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Floor Joist Span Calculator (Metric)

The metric Floor Joist Span Calculator uses millimetres, centimetres, metres, or square metres as shown beside each field. It applies planning screen = span ÷ depth, places a complete metric result in the served HTML, and updates locally in the browser. Verify field measurements, local standards, material data, and structural assumptions before construction.

Enter measurements

mm

Enter a positive field measurement in mm.

mm

Enter a positive field measurement in mm.

mm

Enter a positive field measurement in mm.

Calculated result

15.3 span/depth

A conservative depth screen compares clear span with joist depth; it is not a structural design.

Clear span
3,600.0 mm
Spacing
400 mm o.c.

Planning check passes the conservative depth/18 screen. Verify species, grade, loads, and the current AWC/IRC table.

Floor Joist Span scaled diagram Scaled planning diagram using the server-rendered default values. span 3600 mm depth 235 mm AWC/IRC lookup required
Scaled visual aid. Dimensions in the result table govern; do not scale a cut directly from the drawing.

Printable result and cut list

Floor Joist Span default worked result
ItemValueJob note
Primary result15.3 span/depthVerify before cutting or ordering.
Clear span3,600.0 mmCalculated from the entered dimensions.
Spacing400 mm o.c.Calculated from the entered dimensions.
Formula used by this calculator
FormulaExpressionNote
Primary formulaplanning screen = span ÷ depthRound the final result, not each intermediate value.

How the floor joist span calculation works

Enter the job dimensions in the form. The calculator normalizes the values, applies planning screen = span ÷ depth, and updates the main result, supporting values, diagram labels, and print table together.

The first displayed example is calculated during the static build. That makes the formula and a complete numerical result readable in page source, while the browser script handles later input changes.

Worked example

The prefilled values form a complete span-to-depth planning ratio example. Change one field at a time and compare the updated result with the formula table to catch unit or transcription mistakes.

Common errors to avoid

  • Mixing nominal and actual material dimensions.
  • Entering diameter where the formula expects radius, or total run where it expects one tread.
  • Rounding every intermediate value instead of rounding the final purchase or cut quantity.
  • Treating a planning result as a stamped structural design or local-code approval.

Frequently asked questions

Does the floor joist span calculator work without JavaScript?

The default worked result, formula, diagram, and result table are included in the served HTML. JavaScript is only needed to recalculate after you change an input.

Which unit should I use?

Use the imperial page for US customary inputs or open the metric twin for millimetres, centimetres, and metres.

Can I use the result for purchasing material?

Use it as a quantity and layout check, then add the stated waste allowance and confirm package coverage, stock dimensions, and field conditions.

Does this replace building code or an engineer?

No. The calculator is a planning aid. Current local code, product documentation, and a qualified engineer govern structural and life-safety decisions.

Floor Joist Span metric calculator Metric inputs and rewritten metric result. Floor Joist Span reference chart Printable HTML and PDF reference. Construction calculators Browse all 32 core tools. Editorial standards Sources, formulas, review dates, and safety boundaries.

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iPhone companion

Keep jobsite math available offline

The website calculators and charts stay free. The BuildMath iPhone app keeps its supported construction calculations available when the jobsite has no connection.

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Last reviewed .