for your home or investment. From painting projects to full renovations, this knowledge empowers you.

At La Quinta Fachada Arquitectura, we believe in making architecture accessible through clear, useful, and practical information. If you found this article helpful, feel free to share it—or get in touch if you need a professional measurement. We're here to help!