Job costing is attaching every dollar you spent to the job that caused it, so that you can say what each job made instead of only what the year made.
Written by JD Davenport, who still sells for a living. Updated 4 November 2026.
That is the whole definition. Everything else is the practical question of how a dollar gets attached, which is where it goes wrong.
You sell a roof for $18,000. Material from the supplier comes to $5,200. The crew is paid $4,100. The dumpster is $480. The permit is $190. Nothing else touched that job.
Direct cost is $9,970. Gross profit is $8,030, which is 44.6 percent of the contract.
That is job costing. Four costs, one job, one number at the end. The difficulty is never the arithmetic.
It is not your profit and loss. The P and L tells you what the business did over a period. Job costing tells you what each job did. You can have an accurate P and L and still have no idea which work is worth selling more of, which is the decision you actually have.
It is not net profit. Job costing stops at the costs that exist because the job existed. Rent, insurance, the truck payment and your own salary are real and they are not job costs. Mixing them in makes every job look worse and tells you nothing about which ones to sell.
It is not estimating. The estimate is what you thought it would cost. Job costing is what it did cost. The gap between those two is the most useful number in a contracting business and almost nobody writes it down.
A job's cost is not a number you hold. It assembles itself over weeks, from four directions, and it arrives late.
Cost lands late. The job closed in March. The disposal invoice arrives in May.
Cost lands in the wrong column. A labor invoice with dump fees on it goes entirely to labor, so labor looks high and disposal looks free.
Cost lands on the wrong job. Two jobs on the same street, one purchase order. The material goes to whichever job number got typed.
Credit never lands. Returned material is credited to the account instead of the job, so the job still carries material that came back on the truck.
None of these is carelessness, and none of them is fixed by asking anyone to be more careful. They are what happens when cost information arrives from four systems on four timetables and nothing forces them to agree.
Because an average hides a tail, and the tail is where the money is.
A company averaging 28 percent might be running a steady 27 to 29 on everything, or it might be running 40 on most jobs and losing money badly on five of them. Same average, different business, different fix. The first has a pricing problem. The second has an execution problem on a handful of jobs and cannot name which ones.
Job costing is how you tell those two apart. That is the entire payoff and it is enough.
Take the last ten jobs you finished. For each one, write the contract value and every cost you can attach to it, one row per payment rather than one row per invoice batch. Mark any cost you could not attach to a job. Mark any payment you cannot produce a document for.
Two things come out of that afternoon. A margin for each of ten jobs, which you probably did not have. And a list of what did not reconcile, which is more valuable, because it tells you exactly where your cost information is leaking.
Do not chase the profit number first. Chase the short list.
The method, the twelve tests that find what did not reconcile, and the workbook are The Job Cost Standard. There is a free calculator on the free tools page. More answers on the questions page.