In QuickBooks Online, turn on Projects in your settings. In QuickBooks Desktop, create Jobs under each Customer. Either version lets you assign expenses to specific projects so costs accumulate in the right bucket. The setup takes a few minutes and there are plenty of tutorials for that part.

The real challenge is building a process that captures every material purchase with the right job code. When you enter a bill from your lumber supplier or record a credit card charge from the hardware store, there’s a Customer or Project field. Fill it in. Every time. If you skip that field, the expense exists in your books but doesn’t appear in job cost reports. Your overall numbers look fine while your per-job visibility is useless.

Bulk purchases trip people up. You buy $1,200 in materials and some goes to three different jobs. Split the expense across multiple lines, each assigned to the correct project. Yes, this takes an extra minute. The alternative is material costs that don’t reflect reality on any of your jobs.

Crew purchases need a system. Your guys stop at Home Depot and pay with petty cash or their own cards. Those receipts need to make it into your books with job codes attached. Have crews write the job name on every receipt. Collect receipts weekly and enter them. If receipts disappear, you’re missing costs and your job margins are wrong.

If you stock common materials and pull from inventory for jobs, that’s a more complex setup involving inventory items and assemblies. Many smaller contractors skip formal inventory tracking and just expense materials when purchased, which works fine as long as you’re coding to the right job at purchase time.

Run a Job Profitability report or filter your Profit and Loss by Project to see what you’ve spent on materials for each job. Compare that to your estimate. If framing materials ran 20% over on the last three projects, you know your estimates need adjustment.

A bookkeeper for small business owners who understands construction can set this up correctly and show you how to maintain it. The mechanics of job costing aren’t complicated, but getting useful data out requires consistent coding from the start. Fixing it later means going back through months of transactions and guessing which job that hardware store charge was for.