Room Diagonal Calculator

Calculate the diagonal distance across a rectangular room using its length and width—ideal for verifying flooring runs, trim pieces, carpet rolls, or moving large furniture.

Measure the longest wall. Convert feet and inches to decimal feet (e.g., 18'6" = 18.5 ft).
Measure the perpendicular wall using the same units as the length.

Always verify critical measurements on site with a tape or laser, especially in older buildings where walls can taper or bow.

Examples

  • 18 ft × 12 ft living room ⇒ 21.63 ft diagonal span
  • 5.5 m × 4.2 m studio ⇒ 6.92 m diagonal
  • 10 ft × 10 ft square room ⇒ 14.14 ft diagonal
  • 32 ft × 14 ft hallway ⇒ 34.87 ft diagonal

FAQ

Does the calculation assume perfect right angles?

Yes. It models a true rectangle. For irregular shapes, measure the longest straight line with a tape or laser instead of relying on this formula.

Can I mix imperial and metric units?

No. Convert both inputs to the same measurement system first—either both in feet or both in meters—for a correct diagonal.

How should I use the diagonal when ordering materials?

Compare the diagonal to the maximum available length of flooring planks, trim, or carpets to ensure they fit without seams or splicing.

What about ceiling or staircase clearances?

This tool only handles the floor plan. Include room height in a 3D calculation if you need corner-to-ceiling diagonals or stairway clearance measurements.

Additional Information

  • Formula: Diagonal = √(length² + width²), the classic Pythagorean relationship for rectangles.
  • Keep both measurements in the same unit—feet, meters, or centimeters—so the diagonal comes out in that same unit.
  • To convert the decimal result into feet and inches, multiply the fractional part of the feet value by 12 to obtain inches.

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