In the used-car business, time is the enemy of gross. Every day a unit sits, it costs you floorplan interest, depreciation, and lot space — and it earns you nothing. The two numbers that tell you whether your inventory is moving fast enough are days supply and turn. This guide covers how to calculate both, why the store-wide totals can lie to you, and what a healthy turn rate looks like.
Days supply: how many days of inventory you're carrying
Days supply answers a simple question: at your current sales pace, how many days would it take to sell everything on the lot?
Days supply = Units in stock ÷ (Units sold in period ÷ days in period)
Or, more simply: units in stock divided by your units sold per day. If you have 120 used units in stock and you sold 78 last month, your sales pace is 2.6 units/day, and your days supply is about 46 days.
Lower is generally better — it means capital is turning quickly and units are selling before they age. But there's a floor: too little supply and you can't offer selection. Most stores target a days supply in the 30–45 day range, adjusted for their turn strategy and market.
Turn: how many times your lot refreshes per year
Turn (or turn rate) is the annualized version of the same idea — how many times your inventory "turns over" in a year.
Annual turn = 365 ÷ Days supply
At 46 days supply, that's about 7.9 turns per year. You can also calculate it as annual units sold divided by average units in stock; both get you to roughly the same place.
What's a good turn rate? A common benchmark for most markets is 12 to 16 turns per year, with the strongest operators pushing well past 20. Any store consistently below 10 almost always has a process problem — stale pricing, reactive stocking, or no weekly review cadence — rather than a market problem. For reference, the NADA used-car guidance has long pointed toward a roughly 30-day turn as the target, which implies a turn in the low-to-mid teens.
Why the totals hide your real problem
Here's the trap: your store-wide days supply can look perfectly healthy while specific segments quietly age out. You might show 38 days overall — and be sitting on 80 days of full-size trucks and 15 days of compact SUVs. That imbalance is invisible in the total and obvious in the segments.
The best used-car managers live in the segments, not the totals. Break days supply down by body style and price band on a regular cadence (weekly is ideal), so you can redirect your buying into the short-supply segments before the oversupplied ones age into wholesale losses. A single blended number is a starting point; segment-level days supply is where the decisions get made.
Aging buckets and the danger zone
Turn and days supply tell you about velocity. Aging buckets tell you where the risk is concentrated right now. Sort every unit by how long it's been in stock — 0–30, 31–60, 61–90, and 90+ days — and watch the percentage sitting in the danger zone.
The metric that matters isn't your average age; it's the share of your lot past 60 days. The NADA guidance is to aim for no used units over 60 days. If a meaningful chunk of your inventory is over 60 — say 15–20%+ — you have a stocking or pricing problem, not bad luck. And the fix is catching units before they cross the line, not marking them down after.
Turn the numbers into a plan
Calculating these once is diagnostic; calculating them regularly is management. The workflow that separates fast-turning lots from slow ones is simple and repeatable: measure days supply and turn (in total and by segment), watch the over-60 percentage, and act on aging units early with pricing and merchandising rather than waiting for the wholesale cliff.
Our Used-Car Inventory & Margin Tool runs this for you: enter your aging buckets and last month's sales, and it shows your days supply, turn, aging mix, front gross per unit, and recon time against benchmarks — plus the holding cost your aged units are burning and the realistic gross you can recover. Excel or Google Sheets, with every benchmark editable to your market.
For what those aging units are actually costing you, see The Real Cost of Aged Used-Car Inventory. For a repeatable policy to keep units moving, see A Used-Car Aging Policy That Actually Works.
FAQ
How do you calculate days supply for used cars? Divide units in stock by your units sold per day (units sold in the period ÷ days in the period). 120 units and 78 sold last month is about 46 days supply.
What is a good used car turn rate? Roughly 12–16 turns per year for most markets, with top operators higher. Below 10 usually signals a pricing or stocking problem.
How do you calculate inventory turn? 365 divided by days supply, or annual units sold divided by average units in stock.
Why does my total days supply look fine but cars are still aging? The total hides segment imbalances. Break days supply down by body style and price band — you're likely oversupplied in some segments and short in others.
Benchmarks in this article are directional and vary widely by market, segment, and floorplan terms; figures reference the NADA and used-car industry sources. Validate against your own lot's numbers.