Joist Calculator
Estimate joist count, bays, actual maximum spacing, and linear material.
What this calculator does
Spacing divisions round up, then one outside joist is added.
Joist spacing plan
Count starts with a joist at both outside edges. Enter the dimension across the joists as layout length.
Your plan updates instantly
Drag with a mouse or finger. Pinch or scroll to zoom.
Spacing divisions round up, then one edge joist is added.
The arithmetic uses the exact values shown above. Verify field measurements, product pack sizes, layout constraints, and local requirements before ordering or building.
How to interpret the result
Spacing divisions round up, then one outside joist is added.
How the result is calculated
Joists = ceil(layout length ÷ spacing) + 1.
Worked example
Across 20 ft at 16 in on center, 16 joists cover both edge positions.
Practical guidance
- Measure the project at least twice before ordering.
- Use actual product dimensions and package coverage.
- Keep a saved or printed copy of the inputs used for the estimate.
Assumptions and limitations
This does not select joist size, grade, span capacity, beams, blocking, or connections.
Methodology
Calculations use standard geometry and normalized units. Countable products are rounded up where partial products cannot normally be purchased.
Read the complete calculation methodology and testing policy.
Frequently asked questions
Are results rounded for purchasing?
Countable materials and packages are rounded up. Measured area, length, volume, and cost remain decimal values where appropriate.
Is this a final construction specification?
No. It is a transparent planning estimate. Verify product instructions, site conditions, plans, and local requirements.