After years working with commercial builders, I've seen the same estimation mistakes cost teams thousands on nearly every project. Here's what I've learned:

1. Concrete — Most PMs underestimate by 10-15% because they forget waste factors and pump loss. Always add a 10% buffer on pours over 50 yards.

2. Rebar — Lap splices, hooks, and development lengths add up fast. Standard takeoffs miss 8-12% of actual rebar needed.

3. Drywall — The magic number is 1.12x your net wall area. Corners, cuts, and damage during install always eat more than you think.

4. Lumber for formwork — Reuse rates are optimistic in most bids. Plan for 60% reuse max on wood forms, not 80%.

5. Fasteners & hardware — The "miscellaneous" line item that blows budgets. Track joist hangers, anchor bolts, and tie wire separately.

The fix? Systematic takeoffs with accurate waste factors built in — not gut-feel estimates from the field.

This is exactly why I built ConstructCalc. It handles the math so you can focus on building.