Floor Joist Calculator: How Many Floor Joists Do I Need?
The number of floor joists you need depends on the width and spacing of your floor.
Sixteen inches on-center is the most common joist spacing for residential buildings.
Your local contractor or framing contractor can install floor joists.
The cost of installing floor joists can range from $25 to $40 per hour.
Let’s face it—floor joists are the unsung heroes of a solid, squeak-free floor, and knowing how many you need to frame your space is priority No. 1 in keeping your floors level (literally). Whether you’re framing a brand-new room or replacing older joists, our floor joist calculator provides a smart shortcut to get it done right.
How Many Floor Joists Are Needed For Square Footage
Proper joist planning helps ensure you have a strong, stable foundation for your space. The table below shows the number of joists required for standard floor sizes, assuming a 16-inch on-center spacing.
| Square Feet | Joist Width (feet) | Number of Joists Needed |
|---|---|---|
| 100 | 10 | 10 |
| 300 | 10 | 30 |
| 500 | 10 | 50 |
| 1,000 | 12 | 75 |
| 1,500 | 14 | 94 |
Calculating Floor Joists
Use this floor joist calculator to eliminate the guesswork from your framing. All you need is the width of your floor, your joist spacing, and a calculator to churn out the number you need.
Number of joists = ((Floor width in inches) ÷ (Floor spacing in inches)) + 1
How to Measure Floor Width and Spacing
Since floor joists run perpendicular to the width, knowing this measurement helps determine how many joists you’ll need and how to space them properly. Thankfully, the measuring process is pretty straightforward and just requires a tape measure and a calculator.
How to Measure Floor Width
To measure your floor’s width, start by considering the direction your joists will run. This is key, as joists are installed perpendicular to the span they support. Next, measure the distance across the floor in the direction perpendicular to the joists—this will be your floor width. Use a tape measure to determine the total distance from one wall to the opposite wall, ensuring you measure in inches.
Who to Hire to Install Floor Joists
Unless you’re a licensed and insured contractor, this is a job best suited for outsourcing to a professional. The question is, which type of expert is best? In most cases, your local contractor can install floor joists as part of a framing job, as they likely have the necessary experience, skill set, and crew to make it happen. You can also consider hiring a framing contractor who specializes in installing floor joists, which may be the best option if your project is particularly complex.
Frequently Asked Questions
Floor joists provide support and stability to the flooring installed above them, whether it be hardwood, tile, or carpet. They’re essentially horizontal beams that hold up the subfloor and everything that lies on top of it. Floor joists also help evenly distribute weight across their structure so that it’s shared between walls, beams, and your foundation.
If you’re just replacing a few broken or damaged joists, you can expect to pay anywhere from $100 to $300 per joist. However, a complete installation for a home or room will take into account the cost of labor, which ranges from $25 to $40 per hour. Floor joist installation for an entire room, for example, can cost between $5,000 and $10,000, depending on the room's size and the materials used.
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