A repair shop is really three businesses stapled together: you sell parts at one margin, labor at a completely different one, and you pass through work you did not do at almost no margin at all. Most shop books stack all of that into a single "sales" line, and once that happens the P&L can only tell you whether the month was up or down. It cannot tell you which half of the shop is making money.
Here is how to structure the books so the reports answer the questions an owner actually has: is my labor rate high enough, are my parts being marked up properly, and where did the cash go.
Split revenue before anything else
Set up separate income accounts for parts, labor, tires, sublet, and shop fees, with a matching cost account for each one that has a cost. That single change turns a flat sales number into gross margin by revenue type, which is the number that runs a shop.
The reason it matters is that the two main lines behave nothing alike. Parts margin is set by your purchase price and matrix and moves when a supplier changes pricing. Labor margin is set by your posted rate against what you pay technicians and moves when the mix of work changes. Blend them and a collapse on one side is invisible for months because the other side covered it.
Sublet deserves its own pair of accounts more than anything else on the list. Towing, machine work, glass, alignments, and diagnostics you send out get billed through with little or no markup. Run them through general sales and you inflate revenue with money that was never yours and push your overall margin percentage down for no real reason. Kept separate, sublet is visible as what it is: volume that passes through the shop.
Cores, fees, and sales tax
Core charges are not sales. You bill a deposit on a rebuildable part and refund it when the old unit comes back, so most of the money reverses. Booked as parts revenue, cores inflate sales in one month and dent them in another, and they quietly wreck parts margin in both. Give cores their own account so charges and refunds net against each other and you can see whether cores are actually being returned to the supplier.
Shop supply and hazmat disposal fees are genuine income and should sit in their own account, with the disposal contract as its own expense. That pairing tells you whether the fee is covering the cost it exists to cover, which is a question most shops have never actually checked.
Sales tax is the one that causes real damage. Tax you collect is not revenue at any point — it is money you are holding for the state, and it belongs in a liability account from the moment it is charged. Recorded as income it makes a good month look better than it was, and the bill arrives against cash you have already spent. Which parts of a repair order are taxable varies by state, and labor is treated differently from parts in many of them, so confirm how your state handles it rather than assuming your software's default is right.
Technician pay and your real labor rate
Technician wages are a cost of the labor you sell, not office overhead, and they belong in cost of sales. Put them in overhead and gross profit becomes a meaningless number, because the largest cost of delivering the work is not sitting against the work.
Once that is right, you can calculate the figure that matters more than your posted rate: labor revenue divided by the hours you actually paid for. Under flat rate you bill book hours but you pay for the clock — vacation, training, cleanup, waiting for parts, and every comeback. The gap between the two is your effective labor rate, and it is usually well below the number on the wall.
Warranty and comeback work is where that gap opens up. Those hours consume payroll and produce no invoice, so they never appear as a cost of anything unless you track them. A shop that quietly gives away six hours a week is losing real money that no report will show if the time is buried in general payroll.
Keeping it clean month to month
None of this survives without reconciliation. The parts account only means something if the supplier statements agree with what is on the books, and card processing deposits arrive net of fees, so revenue is understated every month the fee is not recorded separately. Customer deposits on big jobs are a liability until the work is done, not income on the day the money lands.
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