Slab Opening Setout Calculator — Locate Openings From Reference Edges

Q: Can the opening be checked against slab edges?

Yes. The calculator reports edge clearances and warns when an opening extends outside the entered slab.

Q: Does this verify structural requirements?

No. It is a geometric setout tool and does not determine structural reinforcement, strengthening or approval requirements.

Slab and opening dimensions

Slab X dimension

Slab Y dimension

Opening center offset from left edge (X)

Opening center offset from bottom edge (Y)

Opening width (X)

Opening length (Y)

Unit

The entered center offsets are measured to the opening center, not to the opening edge.

Enter slab and opening dimensions.

Opening center—measured from left and bottom slab edges

—Left clearance

—Right clearance

—Bottom clearance

—Top clearance

Location	X	Y

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How slab opening setout is calculated

The center point is positioned using the two entered offsets. Half of the opening width and length is then added or subtracted from the center to determine each opening edge.

Left edge = center X − opening width ÷ 2
Right edge = center X + opening width ÷ 2
Bottom edge = center Y − opening length ÷ 2
Top edge = center Y + opening length ÷ 2

Why edge clearances matter

An opening can have a mathematically valid center but still extend beyond the slab perimeter. The four clearance values make this error obvious before the dimensions are transferred to a drawing or site.

Structural limitation

This calculator does not assess reinforcement interruption, trimming steel, sleeves, penetrations, post-tensioning, fire requirements or structural strengthening.

Frequently Asked Questions

How is an opening center located?

Measure the specified center offset from the selected reference edges.

Can the opening be checked against slab edges?

Yes. Negative clearances indicate that the calculated opening extends outside the slab.

Does this verify structural requirements?

No. It is a geometric setout calculator.

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