Guide
Most solo tradesmen either carry too little and burn hours on return trips, or too much and quietly bleed cash on parts that sit in a bin for a year. Here's a sizing rule and a per-part test that gets you to the sweet spot.
The instinct is to carry one of everything so you never turn a van around. That instinct is expensive. Getting from 85% first-time fix to something close to 100% typically means doubling the value of stock on the van, because the last 15% of jobs are the odd, low-frequency parts that only come up once or twice a year each.
A solo operator who accepts 85% first-time fix and treats the other 15% as a scheduled return trip or a supplier run will carry far less dead weight than one chasing 98%. The lost time on that 15% is real, but it's cheaper than the carrying cost of stocking for every edge case.
A workable rule: carry two weeks' worth of your top 20 parts by usage frequency. Pull your invoices or job sheets from the last three months, rank parts by how often you actually fitted them, and stock enough of the top 20 to cover roughly ten working days without reordering.
For most solo vans this lands between $1,500 and $3,500 in stock value. That's well under the $2,000-$10,000 often quoted for multi-van fleets, because you're only sizing for one person's workload and one van's storage, not for shared jobs or backup crews.
If your number comes out above $3,500, check whether you're carrying stock for jobs you rarely do any more, or specialist items that belong in a supplier order rather than the van.
Stock on the van costs money even when it never moves. Three things eat into it:
Across a working year, these typically run 20-25% of your stock's value. So $4,000 of van stock costs you roughly $800-$1,000 a year just to hold, before a single part is ever used. That's not a reason to carry nothing. It's a reason to be deliberate about what earns its space.
| Stock value on van | Carrying cost per year (20-25%) |
|---|---|
| $1,500 | $300 - $375 |
| $2,500 | $500 - $625 |
| $3,500 | $700 - $875 |
| $4,000 | $800 - $1,000 |
| $8,000 | $1,600 - $2,000 |
Instead of guessing, run each part through a simple test:
Stock it if: uses per year > (part cost × 0.25) ÷ cost of one return trip
The part cost × 0.25 is a stand-in for the annual carrying cost of that one item (using the 25% figure from above). The cost of one return trip is your fully loaded cost of stopping the job, driving to a supplier, and driving back — fuel, time, and the labour you'd otherwise be billing. If a part clears the bar more often than that number of times a year, it earns its spot on the van. If it doesn't, order it in as needed.
Say a return trip to your usual supplier costs you $45 in fuel and lost billable time — a rough figure you can pull from your own job profitability numbers.
Part A: a $60 pressure valve.
N = (60 × 0.25) ÷ 45 = 15 ÷ 45 = 0.33.
You only need to use it once every three years for it to be worth stocking. Carry it.
Part B: a $400 specialist control board.
N = (400 × 0.25) ÷ 45 = 100 ÷ 45 = 2.2.
Unless you fit more than two of these a year, don't carry it. Order it when the job comes up, even if that means a scheduled return visit.
This is why cheap, common parts almost always belong on the van, and expensive, occasional ones almost never do — even if it feels wrong to drive back for something you know you'll eventually need again.
Use the two together, not one instead of the other:
Re-check this list every quarter. Job mix shifts — a run of boiler work versus a run of bathroom fit-outs changes which 20 parts matter.
With a properly sized top-20 list, most solo operators land at 80-90% first-time fix without knowing it — because the parts that cause the bulk of return trips are usually the cheap, common ones the rule catches automatically. The remaining misses are the genuinely unusual jobs, which is a trip cost you were always going to pay somewhere, either in stock or in fuel.
If you want to check whether your current pricing already covers the cost of occasional return trips, run the numbers through a break-even calculator — it's a quick way to see if a few extra trips a month actually move your margin or if they're noise.
None of this needs software. A spreadsheet with part name, cost, uses per year, and a return-trip cost column does the job. If you want a template already built for tracking this alongside job costing, the small business operations kit has one you can drop your own numbers into rather than starting from a blank sheet.
The goal isn't a perfect van. It's a van stocked with the parts that pay for themselves, sized so the cash isn't sitting there doing nothing.
The Small Business Operations Kit ($19) turns guides like this one into fill-in-once worksheets: rate card, quoting sheet, job costing, payment terms and follow-up templates.