The 12 Most Common Estimating Errors — And How to Catch Them

Most estimating errors are not exotic. They repeat. A short checklist run before the estimate goes out catches the large majority.

Quantity errors

1. Wrong unit — m² where m³ is needed, or per-piece where per-set. Check: unit sanity pass against the method of measurement.
2. Missing work items — fittings, access panels, sundries, testing. Check: trade-by-trade completeness list.
3. No wastage / laps — net quantity ordered. Check: every quantity has an allowance line.
4. Double-counting — an item measured in two trades. Check: scope matrix between trades.
5. Scale / drawing revision — measured off a superseded drawing. Check: drawing register with revision numbers.
6. Girth ignored — profiled or moulded surfaces measured flat. Check: multiplier applied to irregular surfaces.

Pricing errors

7. Stale prices — last year’s rates. Check: price date on every material.
8. Labour as a flat percentage — hides productivity reality. Check: output-rate build-up for major trades.
9. Preliminaries buried in rates — rates not comparable. Check: preliminaries as a separate section.
10. Currency / regional mismatch — imported prices not converted or localised. Check: conversion date noted.

Process errors

11. Arithmetic and transcription slips — a mistyped extension. Check: independent recheck of the 20 highest-value lines (they are usually 80% of the cost).
12. No top-down cross-check — bottom-up total never sanity-checked. Check: cost per m² of GFA against a benchmark; investigate any variance over 15%.

The habit that matters most

Every important quantity should be reachable two ways. If the two numbers disagree by more than a few percent, find out why before the estimate leaves your desk.