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Calculation Machine

Joist Span Calculator

Check the maximum floor joist span for a given size, spacing and load.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Maximum span
18 ft 0 in
Span (decimal feet)
18 ft
Spacing
16 in o.c.
Design load
40 psf live + 10 psf dead

Working

  1. 1base 18 ft x spacing 1 x load 1 = 18 ft

Quick answer

Allowable joist span grows with depth and shrinks with spacing and load. This tool scales a published base span by spacing and load factors to give a working span in feet and inches.

The formula

span = base(depth) x sqrt(40/live load) x spacing factor

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Joist size2x10
  • Spacing on centre16 in
  • Live load40 psf

Result

  • Maximum span18 ft 0 in
  • Span (decimal feet)18 ft
  • Spacing16 in o.c.
  • Design load40 psf live + 10 psf dead

How to use the Joist Span Calculator

  1. 1Pick the joist size you plan to use and the spacing on centre.
  2. 2Enter the live load: 40 psf for living areas, 30 psf for bedrooms, 60 psf for decks with heavy use.
  3. 3Read the maximum span, then confirm against your local code table before cutting.

Frequently asked questions

Is this a code-approved span table?

No. It is a planning estimate based on typical No. 2 softwood values. Always confirm the final span with your local building code table or an engineer.

Does spacing really matter that much?

Yes. Going from 16 in to 24 in centres cuts the allowable span roughly 13%, while 12 in centres buys you about 10% more.

About this calculator