Joist Calculator
Estimate how many joists you need for decks, floors and framing projects, plus total linear footage and material cost.
This joist calculator converts your project's length and width, along with your chosen joist spacing, into the number of joists you need to buy. Enter your dimensions, pick a waste percentage for trimming and cuts, and optionally add a price per joist to estimate total material cost.
Project length is the distance across joists between the first and last centerlines; Project width is each joist’s length. Use spacing confirmed for your project.
Your Estimate
Project Area
240.0 ft²
Joists Needed
16 joists
Joists Including Waste
18 joists
Total Linear Feet
192.0 ft
Linear Feet Including Waste
216.0 ft
Assumes joists run across the project width (12 ft) with 20 ft measured between the first and last joist centerlines. Count includes both end joists; the final interval may be shorter than the selected spacing.
Results Actions
Estimates are approximate. Actual joist counts, spans and spacing depend on lumber species, load requirements and local building code — always confirm with a span table or licensed contractor.
Common Joist Spacing
Joist spacing depends on joist size, species, span length and expected load — always confirm with your local building code and a lumber span table before finalizing your framing plan.
How to Use the Joist Calculator
- 1Measure the length and width of your deck, floor or framing project in feet (or meters).
- 2Choose your joist spacing (12, 16 or 24 inches on center, or enter a custom spacing).
- 3Choose a waste percentage — 10% for straightforward layouts, 15%+ for complex framing.
- 4Optionally enter a price per joist to see an estimated material cost.
- 5Review the results: joists needed, joists with waste, and total linear footage.
- 6Always double-check joist size and spacing against local code and span tables before buying.
Example Calculation
For a 20 ft centerline-to-centerline layout across joists, 12 ft joist lengths, 16 in spacing and a 10% waste factor. Project area = 20 × 12 = 240 sq ft. Joists needed = ceiling(240 in ÷ 16 in) + 1 = 16 joists, each spanning the 12 ft width. With 10% waste, joists including waste = 18 joists, for a total of about 216 linear feet.
Accuracy & Assumptions
- Results are estimates only.
- Project length is measured between the first and last joist centerlines. Both end joists are counted; the last spacing interval may be shorter. Rim boards, doubled members and openings are separate.
- Actual spacing, joist size and span limits vary with lumber species, grade and load requirements.
- The purchase total already includes your selected waste allowance. Choose that allowance for cuts and damaged boards; do not add the same waste percentage again.
- Consult a licensed contractor or engineer and local building code before finalizing structural framing.
Frequently Asked Questions
How many joists do I need for my deck or floor?
Measure between the first and last joist centerlines, convert to the spacing unit, divide by the chosen spacing, round up, and add one. A 10 ft run at 16 in spacing needs ceiling(120 ÷ 16) + 1 = 9 joists; an exact 20 ft run needs 16.
What joist spacing should I use?
16 inches on center is the most common spacing for residential decking and framing. Heavy decking, diagonal boards, tile floors or extra loads often call for 12 inches on center, while light-duty framing may allow 16-24 inches. Always confirm with your local building code and lumber span tables.
Does joist spacing affect deck board direction?
Yes. Joists run perpendicular to the direction your deck boards or flooring will run, and closer spacing (12 in) is typically required under diagonal decking patterns to keep board ends properly supported.
How much waste should I add for joist lumber?
A 10% waste allowance covers typical trimming, end cuts and occasional damaged boards. Increase to 15% or more for complex framing layouts with cantilevers, stairs or multiple levels.
Can I use this calculator for floor framing, not just decks?
Yes. The same length, width and spacing logic applies to floor joists and general framing — just be sure to check your local building code and span tables for the correct joist size and spacing for the load your floor needs to support.
This calculator provides an estimate for planning purposes only. Actual joist counts, spans and spacing vary by lumber species, load requirements and local building code — always confirm final quantities with a licensed contractor or engineer before purchasing or building.