Calculation results
Structural utilisation
Important warnings
Floor framing layout
Joist size and spacing comparison
| Joist option | Spacing | Bending | Shear | Deflection | Estimated cost | Status |
|---|
Material cutting and purchase list
| Item | Quantity | Length | Estimated cost |
|---|
Calculation summary
Formula used
Joist count = ceil(layout width ÷ spacing) + 1 Uniform line load = area load × tributary spacing + self-weight Maximum moment = wL² ÷ 8 + PL ÷ 4 Maximum shear = wL ÷ 2 + P ÷ 2 Bending stress = M ÷ S Shear stress = 1.5V ÷ A Uniform deflection = 5wL⁴ ÷ 384EI Point-load deflection = PL³ ÷ 48EI Bearing stress = reaction ÷ bearing area
These equations model simply supported joists under idealised loads. Real framing can redistribute forces through connected building components. Final design requires approved tables and qualified structural review.
How to use this calculator
- Select the preferred measurement system and calculation mode.
- Enter floor dimensions, joist spacing, and member properties.
- Add distributed loads, point loads, openings, and blocking.
- Enter prices for materials, labour, delivery, and tax.
- Calculate, review warnings, and compare alternative joist layouts.
Start with conservative dimensions and verified material properties. Review every warning before relying on any displayed result. Keep supplier span tables beside this planning calculation always.
Example data
| Input | Example | Purpose |
|---|---|---|
| Floor size | 20 ft × 16 ft | Defines the framing area. |
| Clear span | 14 ft | Controls bending and deflection. |
| Joist size | 2×10 actual 1.5 × 9.25 in | Defines member section properties. |
| Spacing | 16 in on centre | Defines quantity and tributary width. |
| Loads | 15 psf dead plus 40 psf live | Creates the design line load. |
| Deflection limit | L/360 | Sets the serviceability threshold. |
Saved recent calculations
Frequently asked questions
How does the calculator count floor joists?
It divides the layout width by on-centre spacing. The result rounds upward before adding the final edge joist. Openings and doubled members increase the purchase quantity further.
Does this replace an official span table?
No, the structural model is a preliminary screening tool. Official span tables include factors missing from simplified equations. Always follow local rules and manufacturer instructions for construction.
What is joist spacing on centre?
On-centre spacing measures between neighbouring joist centre lines. Common values include twelve, sixteen, and twenty-four inches. Smaller spacing normally improves stiffness and increases material quantity.
Why are live-load and total-load deflections separate?
Live-load deflection concerns movement from changing occupancy loads. Total-load deflection includes permanent and variable loads together. Both limits can govern comfort, finishes, and serviceability performance.
Can engineered I-joists use these formulas?
The placeholder provides only a rough comparison. I-joists require manufacturer-specific stiffness, shear, and hole requirements. Use published product tables before specifying engineered floor members.
How are openings included?
The calculator estimates interrupted joists, headers, and trimmers. Actual header reactions depend on opening position and framing. Large openings require detailed load paths and connector design.
What does utilisation percentage mean?
Utilisation compares calculated demand against the selected allowance. Values above one hundred percent receive a failing status. Lower values still require complete design and verification checks.
How does wet service affect results?
Wet service reduces allowable properties using a simple factor. Actual adjustment factors vary by material and design standard. Replace presets with verified project values whenever possible.
Are notches and holes fully checked?
No, only basic proportion warnings are generated. Location, loads, material type, and manufacturer rules still matter. Never cut engineered members without approved written guidance first.
Why can the calculated cost differ?
Supplier lengths, waste, delivery, and taxes vary widely. Labour productivity also changes with access and framing complexity. Update every price field using current local quotations.
Safety and limitations
This calculator provides planning estimates and simplified structural screening. It cannot inspect supports, connections, deterioration, vibration, or local conditions. Obtain qualified approval before purchasing materials or beginning construction.