Estimating Floor and Ceiling Framing

Estimating Floor and Ceiling Framing

To calculate the number of joists, use the formulas shown in the figure below for the appropriate on-center spacing.


To calculate the number of joists needed:

1) Measure the width of the room (ft.);
2) divide by the appropriate o.c. spacing (ft.);
3) add one to start.


Remember to include rim stock in the total joist count. Rim joist stock is calculated as follows:

Building Length (ft.) × 2 ÷ Lumber Length

Lumber lengths should be multiples of the o.c. spacing. For example, use 12s, 16s, 20s, etc., for 16-in. o.c. joist spacing. Depending on the length of the building, it may be more efficient to count these individually, mixing lumber lengths that break evenly on the joists.


The most accurate method of estimating subflooring is to graph scaled lines over the floor plans in 4x8-ft. increments. For a quick alternative, calculate as follows:

  • For each section of subfloor, square off any jogs or cantilevers from the outside perimeter on the plans to form one large perimeter rectangle.
  • For each section, multiply the length by the width of the perimeter rectangle.
  • Divide by 32 (for 4x8 panels).

Floor Area = Length × Width

Sheathing (4x8 panels) = Floor Area ÷ 32

Remember to add a couple of sheets to the total panel count to accommodate cutting error and waste, especially if there are jogs in the floor plan.

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