Joist Size Calculator
Enter the span the joists must cross, their spacing, the timber species, the room use and the floor width. The calculator picks the smallest joist that spans it and counts how many you need.
- Joist size
- Maximum span
- Joists needed
Span is the clear distance between supports, not the joist length.
Joist size depends on span and spacing. A 13 ft span at 16 in centres needs a 2×8 in Douglas fir, which reaches about 13 ft 1 in. A 12 ft wide floor needs 10 joists. At 24 in centres the same span needs a 2×10.
Joist span chart
Douglas fir No. 2, living area. Search for a size.
| Spacing | 2×6 | 2×8 | 2×10 | 2×12 |
|---|---|---|---|---|
| 12 in | 10 ft 11 in | 14 ft 5 in | 18 ft 5 in | 22 ft 2 in |
| 16 in | 9 ft 11 in | 13 ft 1 in | 16 ft 8 in | 19 ft 10 in |
| 19.2 in | 9 ft 4 in | 12 ft 4 in | 15 ft 5 in | 18 ft 1 in |
| 24 in | 8 ft 7 in | 11 ft 4 in | 13 ft 10 in | 16 ft 2 in |
Feet and inches.
Span and joist
16 in centres. Slide the span.
The joists are drawn in section.
How to size floor joists
Span to joist. Step through it.
Span
Measure.
Doubling joists under heavy items such as baths adds stiffness.
Closer or wider spacing
Joist spacing. Compare them.
Stiff.
Standard.
Fewer.
Closer joists feel stiffer and let a smaller size span further.
Before you build
Joist checks. Tick each item.
Tick each item as you go.
Use joist hangers rated for the load at each end.
What you get
Joist sizing. Each tile explains one result.
Nominal.
Maximum.
Joists.
Species.
Room use.
A deeper joist is much stiffer than a wider one.
Frequently Asked Questions
No question matches that search.
What size joist for a twelve foot span?
A two by eight at sixteen inch centres in Douglas fir.
How far can a two by eight span?
About thirteen feet at sixteen inch centres for a living area floor.
How far can a two by ten span?
About sixteen feet eight inches at sixteen inch centres.
Does joist spacing change the span?
Yes. Closer spacing lets each joist span further.
How many joists do I need?
Divide the floor width by the spacing and add one.
Is spruce weaker than fir?
Slightly. Spruce-pine-fir spans a few per cent less.
When do I need engineered joists?
For spans beyond what a two by twelve can reach.
A floor that feels solid
Size your joists for the span and spacing and the floor stays stiff and quiet underfoot.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.