Technical knowledge · TECHNICAL · REV 2026-10-06
How are sightlines designed?
A sightline is a line from a spectator’s eye to the point that must be seen, passing with clearance over the person in front. If the rake is broken or the front row is too high, the paper capacity is a lie.
Definition
In theatre-planning literature the C-value is that vertical clearance. About 90 mm is often discussed as a common target and about 120 mm as a more comfortable view. The final figure is fixed by eye height, rake, and local code, not by this paragraph.
The focal point changes: the stage lip, the actor’s eye line, the middle of a screen, or the touchline. Change the point and the whole rake changes.
Why it matters
A seat without a view is a seat without value, even if the aisle width passes. Complaints after opening are not fixed with a carpet. They are fixed by rebuilding a rake or raising a stage, and both are expensive because the decision was late.
Technical principle
Each row adds height so the clearance survives. In a long hall the rear height becomes large. The remedies are to bring the stage closer, raise it, restart the sequence with a balcony, or accept fewer rows.
A seated eye is not the top of a head. Use a real seated eye and a real head in front, with tolerance for a taller person. One average with no margin makes failed rows at the edges.
Measurement
A section, not only a plan
The sightline is drawn in section through every row to a named point.
A site check
Before the last row is fixed, a rod and eye marks can check the line if the structure can still be adjusted.
Balcony and the dive
The balcony front can cut the line of the rows beneath it. That is a second section, not a decorative detail.
Units
Millimetres of clearance
The C-value is millimetres in section. It is not replaced by a seat count.
Eye height
Fix it in the brief: seated on the specified chair, not standing.
Design considerations
Name the focal point in writing
Stage, screen, or pitch. The unspoken point in each consultant’s head is different.
Tie the seat to the rake
Seat-pan height and thickness move the eye. Changing the model after the rake is poured breaks the line.
Do not add a row by spending the clearance
Capacity that cancels the view is fictional once the room is in use.
Typical applications
Theatre
The point is usually the performer’s face or the stage lip. The sides need a view check, not only a centre section.
Lecture
The point may be the bottom of the board or the middle of the screen. A student writing lowers the eye and changes the sequence.
Stadium
The touchline or the centre of the pitch. A column or a box that cuts the view is counted as lost seats, not as a surprise.
Limitations
Code also governs movement
A rake that is kind to the eye may fail as an aisle or a step. The reverse is also true. Neither is sacrificed in silence.
This page is not your project calculation
90 or 120 mm is a literature range, not an approval.
Horizontal view is not the vertical one
A side column or wall steals the seat even when the rake is excellent.
Common mistakes
Dividing the area
The preliminary capacity calculator on this site warns for this reason. An imagined flat floor is not a rake.
A front row set too high
It lifts the whole sequence, or hides the stage from row two if the rake then falls.
Moving the screen after the seats
A higher or lower screen moves the focal point. Fixed seats do not follow it.
Worked illustration
If the target clearance is 100 mm and the eye sits about 120 mm below the top of the head in front, each row must gain the height that keeps that clearance across the horizontal distance between rows. The figures here are an explanation. Your project uses its real seat height and its real focal point.
Related parameters
Row spacing
The horizontal distance enters both the rake and the clearance.
Wheelchair spaces
The eye height is different, and the location must not be a view from behind standing people only.
Preliminary capacity
The calculator gives an order of magnitude. The section gives an honest capacity.
Using the number
Calculators and teaching numbers on this site are preliminary. They do not replace an acoustic model, a measurement made to the relevant standard, or the designer's responsibility.
Questions designers and owners ask
Is 90 mm mandatory?
Not by this page. It is a range mentioned in planning literature. The code, the brief, and the section are what bind.
Does a larger screen fix a poor rake?
It may raise or lower the focal point. It does not repair a row that sees a head if the image stays behind that head.
Do sliding seats change the line?
Yes if the eye moves horizontally or vertically. The moving system is reviewed with the section, not after it.
What about standing in stadia?
A different sequence: shoulder over shoulder, barriers, and standing waves of people. It is not copied from a seated theatre.
When do I stop adding rows?
When the next row breaks the clearance, the building height, the maximum viewing distance for the image, or the code’s step width.
Technical sources
- Theatre planning literature
The C-value method is common in hall-planning references. Copy the definition from the reference the project adopts.
- The local building code
Steps, aisles, and accessible places govern what the section is allowed to draw.
- An approved section
The only document that knows the eye height of this seat and this stage.
Last editorial review: 2026-10-06. Review means the text was checked, not that a project was measured.
Turn the question into a brief
A study starts from room volume, use, and existing condition, not from a product name. Fill the brief and copy it.
Project study form