Floor joists span between beams or walls at a regular spacing, most often 16 in on centre. A rim or band joist caps the ends of the joists around the perimeter. The joist size depends on the span, spacing, species and load – check the span tables in your building code or the American Wood Council span calculator.
How to use the Floor Joist Calculator
- Floor length (across the joists)
- Joist span (joist length)
- Joist spacing
- Include rim joists
- Joist hangers
- Waste
- Price per joist
- Rim joist price
- Price per hanger
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the floor length (the direction joists are spaced along), the joist span, the spacing, whether to include rim joists and hangers, waste and prices.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Floor length (across the joists) | Units: ft, m, in, yd, cm |
| Joist span (joist length) | Units: ft, m, in, yd, cm |
| Joist spacing | Choose one: 12 in o.c., 16 in o.c., 19.2 in o.c., 24 in o.c. … |
| Include rim joists | Choose one: No, Yes |
| Joist hangers | Choose one: No, Yes |
| Waste | In % |
| Price per joist | Units: /piece |
| Rim joist price | Units: /ft, /m, /yd |
| Price per hanger | Units: /piece |
What each result means
- Joists = ceil((ceil([Floor length (across the joists)]÷[Joist spacing] – 0.000001) + 1)×(1 + [Waste]÷100))
- Rim joist length = [Include rim joists]×(2×[Floor length (across the joists)] + 2×[Joist span (joist length)])
- Total joist length = [Joists]×[Joist span (joist length)] + [Rim joist length]
- Joist hangers = [Joist hangers]×2×[Joists]
- Material cost = [Joists]×[Price per joist] + [Rim joist length]×[Rim joist price] + [Joist hangers]×[Price per hanger]
Example values
How the main result changes when Floor length changes and everything else stays at the example values above:
| Floor length (across the joists) (ft) | Joists |
|---|---|
| 12 | 11 |
| 18 | 16 |
| 24 | 20 |
| 30 | 26 |
| 36 | 30 |
| 48 | 39 |
Formula
Joists = ceil(floor length ÷ spacing) + 1Rim joists = 2 × floor length (+ 2 × span for end joists)Hangers = 2 × joistsWorked example
Inputs — Floor length (across the joists): 24 ft; Joist span (joist length): 12 ft; Joist spacing: 16 in o.c.; Include rim joists: Yes; Joist hangers: Yes; Waste: 5 %; Price per joist: $18 /piece; Rim joist price: $1.5 /ft; Price per hanger: $1.5 /piece.
Results — Joists = 20; Rim joist length = 72 ft; Total joist length = 312 ft; Joist hangers = 40; Material cost = $528.00.
Frequently asked questions
How many floor joists for a 24 ft floor?
At 16 in on centre, 19 joists (18 spaces + 1).
What size floor joist for a 12 ft span?
Typically 2×8 at 16 in o.c. for 40 psf live load in common species; check the span tables for your grade.
Do I need blocking?
Codes require blocking or bridging at supports and often at mid-span for spans over about 8 ft depending on depth.
What formula does the Floor Joist Calculator use?
Joists = ceil(floor length ÷ spacing) + 1.