How do you select target utilisation without using a universal percentage?

Short Answer

Treat target utilisation as a hypothesis you test, not a rule of thumb you copy from the last project — "70% because that's what we always use" is exactly the trap this question is probing. Start from actual counted standard-cell area, then layer on everything that eats into usable space: macro footprint and its dead channels, expected late-stage cell-count growth, physical-only cells (tap/endcap/filler), hold and clock buffering headroom, spare-cell allocation for ECO, and PG-grid reservation.

Technical Explanation

• Voltage areas and local density hot-spots matter too — a block can average 65% utilisation and still have a corner sitting at 95% because a voltage island cramped the rows.
• Keep the numerator (cell area) and denominator (placeable row area) rigorously consistent across candidates you're comparing — mixing definitions is the single most common way this analysis goes wrong.
• Once you have a candidate number, don't just trust it — run a quick placement/route trial and watch congestion and timing, then adjust; utilisation is validated by the tool's behavior, not by the spreadsheet math alone.
• Document every reserved resource explicitly so a reviewer can see why the number is what it is, not just what the number is.

Formula Or Decision Rule

Sensitivity: required area changes from A/U1 to A/U2 when target utilisation changes from U1 to U2.

What To Check

• Warning sign: A copied percentage produces fragmented rows or severe local hot spots.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Choose a lower or reshaped target for the measured constraint, then rerun the same scorecard.

Healthy, Suspicious & Hard-stop Results

• Healthy: Start from counted area, macro dominance, expected growth, physical-only cells, clock and hold buffering, spare strategy, routing access, PG reservation, voltage areas, and local density risk. Then test alternatives.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

” to measurable physical evidence and an owned correction.

Follow-up Question & Model Response

"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that a copied percentage produces fragmented rows or severe local hot spots. ”