# Floor Joist Calculator

URL: https://speedycalculator.com/calculators/construction/floor-joist/
Last updated: 2026-10-25

> The floor joist calculator counts the floor joists, rim (band) joists and joist hangers for a floor from its length, joist span and spacing, and estimates the linear footage and material cost.

## Inputs

- Floor length (across the joists) (ft, m, in, yd, cm)
- Joist span (joist length) (ft, m, in, yd, cm)
- Joist spacing
- Include rim joists
- Joist hangers
- Waste (%)
- Price per joist (/piece)
- Rim joist price (/ft, /m, /yd)
- Price per hanger (/piece)

## Results

- Joists
- Rim joist length
- Total joist length
- Joist hangers
- Material cost

## Formula

```
Joists = ceil(floor length ÷ spacing) + 1
Rim joists = 2 × floor length (+ 2 × span for end joists)
Hangers = 2 × joists
```

## Explanation

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

FieldWhat to enterFloor length (across the joists)Units: ft, m, in, yd, cmJoist span (joist length)Units: ft, m, in, yd, cmJoist spacingChoose one: 12 in o.c., 16 in o.c., 19.2 in o.c., 24 in o.c. …Include rim joistsChoose one: No, YesJoist hangersChoose one: No, YesWasteIn %Price per joistUnits: /pieceRim joist priceUnits: /ft, /m, /ydPrice per hangerUnits: /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)Joists121118162420302636304839

## Worked 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.

## FAQ

**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.


## Sources

- [American Wood Council – Span tables for joists and rafters](https://awc.org/codes-standards/calculators-software/spancalc/)