What is Parametric Estimating?
Parametric estimating multiplies a unit rate by a quantity: cost per square meter × square meters, hours per test case × test cases. When the historical rate is solid and the work scales linearly, it's both fast and impressively accurate.
Its weakness is the assumption of linearity — the 100th kilometer of pipeline may not cost what the first did once terrain changes. The skill isn't the multiplication; it's knowing whether your parameter still applies.
Formula
Estimate = quantity × historical rate per unit
Worked example
A roofing contractor knows installation runs $8.50/sq ft on standard pitches. For a 4,000 sq ft roof: 4,000 × $8.50 = $34,000, quoted in minutes. For the church with the 60-degree steeple, he throws the rate away and estimates bottom-up — the parameter doesn't survive the pitch.