7 Job Costing Mistakes That Are Quietly Killing Your Profit Margins

Most contractors think they're tracking costs. In reality, they're tracking expenses after the fact — not managing costs in real time. The difference between those two things is the difference between a 15% net margin and a 3% net margin.

After working with dozens of subcontractors between $500K and $10M in revenue, I see the same mistakes repeated across trades — electrical, plumbing, HVAC, concrete, framing. Here are the seven most common job costing errors and exactly how to fix them.

1. Lumping Labor Into One Bucket

The most expensive mistake I see: treating all labor as one line item. Your field labor, shop labor, supervision, and drive time all have different cost implications. When you lump them together, you can't tell which jobs are actually profitable.

The fix: Break labor into at minimum three categories — direct field labor, supervision/PM time, and indirect labor (shop, drive time, callbacks). Your job cost reports should show each separately.

2. Ignoring Burden Rate

Your electrician costs you $45/hour in wages. But what does he actually cost you? Once you add payroll taxes (7.65% FICA), workers' comp (8-15% depending on trade and state), health insurance, union benefits, PTO, and tools — that $45/hour employee costs you $62-$78/hour.

The fix: Calculate your true burden rate quarterly. Use it in every estimate. If you're bidding jobs at $45/hour labor cost, you're losing money on every man-hour.

3. Not Tracking Change Orders Separately

Change orders are where margin lives or dies. When you roll change order costs into the original job budget, you lose visibility into whether the base contract was profitable and whether your change order markup is adequate.

The fix: Every change order gets its own cost code. Track revenue and cost separately. Review change order profitability monthly.

4. Monthly Cost Reviews Instead of Weekly

If you're reviewing job costs monthly, you're 30 days too late. A job can bleed $20,000 in a month before anyone notices. By the time you see it on a monthly report, the damage is done.

The fix: Weekly job cost reviews on your top 5 active jobs by dollar value. It takes 30 minutes and saves thousands.

5. No WIP Schedule

Your Work-in-Progress schedule is the single most important financial document for a contractor. It tells you whether you're over-billing (borrowing from the future) or under-billing (financing the GC's project). Without it, you're flying blind.

The fix: Prepare a WIP schedule monthly. Compare percent complete (cost basis) to percent billed. Any variance over 5% needs investigation.

Read more about WIP schedules and financial reporting → [blocked]

6. Estimating Without Historical Data

If your next bid isn't informed by actual cost data from your last 10 similar jobs, you're guessing. And guessing in construction means either leaving money on the table or winning jobs you'll lose money on.

The fix: After every job closes out, do a post-mortem. Compare estimated vs. actual by cost code. Feed that data into your next estimate.

7. Using QuickBooks Categories Instead of Real Job Codes

QuickBooks expense categories (Materials, Labor, Subcontractors) are not job cost codes. Real job costing requires a structured system — phase codes, cost types, and work breakdown structures that map to how you actually build.

The fix: Set up a proper job costing structure in QuickBooks or your construction accounting software. Need help? See our QuickBooks cleanup service → [blocked]

The Bottom Line

Job costing isn't optional for contractors who want to grow. It's the foundation of every pricing decision, every bid, and every profitability analysis you'll ever do. Get it right, and you'll know exactly which jobs make money, which ones don't, and why.

Need help setting up proper job costing? Learn about our fractional controller services → [blocked] or schedule a free consultation.