How to Measure a Round or Angled Space

What you need

A tape measure, a pencil and paper, and a piece of chalk or string for marking lines on the floor. Measure in the unit your supplier uses — feet or metres — and write every number down as you go, with a sketch. A sketch with numbers beats a memory every time, especially when you are standing in a shop an hour later.

Step 1 — Round spaces: measure the diameter

For a round room, measure the diameter — the widest distance straight through the centre. Do not measure the circumference and guess: the area comes from the diameter. The square footage calculator has a circle mode: enter the diameter and it gives you the area in square feet, square metres and square yards. The formula is area = π × (diameter ÷ 2)² — the radius squared, times pi.

Step 2 — L-shaped spaces: split them in two

For an L-shaped room, split it into two rectangles, measure each one and add the areas. Measure each rectangle wall to wall, at floor level, and sketch the split so you can see it. Never measure an irregular room as one big rectangle: you will over-order materials for the part of the rectangle that does not exist, and under-order for nothing at all.

Step 3 — Angled walls: use a diagonal

For a room with an angled wall, measure the longest wall, the width at right angles to it, and a diagonal across the angle. The diagonal is your check: in a square corner, a 3 ft side and a 4 ft side give a 5 ft diagonal (the 3-4-5 triangle). If the diagonal is not 5 ft, the corner is not square, and you should measure the angle as its own small triangle rather than assuming it. A bay window is the same idea: measure it as its own small rectangle and add it to the main room.

Step 4 — Convert the units before you order

Suppliers use different units — square feet, square metres, square yards — and ordering in the wrong one is an expensive mistake. The area converter converts between them: enter the area and the units, and it gives you the exact figure. Convert before you order, not after the delivery arrives.

Worked example

A round room with a diameter of 12 ft has an area of π × 6² = 113.1 ft² (10.51 m², or 12.57 yd²). An L-shaped room split into 12 ft × 10 ft and 6 ft × 4 ft is 120 ft² + 24 ft² = 144 ft² in total. Those are exactly the numbers the square footage calculator gives you, and 113.1 ft² converts to 10.51 m² in the area converter.

Common mistakes

• Measuring the circumference instead of the diameter. The area formula needs the diameter (or radius). A circumference gives you a much bigger number and a much bigger order.
• Measuring an L-shape as one rectangle. The missing corner is not floor. Split the shape and add the real rectangles.
• Forgetting to convert units. Square feet ordered as square metres leaves you short by a factor of ten. Convert before you order.

Safety

Measuring is low-risk work, but watch for trip hazards — loose rugs, cables and tools on the floor — and mind steps and ladders if you are measuring high ceilings or stairwells. Never mark or drill into a wall until you have checked for wires and pipes behind it.

Try these calculators

• Square Footage Calculator — area for rectangles, circles and known areas.
• Area Converter — convert square feet, metres, yards and acres.
• Square Meter Calculator — area in square metres, feet and yards.