Bookkeeping for Contractors: The 5 Mistakes That Wreck Your Job Costing

Contractors and trades get destroyed by bad bookkeeping more than almost any other small business. You’re moving fast, juggling jobs, paying subs, buying materials, and trying to keep clients happy. The books are the last thing you want to think about.

But if your books don’t tell you which jobs made money and which lost money, you’re not running a business — you’re running a lottery.

Here are the five mistakes that wreck contractor bookkeeping most often.

Mistake #1: No Job Costing

This is the big one. Every transaction — materials, subcontractor payments, equipment, labor hours — needs to be tagged to a specific job. Without that, you get a company-wide P&L that tells you whether the business made money, but not which jobs made money.

You bid the kitchen remodel at $42,000. Did it actually profit? If your books can’t answer that, you’ll keep underbidding that kind of work and not know why you’re always broke.

Good job costing in QuickBooks Online looks like this:

• Every customer gets a customer record
• Every job or project gets a sub-customer record under that customer
• Every bill, expense, and timesheet is tagged to the specific job
• You can pull a P&L by job anytime to see margin

Without this, you’re estimating by gut and hoping it works out.

Mistake #2: Treating Materials as a Single Expense Line

Every trip to Home Depot, Ferguson, or the supply house does not belong in one lumped “Materials” category. It belongs in cost of goods sold, tagged to the job it was purchased for.

The mistake happens when contractors let the QuickBooks bank feed auto-categorize every purchase as an expense instead of a job cost. Fast forward 12 months and you have $140K of “Materials” on your P&L and no idea which jobs ate it.

Fix: when you make a purchase, take 30 seconds and tag the receipt to the job. Or have your bookkeeper do it. But it has to happen weekly, not “at tax time.”

Mistake #3: Paying Subs with No W-9 on File

Every subcontractor you pay $600 or more in a calendar year needs a 1099-NEC issued by January 31. To issue a 1099, you need their W-9 with their legal name, address, and tax ID.

If you don’t have a W-9, you have two problems:

1. You can’t file the 1099, which means penalties and potentially losing the deduction
2. You’re supposed to withhold backup withholding (24% of the payment) for vendors without a W-9 — most contractors don’t, which creates IRS exposure

The fix is a workflow: no W-9, no payment. Every new sub gets one before the first check clears. Good bookkeeping sets this up as a standing process.

Mistake #4: Mixing Personal and Business

The Home Depot charge at 6 PM on Sunday that was for your kitchen at home. The lunch you bought your kids that got put on the business card. The boat payment running through the business checking account.

Every single one of these is a bookkeeping problem and a tax problem and a legal exposure problem if you get audited or sued (commingling funds weakens corporate protection for LLCs and S-corps).

The rule is boring and non-negotiable: business accounts pay business expenses. Personal accounts pay personal expenses. When you pay yourself, it’s either a distribution (S-corp), owner’s draw (sole prop/LLC), or W-2 wage. Not “I’ll just run it through the business card.”

Mistake #5: No Work-in-Progress (WIP) Tracking

If you collect deposits or draws on long jobs, you’re carrying revenue on projects you haven’t finished. If your books recognize that deposit as revenue the day it hits the bank, your P&L is lying to you.

The correct treatment for contractor draws:

• Deposits sit in a liability account (Deferred Revenue or Customer Deposits) until earned
• Revenue is recognized as the work is completed
• Costs incurred on an unfinished job sit in a WIP asset account, not in current-period COGS

Without WIP tracking, a good month looks great because you got paid a deposit, and then next month looks terrible because you spent the money on materials with nothing coming in. Your P&L zigzags. You can’t tell how the business is actually doing.

What Good Contractor Bookkeeping Looks Like

• Every customer and job is tracked in QuickBooks
• Every expense is tagged to a job
• Monthly reports show P&L by job, not just company-wide
• WIP and deferred revenue are tracked properly
• W-9s are collected before payment, 1099s go out in January
• Business and personal are completely separate
• You can answer “did this job make money?” in 30 seconds

Bottom Line

Contractors don’t need fancy bookkeeping. They need correct bookkeeping — the kind that tells them which jobs made money, which subs are profitable to hire, and whether the business actually grew this year or just stayed busy.

If your current books can’t answer those questions, it’s time to fix them. Book a free 15-minute call and we’ll tell you what’s missing and what it would take to fix.

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