Timber Joist Span Calculator

Calculator

Enter your joist, loads, and criteria. Results appear above this form after submission.

Unit system

Switching units keeps your selections, not conversions.

Calculation mode

“Check” reports pass/fail and utilization.

Support model

Continuous option is an approximation only.

Species / grade preset

Use “Custom” to enter your own properties.

Joist size

Nominal sizes use typical actual dimensions.

Adjustment factor bundle (C)

Multiply stress values by service/usage factors (0.50–2.00).

Spacing (in o.c.)

Spacing (mm o.c.)

Live load (psf)

Live load (kPa)

Dead load (psf)

Includes joist self-weight if you prefer.

Dead load (kPa)

Includes joist self-weight if you prefer.

Additional total line load (plf)

Example: partition line, heavy strip load (total).

Additional total line load (kN/m)

Example: partition line, heavy strip load (total).

Additional live line load (plf)

Used only for live deflection (optional).

Additional live line load (kN/m)

Used only for live deflection (optional).

Span to check (ft)

Span to check (m)

Stiffness modifier (CE)

Use to reflect creep, moisture, or conservative stiffness.

Custom width, b (in)

Custom depth, d (in)

Custom width, b (mm)

Custom depth, d (mm)

Custom Fb (psi)

Custom Fv (psi)

Custom E (psi)

Custom Fb (MPa)

Custom Fv (MPa)

Custom E (GPa)

Deflection criteria

Deflection often controls residential floors. Choose what you want to enforce.

Tip: Start with live L/360 and total L/240

Enable live-load deflection

Live limit (L/…)

Enable total-load deflection

Total limit (L/…)

Common selections

Residential floors: live L/360, total L/240.
Tile floors: consider live L/480 (project-specific).
Long-term creep: reduce CE for conservatism.

Reset

Example data table

Illustrative scenarios only. Always verify with local requirements.

Scenario	Joist	Spacing	Loads	Criteria	Typical controlling span
Light residential floor	2×10 SPF No.2	16 in o.c.	40 psf live + 10 psf dead	Simple span, live L/360, total L/240	~12–15 ft
Heavier floor or storage	2×12 Hem-Fir No.2	24 in o.c.	60 psf live + 15 psf dead	Simple span, live L/360	~10–13 ft
Metric quick check	38×235 mm (typical)	400 mm o.c.	2.0 kPa live + 0.5 kPa dead	Simple span, live L/360, total L/240	~3.6–4.6 m

Formula used

Uniformly loaded joist model. Supports conservative stress and deflection checks.

Geometry

Section modulus: S = b·d² / 6
Moment of inertia: I = b·d³ / 12

Rectangle assumptions. Not for I-joists or trusses.

Load conversion

Line load: w = (q_live + q_dead) × spacing
Imperial: psf × ft → plf. Metric: kPa × m → kN/m.

Optional strip loads may be added as line loads.

Stress checks

Max moment: M = w·L² / 8 (simple span)
Bending stress: f_b = M / S ≤ F_b,adj
Max shear: V = w·L / 2
Shear stress: τ_max = 1.5·V / (b·d) ≤ F_v,adj

Continuous option applies approximate reduction factors.

Deflection check

Deflection: Δ = 5·w·L⁴ / (384·E·I) (simple span)
Limit: Δ ≤ L / ratio
Solving gives: L ≤ [(384·E·I) / (5·w·ratio)]^(1/3)

Live and total deflection checks are selectable.

This tool is an estimator. It does not include bearing, notching, holes, vibration, lateral stability, concentrated loads, or code-specific adjustment factors. Use professional review for design.

How to use this calculator

A practical workflow for quick span checks.

Select your unit system and calculation mode.
Choose a species/grade preset or enter custom properties.
Select a joist size, spacing, and support model.
Enter live and dead area loads, plus any strip line loads.
Enable deflection checks and pick L/ ratios that match the project.
Press Calculate and review the governing criterion.
Download a CSV or PDF for documentation and sharing.

Related Calculators